INSTRUMENTATION: WATER AND WASTEWATER ANALYSIS INTRODUCTION been dedicated to detect and measure water pollutants and understand their effect on human populations and on the ecological system, as well as on the collection and rectification of wastewater in treatment facilities However, much more remains to be done A realistic primer may help us to visualize the overall effects of water pollution Sitting by an ecologically healthy lake or stream, we observe a proliferation of life—plants and animals familiar and cherished by us Comparing that to our experience of being next to a polluted water body, we would notice different plants, not attractive to us and the presence of foul offensive odors (However for a lake acidified by acid rain, very clear waters, devoid of life, are observed.) The system has changed from being aerobic (presence of dissolved oxygen) to anaerobic (lack of dissolved oxygen) The water body has changed so that it is no longer attractive to us nor can it serve as a water resource A lack of dissolved oxygen in the water has changed the living conditions so that anaerobic fauna and flora can reside there Two conditions can cause this situation: i.e., an excess of nutrients (such as nitrates or phosphates) serving to facilitate the growth of plants and an excess of biodegradable organic matter serving as food for the microbial population These pollutants originate from human biological waste and human activities such as agriculture and industry An excess of biodegradable organic matter leads to an accelerated growth of the microbial population Since they are aerobic and require dissolved oxygen in the water for respiration, a large population could deplete the dissolved oxygen supply leading to the asphixiation of fish, other animals and insects and the death of plants Then anaerobic fauna and flora will flourish producing reduced gaseous substances, such as ammonia and hydrogen sulfide These gases are toxic and unpleasantly odiferous Although water can be reaerated by the air above its surface to provide a supply of dissolved oxygen, the process is very slow allowing for the conditions of oxygen depletion to exist for long periods of time Another mechanism leading to the same result is caused by an excess of nutrients The presence of excessive amounts of nitrates and phosphates spur algae growth in the water body The upper layers of algae shield the lower layers from sun light This situation causes death of the lower layers of algae adding large amounts of biodegradable organic matter to the water body and an explosion in microbiological growth Thus, through the action described above the In the observation of our pollution problems there seems to be an attitude of separation on the part of the human observer from the polluted lake or stream In reality water is so pervasive in our life; it is such a large part of our bodily mass and surrounds us in clouds, fog, rain, snow, lakes, rivers, and oceans We seem to accept its presence without much thought However, we all are part of the ecosystem and, therefore, pollution is an intimate condition of our lives—not something unconnected to us Much of the human population appears to have been separated from their ecological heritage and membership Perhaps this is the reason pollution is so endemic to our world; many people had seen pollution as something displaced from their intimate reality In the last thirty years the threat and cause of damage to ecological and human health from polluting surface and ground water and acid rain and snow, as well as air pollution, global warming, and the destruction of the ozone layer has increasingly occupied our consciousness and our everyday life The society from young school children to adults reading newspapers and watching television are aware that we are heirs to serious environmental problems Polls indicate the great extent of this concern Recently the concerns of various national governments have led to international conferences dealing with the ozone problem and discussion of global warming Perhaps the convergence of several environmental conditions that threaten to change planet earth’s ecological system have awakened the irresponsible amongst the citizenry, government administrators, scientists and engineers, and the industrial establishment to finally realize that we are all part of the ecological system and have a vital interest in the control of pollution The Clean Water Acts of the U.S Congress and environmental action of various States and similar actions in Canada have resulted in some improvement in natural water quality in North America The role of the Green parties and the citizenry has had a similar effect in Western European nations In Eastern Europe there is increasing concern about pollution problems Much remains to be done in the areas of irrigation, non-point source pollution, acid rain and snow, the effect of air pollution on water pollution, protection of ground water from hazardous wastes, and the further reduction of pollution from industrial sources Extensive human effort and resources have 538 © 2006 by Taylor & Francis Group, LLC INSTRUMENTATION: WATER AND WASTEWATER ANALYSIS dissolved oxygen content is greatly reduced and anaerobic conditions develop Another category of pollution are the toxic substances entering water bodies, such as some synthetic organic materials and toxic metals and non-metals: they cause the death of aquatic plants and animals disrupting the water ecosystem Non-biodegradable substances may be toxic, cause problems due to their physical nature, or detract from the beauty of nature From a consideration of the foregoing descriptions of the mechanisms of pollution effects, a number of parameters for the determination and control of water pollution can be listed For example degradable organic matter, non-biodegradable substances, dissolved oxygen, nitrates and phosphates, and toxic metals, non-metals and organic matter are classes of substances requiring methods of analysis However the previous and rather bare outline of the pollution scenario does not expose the complex problems in describing the ecological mechanisms affected by pollution and their attendant solutions The definition of a problem is necessary if one is to prescribe a solution The more complete the definition, the more precise and comprehensive the proposed interpretation can be Unfortunately, we not have the luxury of unlimited time to adequately define the various environmental problems; we must institute actions using the knowledge at hand and update and improve our interim solutions as we approach a more complete definition of each of the problems Indeed, the answers to the problems of water pollution and abatement have been undertaken in this vein The large question, what we measure, brings us to the complexity of the issue, since what we measure is connected to why we measure a particular property or component The attempt to answer these questions cannot be undertaken in this relatively short article, however, a very limited response will be given to these questions This article will describe the operation and use of chemical instrumentation both in the laboratory and in monitoring instrumental systems, for data collection necessary for refining the definition of the environmental water problems, monitoring of processes to treat wastewaters and drinking water, and the ecological monitoring of natural waters WATER AND WASTEWATER ANALYSIS In the last fifty years the advances in electronics have made possible the development of the sophisticated instrumentation and computer systems which serve very well the purposes described herein Development of chemical sensors and their combination with instrumentation has resulted in the laboratory and monitoring chemical measurement instruments so commonly found in laboratories, environmental monitoring systems and manufacturing plants In addition the interfacing of these instruments and computer systems results in effective and creative data handling, computation, and prediction © 2006 by Taylor & Francis Group, LLC 539 A general consideration of the analysis of water and wastewater samples brings forth several factors to consider What characteristics need to be monitored and for what reasons? How we obtain a representative sample of the source to be analyzed and how we preserve its integrity until an analysis is completed? What constitutes our present methodology and with what biological, chemical, physical and instrumental means we carry out these measurements? However, a primary consideration in answering these questions relates to the nature of water and wastewater samples The Nature of Water-Related Samples and Sampling Considerations Nature of Water-Related Samples The category of water and wastewater samples can include water samples, sludges, benthic muds, plant matter and so forth Samples may be taken from a number of systems: for example, natural water bodies, process streams from wastewater treatment and manufacturing plants, benthic environments, marshes, etc Different procedures for sampling can be required for each variety of sample based on their unique chemical, physical, and/or biological nature Water is alluded to as the universal solvent for good reasons; it is the best solvent humans experience In addition to dissolved substances water can also transport insoluble, suspended, and/or colloidal matter Thus, a sample can contain components in a number of physical states: i.e.; dissolved, in ionic and molecular form; insoluble, as are bubbles of gas, and suspended and colloidal chemical substances; and biological organisms in a variety of sizes The determination of the identity and concentration of unique chemical and biological components is important The presence of these components give to the water sample biological, chemical, and physical characteristics—such as physiological qualities, acidity, alkalinity, color, opacity and so forth Sludges and mud samples are heterogeneous mixtures containing water with dissolved matter and the sampling procedure must not change the composition of the mixture Plant matter, having its own unique characteristics, requires the proper procedures for sampling.1 Many samples display time-based changes once taken from the source for a host of reasons Changes are evident over various time scales For example suspended matter settles during a time period as determined by particle size giving a change in opacity and/or color, chemical reactions may occur amongst components, gases and volatile substances may diffuse to the surface of the sample and evaporate, gases or volatile substances in the air space above the sample may condense and dissolve at the sample interface, etc Substances in benthic samples can experience air oxidation and plant matter can lose moisture and so forth All of these changes present a deviation of the sample’s composition and characteristics from the source The seriousness of the changes depend on the purpose of the analysis 540 INSTRUMENTATION: WATER AND WASTEWATER ANALYSIS and its use There must be no perturbation of the sample composition or physical characteristics which will nullify or seriously distort the analysis results and be detrimental to the purposes of the analysis Preservation is a means of preventing decomposition and change of various biological or chemical components in the sample Standard Methods provides a treatment for the preservation of specific components in a variety of samples where preservation is possible (see Table 1).2 Sampling Considerations Two types of changes must be considered in order to define and carry out a proper sampling program, i.e., time-based changes in the source to be sampled and in the sample taken from the source Time-based changes in the source may be short or long term The short-term changes taking place are indicative of biological, chemical, and/or physical interactions in the time span assigned to repetitive sampling Long-term changes in the sources are related to ecological trends It is important to be aware of each of these types of changes so that sample storage changes are not mistaken for sample source changes In order to obtain a legitimate sample of water or wastewater for analysis one must understand the nature the sample and its time-based changes Once a sample taken from the source it may begin to change for many reasons as discussed in the previous section, IIA,1 The sample must be representative of the sample source, that is it must have the same biological, chemical, and physical characteristics of the source at the time the sample is taken Therefore the sampling procedure must not cause a change in the sample relative to the source if there is to be an accurate correspondence between the sample analysis and the nature of the sample source at the time the sample was taken To prevent confusing the two variables, ecological changes in the source and interactions in the sample that can occur in the same time span in a series of samples, preservation of the samples is undertaken to deter changes after sampling However, preliminary sampling and testing may be necessary to indicate the type of time-based changes occurring Then a reliable program can be established with some certainty Three kinds of sampling schemes are undertaken in consideration of the sample source characteristics—grab, composite and integrated samples A grab sample is one taken at a given time and place If the source doesn’t change greatly during a long passage of time or within a large distance in all directions from the sampling point, the grab sample is useful The results from grab samples are said to represent the source for the given values of distance from sampling location and time However a time and place series of grab samples is needed to establish the constancy of analytical values for different times and distances from the original grab sampling point With that information the sampling frequencies and times for a sampling program can be established The sampling of solids, such as, benthic muds and sludges, requires great care in order to obtain truly representative samples © 2006 by Taylor & Francis Group, LLC A series of samples collected and blended to give a time-averaged sample is known as a composite or timecomposite sample In another procedure the volume of each sample of the series collected is proportional to flow of the sample source, namely the water body or waste stream The samples of various volumes are composited to provide the final time-composite sample This type of sample gives an average value over a time period and saves analysis time and cost Sampling frequencies and the total time span of the series depends on the source Composite sampling may be used for process streams to determine the effect of unit processes or to monitor a plant outfall for daily or shift changes However, biological, chemical, and physical parameters that changes on storage during the sampling time period can’t be reliably determined in time-composited samples and another sampling protocol is needed (see Table 1) At times, simultaneous samples are needed from various locations within a given source, such as a river or lake Grab samples are then composited, usually based on volumes proportional to flow, and are called integrated samples These samples are used to determine average composition or total loading of the source which varies in composition in its breadth and depth The sampling program for such sources is complex and requires careful consideration for each unique source Water and Wastewater Parameters A large number of water quality parameters are utilized in the characterization, management and processing of water and wastewater Table lists a number of these parameters separated into three categories—physical, chemical and biological It is obvious that only some parameters are considered to be pollution factors because they indicate conditions of water during processing or in the natural state The STORET system of the USEPA lists more than four hundred parameters separated into six major groups and is used for the analysis, collection, processing and reporting of data.3 Table gives a sampling of groups of parameters in this system Not all of these parameters are used frequently, since many are rather unique to particular waste effluents In actuality, a very small number are used in the analysis of a particular sample Water quality parameters may be divided into two groups, specific and non-specific water quality parameters Specific parameters refer to chemical entities of all types, e.g., ions, elements, compounds, complexes, etc For example, in Table some specific parameters are ammonia, all metals listed, dissolved oxygen, nitrates, sulfates, and so forth Nonspecific parameters are included in three categories and some examples are as follows: chemical (hardness, alkalinity, acidity, BOD [biochemical oxygen demand], TOC [total organic carbon], COD [chemical oxygen demand], chlorine demand), physical (salinity, density, electrical conductance, filterable residue), and physiological (taste, odor, color, turbidity, suspended matter) Many of these non-specific parameters are 541 INSTRUMENTATION: WATER AND WASTEWATER ANALYSIS TABLE Summary of special sampling or handling requirementsa,* Container† Determination Minimum Sample Size mL Sample Type‡ Maximum Storage Recommended/ Regulatory Preservation§ Acidity P, G(B) 100 g Refrigerate 24 h/14d Alkalinity P, G 200 g Refrigerate 24 h/14 d BOD P, G 1000 g Refrigerate h/48 h Boron P 100 g, c None required 28 d/6 months Bromide P, G 100 g, c None required 28 d/28 d Carbon, organic, Total G 100 g, c Analyze immediately: or refrigerate and add H3PO4 or H2SO4 to pH Ͻ d/28 d Carbon dioxide P, G 100 g Analyze immediately stat/N.S COD P, G 100 g, c Analyze as soon as possible, or add H2SO4to pH Ͻ 2; refrigerate d/28 d Chloride P, G 50 g, c None required 28 d Chlorine, residual P, G 500 g Analyze immediately 0.5 h/stat Chlorine dioxide P, G 500 g Analyze immediately 0.5 h/N.S Chlorophyll P, G 500 g, c 30 d in dark 30 d/N.S Color P, G 500 g, c Refrigerate 48 h/48 h Conductivity P, G 500 g, c Refrigerate 28 d/28 d P, G 500 g, c Add NaOH to pH Ͼ 12, refrigerate in dark# 24 h/14 d; 24 h if sulfide present Amenable to chlorination P, G 500 g, c Add 100 mg Na2S2O3/L stat/14d; 24 h if sulfide present Fluoride P 300 g, c None required 28 d/28 d Hardness P, G 100 g, c Add HNO3 to pH Ͻ months/6 months Iodine P, G 500 g, c Analyze immediately 0.5 h/N.S Metals, general P(A), G(A) 500 g For dissolved metals filter immediately, add HNO3 to pH Ͻ months/6 months P(A), G(A) 300 g Refrigerate 24 h/24 h P(A), G(A) 500 g, c Add HNO3 to pH Ͻ 2, 4ЊC, refrigerate 28 d/28 d Ammonia P, G 500 g, c Analyze as soon as possible or add H2SO4 to pH Ͻ 2, refrigerate d/28 d Nitrate P, G 100 g, c Analyze as soon as possible or refrigerate 48 h/48 h (28 d for chlorinated samples) Nitrate ϩ nitrite P, G 200 g, c Add H2SO4 to pH Ͻ 2, refrigerate none/28 d Nitrite P, G 100 g, c Analyze as soon as possible or refrigerate none/48 h d/28 d Cyanide: Total Chromium VI Copper by colorimetry* Mercury Nitrogen: Organic, P, G 500 g, c Refrigerate; add H2SO4 to pH Ͻ Odor G 500 g Analyze as soon as possible; refrigerate h/N.S Oil and grease G, wide-mouth calibrated 1000 g, c Add HCl to pH Ͻ 2, refrigerate 28 d/28 d Kjeldahl* Organic compounds: MBAS P, G 250 g, c Refrigerate 48 h Pesticides* G(S), TFE-lined cap 1000 g, c Refrigerate; add 1000 mg ascorbic acid/L if residual chlorine present d/7 d until extraction; 40 d after extraction Phenols P, G 500 g, c Refrigerate, add H2SO4 to pH Ͻ */28 d (continued) © 2006 by Taylor & Francis Group, LLC 542 INSTRUMENTATION: WATER AND WASTEWATER ANALYSIS TABLE Summary of special sampling or handling requirementsa,* (continued) Container† Determination Minimum Sample Size mL Sample Type‡ d/14 d Electrode Analyze immediately 0.5 h/stat Winkler Titration may be delayed after acdification h/8 h Analyze immediately 0.5 h/N.S G, TFE-lined cap Oxygen, dissolved: G, BOD bottle Ozone G g 300 Preservation§ Refrigerate; add HCl to pH Ͻ 2; add 1000 mg ascorbic acid/L if residual chlorine present Purgeables* by purge and trap ϫ 40 Maximum Storage Recommended/ Regulatory# g 1000 g PH P, G 50 g Analyze immediately h/stat Phosphate G(A) 100 g For dissolved phosphate filter immediately; refrigerate 48 h/N.S Salinity G, wax seal 240 g Analyze immediately or use wax seal months/N.S Silica P 200 g, c Refrigerate, not freeze 28 d/28 d Sludge digester gas G, gas bottle — g — N.S Solids P, G 200 g, c Refrigerate d/2–7 d; see cited reference Sulfate P, G 100 g, c Refrigerate 28 d/28 d Sulfide P, G 100 g, c Refrigerate; add drops 2N zinc acetate/ 100 mL; add NaOH to pH Ͼ 28 d/7 d Taste G 500 g Analyze as soon as possible; refrigerate 24 h/N.S Temperature P, G — g Analyze immediately stat/stat Turbidity P, G 100 g, c Analyze same day; store in dark up to 24 h, refrigerate 24 h/48 h * See text for additional details For determinations not listed, use glass or plastic containers; preferably refrigerate during storage and analyze as soon as possible † P ϭ plastic (polyethylene or equivalent); G ϭ glass; G(A) or P(A) ϭ rinsed with ϩ HNO3; G(B) ϭ glass, borosilicate; G(S) ϭ glass, rinsed with organic solvents or baked ‡ g ϭ grab; c ϭ composite § Refrigerate ϭ storage at 4ЊC, in the dark # Environmental Protection Agency, Rules and Regulations 40 CFR Parts 100–149, July 1, 1992 See this citation for possible differences regarding container and preservation requirements N.S ϭ not stated in cited reference; stat ϭ no storage allowed; analyze immediately a If sample is chlorinated, see text for pretreatment very important in water and wastewater characterization and instruments are available to measure specific and non-specific parameters Methodology The large variety of tests carried out on water and wastewater samples and sources have been codified and are included in the laboratory reference in the United States entitled “Standard Methods for the Examination of Water and Wastewater” and is commonly referred to as Standard Methods This compendia of methods is regularly updated At the present time the 19th edition published in 1995 is in use2 and a supplement was issued in 1996 Supplements are used to update methods on an ongoing basis in order not to unduly prolong the publication of the new edition However not more than one supplement appears to have been published for each edition © 2006 by Taylor & Francis Group, LLC Three professional organizations jointly write and edit this manual—the American Public Health Association, the American Water Works Association and the Water Environment Federation (formerly the water pollution Control Federation) It is published by the American Public Health Association Over five hundred professionals belonging to these organizations and others participate in Standard Methods It was first published in 1905 and an interesting history of its genesis is given in the preface to the 19th edition.2 At one time methods were segregated between water and wastewater test methods, however, since the 14th edition in 1976, that division ceased In the 19th edition, methods are classified in ten groups: Introduction, Physical Aggregate Properties, Metals, Inorganic Nonmetallic Constituents, Aggregate Organic Constituents, Individual Organic Constituents, Radioactivity, Toxicity, Microbiological Examination, and Biological Examination INSTRUMENTATION: WATER AND WASTEWATER ANALYSIS TABLE Some water quality parametersa Physical Chemical Biological Color Acids or alkali Algae Conductivity Ammonia Bacteria Odor Biochemical oxygen demand Radioactivity (BOD) Solar radiation intensity Calcium Suspended solids or Pathogens Protozoa Chloride sludges Temperature Turbidity Viruses Chlorophyll Chemical oxygen demand (COD) Dissolved oxygen Hardness Heavy metals: Chromium Copper Iron Lead Manganese Mercury Magnesium Nitrate, nitrite Organic compounds: Detergents Herbicides Pesticides Phenol Oils and greases Oxidation–reduction potential pH Phosphates Potassium Sodium Sulfate Total organic carbon (TOC) a Reprinted from Ref (4), p 1438 by courtesy of Marcel Dekker, Inc Twenty-five years ago the dearth of instrumentation was used in Standard Methods.4 However, in the present edition the following instrumentation is employed in the methodologies: molecular spectroscopy (visible, uv, ir), atomic spectroscopy (absorption, flame, ICP), chromatography (gas, ion, liquid), mass spectrometry (GC/MS, gas chromatography/mass spectrometry), electro-analytical techniques (polarography, potentiometric and amperometric titrations, selective ion electrodes), radio-activity counters (gas filled and semiconductor detectors and scintillation counters), and automated continuous-flow methods © 2006 by Taylor & Francis Group, LLC 543 Also included in Standard Methods are aspects such as safety, sampling, mathematical treatment of results, reagents, apparatus etc It is fortunate, indeed, that such a comprehensive work is available and that it is regularly revised Enlightened editorial leadership and the many members of the Standard Methods committees in the last twenty years can be credited for the steady increase in the inclusion of instrumentation in Standard Methods In the increasing complexity of environmental and ecological problems and guidance of Standard Methods is a valuable and practical support in obtaining necessary analytical data The American Society for the Testing of Materials, ASTM, is an important compendium for the analysis of raw and finished material products A large section is devoted to the analysis of water and wastewater in the context of processing and usage.5 The EPA has published instrumental methods for the analysis of priority pollutants and other substances controlled by Federal legislation.6 Types and Purposes of Instrumentation and Computer Systems The development of a large variety of analytical instrumentation has been a boon to water and wastewater characterization, research, management, and process control In addition to the requirements of the process industries, the needs of the water and wastewater area have spawned the development of some specialized laboratory, monitoring and process control instrumentation Some examples are the total carbon and organic carbon analyzers, biological oxygen analyzers and the residual chlorine analyzer Monitoring and data acquisition systems, in conjunction with this instrumentation, are increasingly used in wastewater management and plant process control Certainly a number of physical parameters such as temperature, flow rate, pressure and liquid level have been measured instrumentally in the process industries, including wastewater treatment plants, predating the development of this wide variety of analytical instrumentation.7 Instruments utilized in the measurement of parameters important to wastewater analysis, treatment and management can be divided into two categories based on application Monitoring of water bodies and waste treatment processes require monitoring instruments which are characterized by ruggedness and capability of unattended operation and data storage and/or transmission A second category, laboratory instruments, in many instances, may be more sophisticated, sensitive to the surrounding environment and also have data storage and transmission capabilities Each type has its specific utility in the scheme of analysis and data acquisition for wastewater characterization and processing In many instances monitoring instruments are laboratory devices which were ruggedized and prepaared for field use Thus the variables measured and the principles of operation are the same in many cases Some examples of variables measured by laboratory and monitoring instrumentation are pH, conductivity, DO (dissolved oxygen), specific cations and 544 INSTRUMENTATION: WATER AND WASTEWATER ANALYSIS TABLE Number of STORET listings for water analysis Parameters by groups Example Number of parameters in group General physical and chemical Alkalinity, COD, iron turbidity, zirconium Physical observations Algae, foam, oil 149 Radionuclides Gross alpha and beta, strontium-90 Microbiological Coliform by MPH and MF, total plate count 18 Carbon adsorption data Chloroform and alcohol extractables 12 Natural organics Chlorophyll, tannins Synthetic organics ABS, phenols Halogenated hydrocarbons Aldrin, heptachlor, toxaphene 62 Phosphorated hydrocarbons Malthion, parathion 10 Miscellaneous pesticides Silvex Available chlorine 12 141 Organic materials Treatment-related observations anions, and a variety of specific and nonspecific parameters by automated analyzers However, the instrumental appearance and some unique functions related to unattended operations may differ Monitoring and data acquisition systems are also considered in this article In the control of wastewater treatment systems and plants, data may be obtained exclusively from monitoring instrumental systems or a combination including data from laboratory instruments via a laboratory data system and/or from data entered through terminals Analytical instrumentation can be classified according to principles based on various physical phenomena These general categories are spectroscopy, electrochemistry, radiochemistry, chromatography, and automated chemical analysis The instrumentation described in this article is organized according to these categories INSTRUMENTATION Structure of Instruments An instrument is a device that detects a physical property or chemical entity through the conversion of a physical or chemical analytical signal to an energy signal, usually electrical, with subsequent readout of the energy signal Three main parts comprise an instrument: that is a chemical or physical sensor, signal conditioning circuits, and readout devices The sensor develops a signal, usually electrical, in response to a sample property and the signal conditioning circuit modifies the signal in order to allow convenient readout display of the signal Finally, a readout device displays the signal, representative of the sample, in terms of a reading on an analogue or digital meter, a recorder chart, an oscilloscopic trace, etc Figure delineates the three major parts © 2006 by Taylor & Francis Group, LLC and functions of an instrument, sample properties (measurand) to be measured, and instrumental criteria Sensors A sensor, the primary contact of the instrument with the sample, is a device that converts the input energy derived from a sample property to an output signal, usually electrical in nature The relationship between the input energy (measurand), Q1, and the output energy, Q0, is expressed in the form: Q0 ϭ f(Q1) (1) and is known as the transfer function The sensitivity is given in the equation S ϭ dQ0/dQ1 (2) When the transfer function is linear, the sensitivity is constant throughout the sensor’s range However, the sensitivity (gain or attenuation factor) is dependent on the value of the differential fraction in equation The sensor threshold is the smallest magnitude of input energy necessary to obtain a measurable change in the output Readout signals may be digital, D (discrete), or analog, A (continuous), in form and are a function of the nature of the input signal and the sensor and the design of the signal conditioning circuits These signals are interconvertible using A/D or D/A devices Fast reacting sensors and circuits, however, are utilized for producing digital signals, where, formerly, analog signals were obtained Two varieties of sensors, chemical and physical, are in use on various instruments The physical sensor allows the conversion of physical energy from one to another One example is a photocell that converts an impinging light beam INSTRUMENTATION: WATER AND WASTEWATER ANALYSIS Measurand Sample property Light Absorption Emission Thermal Conductivity Temperature Heat capacity Electrical Redox Electrolytic conductivity Ionic activity Mechanical Mass Density Viscosity Surface tension Nuclear (X-ray, b.g) Emission Absorption FIGURE Input Signal Sensor Energy transducer Transducer Signal Light Photocell Photographic plate Thermal Thermocouple Katharometer Thermister Bolometer Electrical Electrode pair Electrodes, AC system Membrane electrode Mechanical Balance force transducer Force transducer Hydrometer Viscosity pipet Nuclear Ionization tubes Scintillation counters Photographic plates Cloud chamber Semiconductor detectors Signal conditioning Signal modification Amplification Arithmetic operation Chopping Comparison to reference Digitization Rectification Stabilization 545 Readout Types Output Signal Analog Meter Oscilloscope Recorder Digital Nixie display Point plotter Printer Tape, paper, or magnetic Instrument Criteria Band width Noise figure Sensitivity Signal-to-noise Time constant Error Hysteresis Nonlinearity Scale Zero displacement Diagram of instrumental functions Reprinted from Ref (4), p 1442 by courtesy of Marcel Dekker, Inc to an electrical signal and is used in spectrometers A second example is a piezoelectric crystal-based sensor that converts a mechanical force to an electrical charge translatable to a potential The piezoelectric effect is reversible; an electric charge will cause a mechanical dislocation in the crystal Another example of a physical sensor is a platinum resistance thermometer where the resistance of a platinum wire is altered by a change in temperature Chemical sensors are devices that allow the analyte or target material through one of its specific chemical parameters to ultimately generate an energy signal, usually electrical, in a transducer through the agency of a selective chemical or physical chemical reaction A transducer is a material structure inside of which or on whose surface the specific chemical or physical chemical reaction takes place leading to the generation of the energy signal Thus, there are two parts to the chemical sensor, the interface zone or area where the selective reaction takes place and the usually non-specific transducer.8 Figure illustrates, functionally, the parts of a chemical sensor An example of a chemical sensor is a potentiometric electrode Here the selective chemical reaction, the redox reaction of the analyte, is in equilibrium at the electrode surface imposing a potential that is proportional to the logarithm of the concentration of the analyte as described by the Nernst equation For example, a copper electrode in a solution of copper ions will take on a potential in response to the concentration of copper ions The logarithm of the copper © 2006 by Taylor & Francis Group, LLC ion concentration is proportional to the electrode potential Another illustration of a chemical sensor is an amperometric electrode, where a current arises due to the redox reaction of the analyte when the electrode is at the appropriate potential The concentration of the analyte is proportional to the magnitude of the current A platinum electrode maintained at the redox potential for the silver/silver ion redox system will detect the concentration of silver ions A membrane electrode is another type of chemical sensor The fluoride electrode consists of a lanthanum fluoride (LaF2), thin, crystal membrane On the outside surface, the sample side of the membrane, the fluoride ions, FϪ, from the sample are attracted electrostatically to the lanthanum ion, La3ϩ, at the surface of the membrane to form a complex The complexed entities not penetrate very deeply into the surface The amount of FϪ complexed is a direct function of its activity (see Section III,B,2,a) and represents a selective physical chemical reaction A membrane potential arises because the opposite side of the membrane is exposed to a standard activity of FϪ giving a net difference in potential between the two sides The membrane potential is the non-specific electrical signal of the sensor Signal-Conditioning Circuits These circuits modify the signal produced by the sensor so as to provide an accurate representation of the sensor signal with optimal electrical characteristics to drive the readout device In Figure a number of signal conditioning modes are given and can be 546 INSTRUMENTATION: WATER AND WASTEWATER ANALYSIS CHEMICAL SENSOR TARGET or ANALYTE FIGURE INTERFACE ZONE selective reaction (chemical or physical electrical signal Chemical sensor placed in four categories—modification of sensor output, amplification, mathematical operation, and signal modification for readout The electrical components used in these circuits are of two types, active and passive elements Active elements, such as solid state devices add energy to a circuit; whereas passive elements, such as resistors, capacitors, inductors, diodes add no energy Both elements are combined to form active and passive circuits Active circuits change signals in a complex way Passive elements are used in active circuits to provide necessary conditions for the proper functioning of active circuits Some active devices are ionization chambers, vacuum phototubes, operational amplifiers and gas discharge tubes Readout Devices The sensor signal modified by the conditioning circuits is ultimately converted into a visual form by the readout device or output transducer The readout signal may be analog or digital requiring a compatible readout device Analog readout devices comprise recorders, meters, oscilloscopes, photographic plates and integrators; printers, computers and digital meters with optical displays provide digital readouts A digital computer may be interfaced to an instrument, in order to compute values from a digital output signal and produce a hard (printed) copy of the data using a printer Analog output signals may be digitized in order to utilize a computer The advantages of digital outputs are the statistical benefit derived from counting and analog outputs are advantageous in feedback control systems Analog Devices The automatic recording potentiometer or potentiometric recorder has been, over the years, the most frequently used readout device providing a continuous trace on a chart of an analog signal Its operation is based on a low power servomechanism utilizing a feedback system The instrumental signal to be measured is compared to a standard reference signal The amplified, difference or error signal activates the pen-drive motor moving the pen on the chart to a position representing the magnitude of the analog signal The control of the pen, based on the error signal, denotes the feedback system and the total system is referred to as a servomechanism.9,10 Two types of recorders, the Y-time or X–Y, allow the recording of a signal, © 2006 by Taylor & Francis Group, LLC TRANSDUCER Y, as a function of time or of two signals representing the ordered (data) pair, x, y, respectively In the Y-time device, a constant-speed motor moves the chart in the x direction while the servomechanism deals with the y signal The X–Y recorder has two servo-systems, one for each signal, x and y However, recorders may be limited by the rate that the data flows from the instrument Some recorders can adequately respond to signals during fast scans For example fast scans in cyclic voltammetry of about volt/sec can be transcribed using a recorder, however, at faster rates an oscilloscope is necessary Almost any instrument can utilize a potentiometric recorder A Y-time analog recorder is commonly used to trace gas and liquid chromatograms; the abscissa, X axis, is for retention volume or time and the ordinate is for the detector response The oscilloscope is a measuring device with complicated circuitry that allows accurate display and measurement of non-sinusoidal or complex waveforms The oscilloscope’s basic part is the cathode ray tube, CRT A CRT is a vacuum tube containing an electron gun pointing to a fluorescent screen at the tube’s end The electron gun provides a beam whose movement is controlled by two sets of deflector plates perpendicular to each other The plates receive the signals representing the waveforms These analog signals are displayed on a fluorescent screen as Y-time or X-Y curves The display is photographed to provide a hard copy of the analog data The oscilloscope can display data that is generated at high rates, since there are no mechanical movements used in manipulating the electron beam Where very fast events must be recorded, an oscilloscope is an effective readout device.11 (See the previous paragraph on the potentiometric recorder.) Oscilloscopes have facilities to store, compare, and manipulate signals Analog meters are based on the D’Arsonval meter movement The electrical current signal passing through a moving coil, to which is fixed a pointer, induces a magnetic field in the coil A static magnetic field from a permanent horseshoe magnet surrounds the coil The interaction between the two fields causes the movement of the coil: the degree of movement is determined by the magnitude of the signal current Analog meters require the analyst to interpret or read the output signal value by the position of the indicator needle or pointer using a calibrated scale mounted on the meter A resistance placed in series with the meter movement allows INSTRUMENTATION: WATER AND WASTEWATER ANALYSIS the measurement of voltage Resistance may also be measured with the meter One weakness of this device is its low internal resistance causing loading errors by high impedance signals.11 A meter is used in the analysis of single samples or samples analyzed, serially, at a slow rate on a spectroscopic instrument at one frequency or wavelength Meters also are employed to indicate proper adjustment of potentials, currents, temperatures, etc for various instruments An electronic voltmeter, EVM, is more sensitive and accurate than the D’Arsonval-based meter previously described, particularly for signals with high impedance The internal resistance is 10 Mohms (megaohms, 106 ohms) or more for d.c (direct current) signals and Mohm for a.c (alternating current) signals The circuits use solid state devices compared to the earlier device, a VTVM (vacuum tube voltmeter) Current and resistance is measurable with the EVM Its application parallels those for the D’Arsonvalbased meter.11 A photographic plate or film may be used to collect data in the time domain where all the data are displayed simultaneously, that is a spectrum in emission spectroscopy The radiation in the dispersion pattern of the sample reflected or transmitted from the prism or grating impinges on the photographic plate Electronic integrators determine the area under a curve and are superior in precision to the ball and disk integrator and the several hand methods widely utilized They may be based on operational amplifier or transistor circuitry Some potentiometric recorders have a second pen controlled by an integrator and the density of the pen’s excursions determine the area under the curve This last type is not as convenient as the electronic integrators that can correct for baseline changes Chromatographic peak areas for GC and HPLC (high performance liquid chromatography), anodic stripping analysis peaks, spectroscopic curves, etc are integrated as a means of quantitation and analysis of an analyte Analog computers are available but are not used now to any great extent Digital Devices The digital computer or microprocessor interfaced to the instrument brings a broad capability to the display and processing of instrumental data Data reception and storage is convenient when real time computation and display are not required Mathematical calculations, including the areas under curves, graphic and tabular displays, correlation with previously collected data, and many other operations can be carried out at one’s convenience Real time processing can be accomplished on a time-sharing basis or with a dedicated computer The visual display is at a video monitor and a printer provides a hard (printed) copy of the raw and calculated data, graphs, and other information Computer devices include microprocessors and micro-, mini-, and mainframe computers The instrument must be carefully interfaced to the computer and this task requires much electronic skill Instruments providing spectral readouts, the need for number crunching and repetitive analyses © 2006 by Taylor & Francis Group, LLC 547 can benefit greatly from a computer interface Some instruments that utilize Fourier transform analysis require a computer capability and many instrumental techniques have been revolutionalized by computer use The use of the computer12 in the reduction of noise in instrumental signals by ensemble and boxcar averaging has greatly improved the quality of instrumental data.12 Digital meters measure analog signals and provide a digital readout A/D conversion of the analog input is accomplished electronically The digital data is displayed as numeric images using solid state devices such as LEDs, light emitting diodes, and LCDs, liquid crystal displays, and lamps such as, NIXIE, neon, and incandescent bulbs The LED is the more convenient device because its seven segment readout display uses lower currents and voltages than the lamp displays The LED’s red image, due to the semiconductor gallium arsenide doped with phosphorus, may be increased in intensity by using more semiconductor in the LED The image color of LEDs may be fabricated to be green or yellow, also.11,13 LCDs operate by means of polarizing light They use reflected light for viewing, a sevensegment and dot matrix readout display, an a.c voltage, consume very little power and are more fragile than LEDs.14 The LCDs and LEDs are the newest and most convenient display devices Digital meters can be used in place of the analog variety The former are more accurate and easier to read Instrumental Parameters and Definitions Instrumental characteristics of operation and data treatment and statistics are defined by a number of parameters A definition of each term is as follows: • • • • The range of frequencies (information) in the signal is called the bandwidth During amplification, some amplifiers cannot respond to the range of frequencies in the signal producing an amplified signal with a narrower bandwidth The baseline is the signal obtained when no sample is being examined and reflects the noise inherent in the instrument Calibration is the process relating instrument response to quantity of analyte In general a series of standard solutions or quantities of analyte are analyzed on the instrument taking reagent blanks into account and using a similar matrix as the sample under consideration The quantityresponse data are plotted to provide a calibration curve where error bars indicate the precision of the method.15 Other calibration procedures such as the methods of standard additions16 and of internal standards17 have advantages in specific situations The former is helpful in ameliorating interferences from the sample matrix and the latter in correcting for changes in instrument response particularly in GC, and ir (infrared) and emission spectroscopy.18 The gain refers to the ability to amplify a signal and is the ratio of the output to input signal The 575 INSTRUMENTATION: WATER AND WASTEWATER ANALYSIS There is an on-line instrument for laboratory and field use to carry out ASV.83 In the cell a glassy carbon electrode is automatically coated with a thin film of mercury (When the replating of the electrode is necessary it is carried out automatically.) A pump and a set of five solenoid streamswitching valves direct sample and reagents to the cell By means of a microprocessor and keyboard all steps and analytical parameters can be facilitated The detection limits are ppb for bismuth, cadmium, copper, lead, thallium and zinc that plate on mercury and 50 ppb for gold and silver that not plate on mercury and mercury itself A.c voltammetry employs a variety of electrodes with similar operations and response described in the polarography section above Figures 26A and B give information relating to the detection limits in terms of the molarity of the analyte and the resolution between peaks or waves expressed as volts Therefore, comparisons can be made among the techniques illustrated (2) Amperometry (a) Dissolved oxygen electrode Dissolved oxygen, DO, in water is determined by DO probes using galvanic or electrolytic type cells These DO probes have their electrodes protected by an oxygen permeable membrane and are referred to as membrane electrode systems These systems contain a cathode working electrode and a Ag/AgCl reference electrode and an electrolyte of 1M KCl In an electrolytic probe the working electrode may be a platinum (Clark electrode84), gold, carbon, or silver microelectrode (see Figure 28) Gold seems to give the best service A membrane electrode system using a gold electrode, an Ag/ AgCl reference electrode with a KCl gel electrolyte, and a polarizing voltage of Ϫ0.80 is commercially available.85 The cell reactions are as follows: Gold cathode O2 ϩ H2O ϩ 4e → 4OHϪ (36) silver anode (ClϪ,a ϭ 1) CONNECTOR PVC BODY SILVER ANODE FILL PORT SCREW ELECTROLYTE CAP THERMISTORS TEFLON MEMBRANE GOLD CATHODE FIGURE 28 Clark DO membrane probe (Courtesy of Rosemount Analytical, Inc., La Habra, CA.) solution is employed because of the insolubility of the lead salt formed A mil, polyethylene membrane separates the cell from the analyte sample Several galvanic electrode systems are available.89,90,91 The DO probe output current, from both types of cells, is sensitive to temperature changes To compensate for this effect, thermisters are placed in series with the load circuit92 or in a bridge circuit.93 (39) (c) Potentiometric, amperometric and conductometric titrations Electrochemical means are used in potentiometric, amperometric and conductometric titrations to determine the endpoint of a titration These titrations are so named to indicate the mechanism of endpoint determination The electrodes used in the potentiometric and amperometric methods are sensitive to analyte and/or titrant redox species; whereas in conductometric titrations total ion content is detected In amperometric titrations the magnitude of current flow is determined as a function of the volume of titrant The electrode potentials are measured as a function of the titrant volume in potentiometric titrations Conductance measurements are used in conjunction with titrant volumes in this third method The electrolyte, M KOH, is used since hydroxide, OHϪ, ion is formed during the reduction of the oxygen For higher sensitivities a saturated potassium bicarbonate electrolyte (1) Potentiometric titrations94 The electrodes discussed in the previous section on potentiometric instruments may be used in potentiomeric 4Ag ϩ 4ClϪ → 4AgCl ϩ 4e (37) The design, based on the Clark electrode, is commonly used by a number of manufacturers.85,86,87 A DO probe operating on the galvanic principle is one developed by Mancy et al.88 In this cell a spontaneous electrochemical reactions occurs as follows: Pb anode 2Pb ϩ 60HϪ → 2PbOOHϪ ϩ 2H2O ϩ 4e (38) Ag cathode O2 ϩ 2H2O ϩ 4e → 4OHϪ © 2006 by Taylor & Francis Group, LLC 576 INSTRUMENTATION: WATER AND WASTEWATER ANALYSIS titrations A reference and the appropriate indicating electrode allows the measurement of the potential for each volume of titrant added The potential changes in response to the changes in the ratio of concentrations of oxidized to reduced species The Nernst equation can be used to calculate the magnitude of these changes A common classical example of a potentiometric titration is an acid-base titration using pH/reference electrode pair The pH electrode indicates the hydrogen ion concentration in terms of pH as a function of volume The titration apparatus and the resulting “S” shaped curve is shown in Figure 29 Another example concerns the complexometric titration of calcium ion with the titrant, EDTA (the sodium salt of ethylene diamine tetraacetic acid) A calcium ion selective and reference electrode are employed The titration curve, is similar in shape to the curve in Figure 29 The pCa value is measured by the calcium electrode as a function of volume of EDTA titrant Automatic titrators allow the analyst to carry out a variety of potentiometric titrations without the consumption of a great deal of time and effort However the reaction must not be too slow and the concentrations of the analyte between 0.1 and 0.001N Two types of automatic titrators are available—recording and non-recording Many commercial titrators have the following features: A delivery system consisting of a buret with a solenoid operated delivery valve or a calibrated syringe whose plunger is motor-driven by a micrometer screw The potential from the indicator-reference electrode pair is compared to a set end point potential by an anticipator, null-sensing amplifier circuit yielding an amplified error signal The error signal controls the buret delivery Anticipation of the endpoint prevents over titrating the endpoint (see Figure 29) Recording titrators plot the titration curve and are completely automatic accepting a number of samples for titration in a serial manner Some titrators contain microprocessors providing computational ability and processing of a number of samples The error signal is used to drive the recorder pen and control buret delivery Advantageously, unknown systems may be titrated yielding a curve that can be interpreted by the analyst Direction selector (0–14 or 14–0) Motordriven syringe Potentiometer On/off Starting pot ("end point") Up Down Electrodes Burette Speed Titrate Chart recorder Amplifier pH meter Error signal Titrant Refill Indicator lamp Switch Pen motor Temperature compensation Chart motor On/off Magnetic Calibra- Selec- On/off stirrer tion tor On/off E, mV E, mV (a) Potentiometer drive on; buret and chart drive off V, mL V, mL (b) (c) Buret and chart drive on; potentiometer drive off (d) FIGURE 29 (a) An automated, curve-recording titrator (b) Theoretical titration curve (c) Recorded curve with endpoint anticipation (d) Enlarge portion of (c) with explanation (From G Svehla, Automatic Potentiomeric Titrations, p 176, Pergamon Press Copyright © 1978 by Prof G Svehla Reprinted with permission.) © 2006 by Taylor & Francis Group, LLC INSTRUMENTATION: WATER AND WASTEWATER ANALYSIS The accuracy of automatic titrators depends on the concentration of the analyte and the sensitivity of their electrode sensing systems Accuracies are 0.1% for a 10Ϫ2 N solution and 1% for the limiting concentration of 10Ϫ3 N Titrimetric methods are given in Standard Methods2 for the following parameters—carbon dioxide, cyanide, COD, sulfite, and ammonia (2) Amperometric titrations50 Amperometric titrations may utilize one or two polarizable micro electrodes configured differently, electrically These several electrode systems will lead to differently shaped titration curves Titration curves show the current flow as a function of the volume of titrant A single polarized electrode may be a DME, a solidstate electrode of carbon, platinum or other noble metal or a rotating noble metal electrode The potential imposed on the indicating electrode is such that the limiting current, iL, is obtained As in voltammetry, the current obtained is proportional to the concentration of the electroactive species Therefore as the titration proceeds, the diffusion current, id, changes The shape of the titration curve depends on the electroactivity of the titrant and the analyte as seen in Figure 30 In general two straight lines are obtained and their interpolated intersection indicates the endpoint Two polarized microelectrodes result in titration curves whose shapes depend on the reversibility of the electrode reactions of the titrant and the analyte When there is a reversible reaction for the analyte, (IϪ/IϪ), and an irreversible one for the titrant, (S2OϪ/S4OϪ), the result is a “deadstop” endpoint where the current ceases to flow When analyte and titrant show reversible electrode behavior, e.g., Fe2ϩ/Fe3ϩ and Ce3ϩ/Ce4ϩ, respectively, the titration curve shows no current flow at the endpoint There is current flow on either side of the endpoint (3) Conductometric titrations95 The instrumental apparatus and conductance cells for conductometric titrations are given in Figures 31 and 32 (a) (d) Conductometric methods96 Conductometric methods detect ionic species in solution The conductivitivity of a solution indicates the presence of ionic species and can, under certain conditions, be used to estimate the concentration of the dissolved electrolyte Conductance is also used in conductometric titrations (see the previous section III,B,2,c,(3)) Solutions containing electrolytes conduct electricity and obey Ohm’s law Electrical resistance or conductance is measured by placing the solution between two electrodes and using a Wheatstone bridge to carry out the measurement Conductivity, Cn, is reciprocally related to resistance, Rs, so that Cn ϭ 1/Rs Units for Rs and Cn are ohms and mhos (ohmsϪ1), respectively (The IS unit for Cn is siemans, S.) Resistance and conductivity of a solution are sensitive to the dimensions of the volume of solution included between the plate electrodes (see Figure 32) Conductivity is proportional to the area of the electrodes and reciprocally related to the distance between them By normalizing conductivity to a given dimension, a cube that is one centimeter on its edge, and designating a new parameter, specific conductance, Csp in mhos/cm, the following calculation can be made, Csp ϭ (1/Rm)(d/ar) ϭ (1/Rm)(kc) mhos/cm Volume of reagent FIGURE 30 Amperometric titration curve © 2006 by Taylor & Francis Group, LLC (40) where Rm is the measured solution resistance in the conductivity cell, d and ar are the distance between and the area of the (c) Analyte and reagent reduced Diffusion current Only analyte reduced Diffusion current Diffusion current In Figure 33 a precipitation and several acid–base titration curves are illustrated Since ions are detected by the electrodes, any change in ionic concentration in the analyte solution during the titration is the basis for a conductometric titration In a neutralization reaction two ions, hydronium, H3Oϩ and hydroxide, OHϪ, are removed to form unionized water, H2O Two ions, silver, Agϩ and chloride, ClϪ, form a precipitate of silver chloride, thereby decreasing the conductivity of the solution in the course of the titration Analogously, the formation of an unionized complex can be the basis for a conductometric titration (b) Only analyte reduced 577 Volume of reagent Volume of reagent 578 INSTRUMENTATION: WATER AND WASTEWATER ANALYSIS electrodes, respectively The cell constant is kc ϭ d/ar cmϪ1 In analytical determinations of solution conductivities, Csp is usually calculated as micromhos per centimeter (␮mhos/cm) or in SI units of millisemens per meter (mS/m) Temperature control during measurements is necessary, since the temperature coefficient for conductance measurements is about 0.5 to 3% per ЊC Conductance data is usually obtained at 25ЊC Temperature corrections can be made, however the further the data temperature is from 25ЊC, the greater the uncertainty Oscillator 60 – 1000 Hz a R2 R1 Null Detector RC R3 Cell b FIGURE 31 Basic Wheatstone ac bridge circuit for measuring conductance (From Gary D Christian and James E O’Reilly, Instrumental Analysis, 2nd edition Copyright © 1986 by Allyn and Bacon Reprinted with permission.) A The measurement of solution resistance requires a conductance cell to contain the solution and a Wheatstone bridge for measurement The Wheatstone bridge, shown in Figure 31, has an alternating current source of to 10 volts with a frequency of 60 to 1000 Hz R2 and R3 are fixed resistors of known values, R1 is a variable resistor with up to decades of resistance, and Rc is the resistance of the analyte solution The null detector may be a headphone used with 1000 Hz, a cathode-ray tube or a micrometer A variable capacitor is connected across R1 and adjusted to balance out any phase shift in the a.c signal caused by the capacitance of the electrode surfaces This adjustment is made to provide the sharpest minimum in the null signal A conductivity cell essentially consists of two square plates of platinum of the same area, ar and platinized with platinum black to prevent polarization (see Figure 32) The plates are arranged parallel to each other at a fixed distance, d (Other durable metals such as stainless steel and nickel are used in field and continuous monitoring operations.) The cell constant is not easily nor accurately obtained by measurement of the area and distance A standard procedure is to fill the cell with a potassium chloride solution of known molarity and specific conductivity, Csp The measured resistance, Rm, along with Csp, when substituted in equation 40, will give the cell constant, kc Standard Methods gives an excellently, detailed description of the measurement of conductivity.2 The measurement of specific conductances has a number of uses in water analysis: i) Ascertaining the mineral content of water in order to determine the effect of total ionic content on corrosion rates, physiological effects on animals and plants and the effects on chemical equilibria ii) Appraising daily and seasonal variations in the mineral content in raw waste and natural waters iii) Estimating the mineral content of high purity (distilled and deionized) water iv) Checking the results of chemical analysis and estimating sample size v) Determining the endpoint in conductometric titrations vi) Estimating the total dissolved solids content by multiplying the specific B C FIGURE 32 Three types of conductance cells, A: Precision conductance cell B: Conductometric titration cell C: Concentration dip cell (From Gray D Christian and James E O’Reilly, Instrumental Analysis, 2nd edition Copyright © 1986 by Allyn and Bacon Reprinted with permission.) © 2006 by Taylor & Francis Group, LLC INSTRUMENTATION: WATER AND WASTEWATER ANALYSIS (a) 579 (b) Titration of a weak acid Conductance Conductance Titration of a weak acid Volume of NH3 Volume of NaOH (c) (d) Titration of the chloride ion Conductance Conductance Titration of a salt of a weak acid Volume of HCL Volume of AgNO3 FIGURE 33 Examples of conductometric titration curves The first three curves a, b, and c represent acid/base titrations and curve d a precipitation titration conductance by developed empirical factors.2 Conductivity is also used to control plant processes, such as fluoride additions to drinking water and coagulant dosage control.97 (e) Coulometric methods98 Coulometry embraces three methods coulometric titrations of constant-current (amperostatic) coulometry, constantpotential (potentiostatic) coulometry, and electrogravimetry In these methods the oxidation or reduction of an analyte to a new species or the reaction of an analyte with an electrolytically generated substance takes place In the two coulometric methods the number of coulombs, necessary to directly or indirectly affect and electrochemical change of the analyte or a species that reacts with the analyte, is determined In electrogravimetry the analyte undergoes an electrolytic reaction and the new species is deposited on an electrode and measured gravimetrically The analyte concentration is calculated from a knowledge of the chemical composition of the deposit (1) Electrogravimetry The analyte, metallic cations, halide ions, etc., are completely removed from the solution by plating on an inert © 2006 by Taylor & Francis Group, LLC electrode, usually platinum, in electrogravimetry The increase in electrode weight is due to a species containing the reduced or oxidized form of the analyte Electrolytic deposits can be pure metals on the cathode or compounds such as lead peroxide, thallium oxide and manganese dioxide on an anode, and silver halide salts on a silver anode Complete removal of the analyte by electrolytic deposition from solution requires an unpolarized process where large electrodes and vigorous stirring are used Two conditions are necessary concerning the electrolytic deposit: The deposit must adhere well to the electrode so that an accurate weight of the plated substance can be obtained, and a predictable, constant chemical composition of the deposit is required The composition of the electrolyte solution does have, at times, a dramatic effect on the nature of the deposit Substances are added to the electrolysis solution to improve the adherence of the plated substance to the electrode Electrolysis in electrogravimetry can be achieved under two conditions—constant current through the cell or at a controlled working electrode potential Constant-current electrolysis has the advantage of a constant rate of reaction and is faster than the controlled-potential 580 INSTRUMENTATION: WATER AND WASTEWATER ANALYSIS method The limitation is an uncontrolled, working electrode potential As the reaction progresses, the voltage must be increased to provide a constant current because of polarization effects This can lead to the deposition of other species However, this method is advantageously applicable when the species is alone in the solution, more readily reduced than hydrogen ion, and therefore, not requiring a controlled working electrode potential Some of the cations amenable to this technique are cadmium(II), cobalt(II), copper(II), iron(III), lead(II), nickel(II), silver(I), tin(II), and zinc(II) If interfering ions are present they may be complexed in order to prevent electrodeposition or removed by prior chemical precipitation Another application of this technique is the removal of a species which would interfere in a subsequent analytical operation A mercury electrode is frequently used in this last application Controlled, working electrode potential electrolysis has the greatest applicability to mixtures of analytes The control of the working electrode permits selective deposition of metals with standard potentials that differ by several tenths of a volt A schematic diagram in Figure 34 embodies the elements of the apparatus One example is the analysis of a sample containing copper, bismuth, lead, cadmium, zinc, and tin The first three metallic ions are deposited at selective potentials; tin is held in solution as the tartrate complex Cadmium and zinc are selectively deposited from ammoniacal solutions Finally, the solution is acidified to decompose the tin-tartrate complex and tin is deposited.99 Table 10 lists analyses amenable to this technique.100 Controlled, working electrode potential electrolysis is a slower process than the constant current method, since the current flow tends to decrease in the controlled-potential technique due to polarization and other effects (2) Coulometry In coulometry one measures the quantity of electricity (number of coulombs) that is required to carry out a redox reaction of the analyte or generate a reagent that reacts with the analyte Two general methods are used in coulometry, namely controlled-potential (potentiostatic) and controlledcurrent (amperostatic), commonly known as coulometric titrations All the current must be used, solely, either directly or indirectly, for the reaction concerned with the analyte, that is, the current efficiency must be 100% for a quantitatively accurate, analytical result In these methods standards are not needed; the proportionality constant (Faraday’s constant) relating the number of coulombs and the amount of the analyte is derivable from known physical constants In the cell the effects of polarization are decreased by using electrodes with large surface areas and by vigorous stirring of the solution during electrolysis Faraday’s law relates the amount of electricity in coulombs to the number of equivalents of reactant in a redox reaction Faradays’s constant, 96,487 coulombs/equivalent, is the stoichiometric factor relating electrical charge passing in an electrolysis and the equivalence of substance reduced or oxidized Since one ampere is the rate of flow of one coulomb per second, the time integrated flow of the current during a redox reaction will yield the number of coulombs, Q When the current varies over the reaction time period an integration is necessary With constant current the number of coulombs can be calculated by the equation, it ϭ Q ϭ 96,487 W/ew The quantities are i, amperes; t, seconds; W, sample weight in grams, and ew, equivalent weight in grams per equivalent The equivalent weight is the atomic weight, aw, or formula weight, fw, divided by the number of electrons in the redox or half cell reaction For example in the reactions, Potentiostat Potential measuring device Fe3ϩ ϩ e ϭ Fe2ϩ Variable voltage source Potential control Metal Separated Metal Matrix Sb Pb, Sn Bi Sb, Cd, Cu, Pb, Sn, Zn Cd Reference electrode Zn Cu Schematic for a controlled-potential electrolysis Sb, Bi, Cd, Pb, Ni, Sn, Zn Pb Al, Cd, Fe, Mn, Ni, Sn, Zn Ni Counter or auxiliary electrode © 2006 by Taylor & Francis Group, LLC (42) TABLE 10 Controlled-potential electrolysis for metal separation and determination Working electrode FIGURE 34 apparatus (41) Al, Fe, Zn Ag Cu and more active metals INSTRUMENTATION: WATER AND WASTEWATER ANALYSIS and MnOϪ ϩ 8Hϩ ϩ 5e ϭ Mn 2ϩ ϩ 4H O (43) the ew of Fe3ϩ is the aw/1 and ew of MnOϪ or KMnO4 is the fw/5 The number of equivalence is represented by the fraction W/ew, in equation 41 (a) Potentiostatic coulometry The working electrode is maintained at a constant, present potential in potentiostatic coulometry In this process the analyte changes oxidation state reacting quantitatively with the current: i.e., at 100% current efficiency Redox reactions with less reactive species in the analyte solution are precluded Maintenance of 100% current efficiency allows the amount of analyte present to be determined unambiguously by the quantity of coulombs consumed application of Faraday’s law (see equation 41) As the redox reaction proceeds, the current and, therefore, the reaction rate decreases because of increases in polarization effects, changes in solution resistance, etc Nevertheless the working electrode potential is maintained restricting the rate of the redox reaction of the analyte The amount of current consumed usually decreases with time and is measured by an integrating device: namely, an electronic, chemical or electromechanical coulometer An electronic controlled-potential coulometric titrator developed by Kelley et al.101 includeds a potentiostat, which provides and maintains a constant potential, a d.c current source, an electrolytic cell, and an integrating coulometer providing the readout in coulombs This device has a range of 10 microamperes to 10 milliamperes, an accuracy of 0.01%, and a response time of 10 milliseconds An application of this method is the analysis of a mixture of antimony(III) and antimony(IV) accomplishable in two steps.102 From the voltammetric data, the centered plateau reduction voltages for the following reactions are as follows: Sb5ϩ ϩ 2e ϭ Sb3ϩ, Ϫ0.21 V vs SCE (44) Sb3ϩ ϩ 3e ϭ Sb0, –0.35 V vs SCE (45) and The supporting electrolyte (6M hydrochloric acid plus 4M tartaric acid) is reduced at Ϫ0.35 V, before addition of the sample to remove impurities Upon addition of the sample of antimony species, the solution is deaerated with nitrogen The potentiostat described previously101 is set at Ϫ0.21 V and the experimental voltage rises slowly to that value during the reduction of Sb5ϩ The current starts to decrease when the set voltage is reached because of the decreasing concentration of Sb5ϩ, other polarization effects, etc When all the Sb5ϩ has been reduced to Sb3ϩ, the current will decrease to a negligible value signaling the end to the first analytical step The second reduction is carried out by setting the potentiostat at Ϫ0.35 V and repeating the process In calculating the © 2006 by Taylor & Francis Group, LLC 581 Sb3ϩ content of the sample a correctin for the Sb3ϩ generated in the first reduction must be made When interfering substances are present, e.g., the analysis of plutonium in the presence of iron, an indirect approach can be used.103 Potentiostatic coulometry has the same advantages of controlled-potential electrogravimetry In employing this method rather than electrogravimetry, the attendant problems of poorly adhering electrolytic deposits are eliminated In addition analytes that not yield electrolytic deposits, but are amenable to coulometric analysis can be analyzed This technique requires a longer analysis time compared to amperostatic coulometry because of the decrease in current flow as the reaction proceeds (b) Coulometric titrations Coulometric titrations (amperostatic or controlled current coulometry) are carried out at constant current (see Figure 35) At the working electrode a reagent is generated that reacts with the analyte in one of several types of reactions namely, oxidation/reduction, acid/base, complexation or precipitation (see Figure 36) When all the analyte has reacted with the generated reagent, the endpoint or completion of the reaction may be detected by potentiometry, indicator color changes, amperometry, or conductance In some coulometric titrations, however, part of the current arises from direct reduction or oxidation of the analyte at the electrode and the remainder through the generation of the reagent Subsequent reaction of the reagent and the remaining unreacted analyte ends the titration and 100% current efficiency is maintained The measured quantity for a coulometric titration is the number of coulombs necessary to generate the reagent; it is comparable to the volume of titrant in a classical titration (A current efficiency of 100% is, therefore, required.) Classical and coulometric titrations are comparable in a number of ways; these two methods have similar endpoint detection methods and stoichiometric reactions between titrant and analyte must be rapid, complete, and free of side reactions Since constant current is used in this technique, an accurate timer is used in the coulometric titrator in Figure 35 The product of time and current (see equation 41) will give the number of coulombs An integrator is not needed, as in potentiostatic coulometry, where the current can change with time The generation of reagents can be internal, in the cell containing the analyte, or external to the cell Outside generation of reagents is often convenient for several reasons: Electrolytic interference of substances in the sample solution and incompatibility in conditions fostering efficient generation of the reagent can occur in internal generation (x) Internal generation Two modes of internal generation are utilized and are referred to as primary and secondary coulometric titrations In the primary mode the analyte reacts directly with a species generated from the electrode material Therefore, no other species should be present which will react electrolytically with the working electrode within about 0.5 V of the 582 INSTRUMENTATION: WATER AND WASTEWATER ANALYSIS Constant current source Potentiometer for current measurement ED Counter or auxiliary electrode (in separator) Detection circuit Ganged switch Clock Detection electrodes Working or generating electrode FIGURE 35 Coulometric titration apparatus Electrolyte solution Auxiliary electrode Sintered glass frit Generator electrode Stirrer FIGURE 36 Coulometric titration cell potential of the analyte reactivity An example of this mode is the reaction of ionic halide ions, mercaptans, or sulfhydryl groups with electrolytically generated silver ion from a silver working electrode The endpoint may be determined amperometrically In the secondary mode an active intermediate is generated which reacts directly with the analyte The intermediate © 2006 by Taylor & Francis Group, LLC Titration vessel is generated at a potential between that of the potential of the analyte redox reaction and the potential of an interfering, unwanted reaction The interfering reaction can be due to electrolysis of the supporting electrolyte or another electroactive substance in the sample An illustration of a secondary titration is as follows: The coulometric titration of ferrous ion uses the Ce3ϩ/Ce4ϩ system as the intermediate reactant At the onset of the titration the ferrous ion is directly oxidized at the platinum working electrode Since the current is constant, the potential increases as the concentration of ferrous decreases At the oxidation potential of cerous ion and ceric ion is generated and in turn oxidizes the ferrous ion The summation of number of coulombs consumed by each electrolytic reaction, direct and ceric oxidation, is that needed to titrate the ferrous ion present The endpoint chosen is the presence of excess ceric ion indicating the complete oxidation of ferrous ion The ceric ion can be detected photometrically or amperometrically Since an excess of cerous ion is present, the working electrode is maintained at the unique potential of the cerous/ceric couple The potential of the working electrode is prevented from increasing to a value where the interfering substances would be electrolyzed In secondary coulometric titrations reagents can be generated that are inconvenient to use in classical titrimetry Some examples are the generation of hydroxide ion free of contamination by carbonate and the reactive titrants bromine, chlorine and titanous More on this subject is given in the section on external generation of titrants (y) External generation External generation of reagents can be necessary because of the presence of interfering substances in the sample In Figure 37 a double-arm apparatus for the outside generation of reagents is shown.104 Specifically the equations for the generation of acid (hydrogen ion) or base (hydroxide ion) INSTRUMENTATION: WATER AND WASTEWATER ANALYSIS TO GENERATOR ELECTROLYTE SUPPLY RESERVOIR PL ELECTRODES (SPIRALLY WOUND) GLASS WOOL + – CATHODE DELIVERY TIP ANODE DELIVERY TIP FIGURE 37 Generator cell for external production of titrants for coulometric titrantions (Adapted with permission for reference 104 Copyright 1931 American Chemical Society.) For example in acid/base titrimetry: (a) generation of OHϪ at the cathode: 2H2O ϩ 2e → H2 ϩ 2OHϪ (b) generation of Hϩ at the anode: H2O → 1/2O2 ϩ 2HϪ ϩ 2e are illustrated A single arm electrolytic cell is also available.105 Unstable reagents, namely, bromine, dipositive silver, titanium(III), chlorine, etc., can be conveniently generated and utilized in this technique In Table 11 a number of applications are listed The ability to utilize these unstable reagents expands the horizon of tritrimetry and analytical methodology (3) Measurement of radioactivity106–8 a Introduction Radioactive species in water and wastewater samples arise from natural and anthropogenic sources The radiation emitted from radioisotopes is deleterious to all life forms and has led to the development of a number of radiochemical methods Standard Methods2 includes procedures for the detection of tritium, radium, and radioactive species of cesium, iodine, strontium, and uranium, as well as, the general measurement of the amount of ionizing radiation Several kinds of ionizing radiation emanate from radioisotopes They include gamma and x-rays, beta rays (electrons and positrons), and alpha rays (helium nuclei) Ionizing radiation, when penetrating matter, causes the formation of ions that are dangerous to life forms leading to cancer in people and animals Gamma and x-rays are at the high frequency (Ͼ106 Hz) end of the electromagnetic spectrum The former arise from decay events occurring in the nucleus and are more energetic higher frequency than © 2006 by Taylor & Francis Group, LLC 583 x-rays that arise from rearrangement of electrons surrounding the decaying nucleus Both kinds of radiation are highly penetrating due to their high energy, and therefore are quite dangerous Gamma and x-rays are emitted as continuous spectra Beta particles are of two types; negatively charged electrons or negatrons and positively charged positrons They arise from decay processes in nuclei A positron has a very short life time and is annihilated on collision with an electron to form two gamma photons Beta particles have a greater penetrating ability than alpha particles and are poorer at ionizing matter Since beta particles are scattered in air, their range is difficult to assess Beta particles, emitted during nuclear decay, yield a continuous spectrum of particle energies Alpha particles are the nuclei of helium atoms and have a positive charge of two They are the product of the decay of natural isotopes, such as uranium, radium, and others Alpha particles have high energies and, therefore, high ionizing power Their penetrability, however, is low, about 5–7 cm in air Alpha particles emitted from radioisotopes not yield continuous spectra The particles are monoenergetic or have a group of several discrete energies b Instrumentation Ionizing radiation interacting or colliding with matter causes the production of charged particles or energetic species; measurement of these entities is the basis of radiation detectors Two phenomena are utilized in detecting ionizing radiation: namely, ionization of gases and solids to yield an ion current pulse and the excitation of crystals to provide a luminescence pulse The ionization of gases is used in ionization chambers, proportional counters, and GM (Geiger Muller) tubes Semiconductor radiation devices produce electron–hole pairs on the impingement of ionizing radiation Crystal and liquid scintillation detectors are employed for insoluble and soluble samples, respectively An instrument for radiation measurement consists of several modules: namely, the detector responding to the radiation, circuits that count pulses, and pulse height analysis circuits that discriminate between the energies of the pulses The nature of the pulse must be understood In a number of detectors the pulse height of the radiation event is proportional to the energy of the particle or radiation and independent of the applied potential, whereas in other detectors the pulse height is independent of energy The pulse of photoelectrons or luminescence has a pulse width due to the random nature of the collision process This fact leads to small statistical differences in the pulse heights for a series of radiation events with the same energy The smaller the pulse width the greater the resolution of the detector Radiation detectors operate under two modes, pulse and mean level Of interest here is the pulse type that detects the interaction of radiation with the detector as a unique event These pulses are counted and measured in counts per unit time, usually, counts per minute, cpm The type of radiation (beta, gamma, etc.), the frequency or wavelength and the intensity of the radiation can also be determined 584 INSTRUMENTATION: WATER AND WASTEWATER ANALYSIS TABLE 11 Typical electrogenerated titrants and substances determined by coulometric titration* Electrogenerated Titrant Generating Electrode and Solution Typical Substances Determined Bromine Pt/NaBr As(III), U(IV), NH3, olefins, phenols, SO2, H2S, Fe(II) Iodine Pt/KI H2S, SO2, As(III), water (Karl Fischer), Sb(III) Chlorine Pt/NaCl As(III), Fe(II), various organics Cerium(IV) Pt/Ce2(SO4)3 U(IV), Fe(II), Ti(III), IϪ Manganese(III) Pt/MnSO4 Fe(II), H2O2, Sb(III) Silver(II) Pt/AgNO3 Ce(III), V(IV), H2C2O4 Iron(II) Pt/Fe2(SO4)3 Mn(III), Cr(VI), V(V), Ce(IV), U(VI), Mo(VI) Titanium(III) Pt/TiCl4 Fe(III), V(V, VI), U(VI), Re(VIII), Ru(IV), Mo(VI) Tin(II) Au/SnBr4(NaBr) I2, Br2, Pt(IV), Se(IV) Copper(I) Pt/Cu(II)(HCl) Fe(III), Ir(IV), Au(III), Cr(VI), IOϪ Uranium(V), (IV) Pt/UO2SO4 Cr(VI), Fe(III) Chromium(II) Hg/CrCl3(CaCl2) O2, Cu(II) Silver(I) Ag/HclO4 Halide ions, S2Ϫ, mercaptans Mercury(I) Hg/NaClO4 Halide ions, xanthate EDTA Hg/HgNH3Y2 Ϫ a Metal ions Cynaide Pt/Ag(CN)Ϫ Ni(II), Au(III, I), Ag(I) Hydroxide ion Pt(Ϫ)/Na2SO4 Acids, CO2 Hydrogen ion Pt(ϩ)/Na2So4 Bases, CO2Ϫ, NH3 a Y4Ϫ is ethylenediamine-tetra-acetate anion * From A.J Bard and L.R Faulkner, Electrochemical Methods, p 390 (Copyright 1980 by John Wiley & Sons, Inc., with permission.) (1) Detectors Radiation detectors operate on two principles; ionization of a gas or solid to generate a small ion current and excitation of a substance to cause a short term luminescence in a crystal or a solution Ionization detectors are of two varieties; those that use gas as the ionization medium, i.e., gas-filled detectors and those that use a crystal, i.e., semiconductor detectors Measurements of luminescence due to radiation are made in crystal and liquid scintillation counters (a) Ionization and detectors (i) Gas-filled detectors Gas-filled detectors respond to ionizing radiation by the formation of ion pairs, a photoelectron and a positive ion (cation) by the gas molecules The gas in the detector tube may be a mixture: e.g., argon and a low concentration of an organic substance or methane In Figure 38 the gas-filled detector is shown with a central electrode (anode) to which is applied a voltage The pulse of photoelectrons migrate to the electrode due to the electric field, are collected, and produce a small ion current The current represents the radiation event or particles © 2006 by Taylor & Francis Group, LLC deposited in the detector Several types of gas-filled detectors are available and are operated in a number of voltage ranges leading to different kinds of detector responses; the operation potentials of these detectors are illustrated in Figure 39 In the saturation (ionization) chamber region a radiation event of given energy gives rise to a number of photoelectrons independent of the applied potential They are collected on the electrode yielding an ion current or pulse height proportional to the energy of the radiation event The pulse of photoelectrons, however, has a pulse width due to the random nature of the collision process This situation leads to a statistical variation in pulse heights for the same energy source defining the resolution of the detector An ionization chamber in combination with a pulse height analyzer can operate as an alpha particle spectrometer When the applied voltage is increased, producing a field greater than 200 V/cm, the electrons formed in ion pair formation are accelerated These accelerated electrons collide with gas molecules causing increased ionization leading to an increase in the collected electrons This effect is called gas amplification and is present in the proportional counter and GM tube regions INSTRUMENTATION: WATER AND WASTEWATER ANALYSIS 585 GAS X-RAYS INSULATOR WINDOW END SIDE TO PREAMPLIFIER X-RAYS ANODE CATHODE R +2 kV – DETECTOR TUBE POWER SUPPLY ARC DISCHARGE IONIZATION CHAMBER REGION GLOW DISCHARGE REGION PROPORTIONAL COUNTER REGION MIGRATION 10 GEIGER COUNTER REGION REGION OF LIMITED PROPORTIONALITY 105 REGION OF UNSATURATION GAS AMPLIFICATION FACTOR 1010 AVALANCHING DISCHARGE FIGURE 38 Structure of the gas-filled x-ray detector (proportional and Geiger counters, and ionization chamber) The detector is shown to have both end and side windows for illustration only APPLIED POTENTIAL FIGURE 39 Gas-amplification factor as a function of applied potential for the gas-filled detector The proportional counter region yields a pulse height that is proportional to the energy of the radiation or particle The amplification factor is 500 to 10,000 and the detector has a dead or non-conducting time of 0.5 ␮sec Alpha and beta particles can be measured separately by using two different voltages and a pure alpha emitter in a proportional counter The limited proportional region is not useable because the presence of secondary charges hinders the gas amplification process In the GM tube region the amplification factor is 109 and the pulse height is independent of the energy and type of radiation Therefore pulse height analysis can not be performed using the GM tube but it can be used for general counting The plateau is 300 V in length and the counting rate increases less than 3% for a 100 V increase in applied voltage The positive space charge formed in the detector © 2006 by Taylor & Francis Group, LLC causes a non-conducting or dead time of 250 microseconds leading to a loss in radiation events Serious rate errors are experienced when counting rates are larger than 104 cpm A correction to the rate may be calculated using the resolving time This detector is useful for the analysis of separated radionuclides (ii) Semiconductor detectors There are several types of semiconductor detectors, namely the surface barrier and p-n junction detectors and the lithium drifted silicon or germanium detectors They all function, generally, according to the following model Semiconductor detectors are analogous to gas-filled detectors in their principle of operation In the semi-conductor detector a radiation event leads to an ion pair formation of an electron–hole pair, whereas in the gas-filled detector electron– cation pair are formed A model of a semiconductor detector, a lithium drifted detector, is given in Figure 40 A central zone of ultrapure intrinsic semiconductor is flanked by thin layers of p and n type semiconductor material A bias voltage is imposed across the ensemble to form a high field The radiation event causes the formation in the central zone of a highly energetic photoelectron which gives rise to a large number of electron–hole pairs The large number of highly mobile electrons are raised to the conduction band due to the transfer of kinetic energy from the photoelectron The electrons and holes “move” to the p and layers, respectively, and are collected under the effect of the high field giving a current pulse The size of the current pulse is proportional to the energy of the radiation event as in the proportional counter and has a pulse width due to the randomness of the process as discussed previously In the lithium-drifted detectors lithium, an n-type substance, is used to form the ultrapure intrinsic semiconductor from p-type silicon or germanium Ultimately lithium becomes the doppant for the n-type semiconductor after several involved processes These detectors function more efficiently if the detector and preamplifier are kept at liquid 586 INSTRUMENTATION: WATER AND WASTEWATER ANALYSIS better resolved in a crystal spectrometer Energy dispersive systems have excellent resolution for wavelengths below one Angstrom but demonstrate poor resolution for wavelengths above Å To preamplifier Electrons Holes Gold contact surface (~2000 Å) –500V N-type region Lithium-drifted Intrinsic region P-type region (dead layer~0.1mm) X-rays Gold contact surface (~200 Å) FIGURE 40 Cross-section of a typical lithium-drifted silicon, Si(Li), detector, x-rays create electron-hole pairs in the intrinsic region of the semiconductor; these charge carriers then migrate to the electrodes under the influence of an applied bias voltage (Courtesy of the Kevex Instruments, Inc.) nitrogen temperature, Ϫ196ЊC At this low temperature thermal electronic noise is reduced and the lithium is prevented from diffusing into the central zone of intrinsic semiconductor material affording increased resolution capabilities The energy required for an electron-hole pair generation for several radiation detectors are as follows: 2.9 eV (eV, electron volt) for germanium, 30 eV for a gas ionization chamber and 500 eV to produce a photoelectron in a NaI(T1) scintillation detector Thus the germanium detector produces 170 times the number of ion pairs than the scintillation detector per unit of eV and the resolution is better by 13 times Different pulse widths from each type of detector caused by the randomness of the process lead to the differences in energy resolution For electrons, x-rays, and photons the energy resolution in the germanium detector is 3.8, 0.6 and 20 keV, respectively The line widths for the germanium and scintillation detectors are 3.3 and 46 keV (keV, 1000 ev), respectively The germanium detector in conjunction with a pulse height analyzer can be used to measure x- and gamma rays in energy dispersive spectrometers In these systems radiation with a spectrum of energies can be resolved, since the germanium detector produces a current pulse proportional to the energy of the radiation One disadvantage of this system is that radiation with a wave-length above one Angstrom is © 2006 by Taylor & Francis Group, LLC (b) Scintillation counters Some inorganic crystals and organic crystals or molecules emit a pulse of light on interaction with ionizing radiation The energy of the radiation causes ionization or activation of the scintillating substance; it relaxes emitting a fluorescent or phosphorescent light pulse with life times of about 10Ϫ8 and 10Ϫ4 seconds, respectively, in the visible or near uv region The number of photons emitted in each light pulse is proportional to the energy of the radiation event Some crystals, namely sodium iodide, are doped with an activator to shift the light pulse to a longer wavelength (i) Crystal scintillation counters Scintillation counters utilize a crystal optically coupled to a photomultiplier tube that converts the light pulse to a current pulse (see Section III,B,1,b,(4),(a),(iii)) Further coupling to a pulse height analyzer results in an energy dispersive spectrometer Inorganic scintillation crystals of alkali halides doped with thallium, lead or europium as activators, are commonly used Potassium halides containing the natural radioisotope, 40K, which radiates electrons, positrons, and g rays, are not used in scintillation detectors A sodium iodide crystal doped with 0.1 to 1% of thallium (I) iodide, NaI(T1), is most widely used The NaI has a high density that absorbs gamma radiation and the iodide ion provides an efficient conversion of radiation to light The radiation event first activates the iodide ion that emits a light pulse in the uv region The uv pulse excites the thallous ion that on relaxation emits fluorescent light at about 410 nm It is compatible to the photomultiplier tube Since NaI is hygroscopic, the crystal must be sealed well For most efficient counting a “well type” detector is frequently used Isotopes emitting x-, gamma, beta, and alpha radiation are detected by NaI(T1), and in particular x- and gamma radiation are most beneficially measured Energy dispersive spectrometers using this detector are employed in x- and gamma-ray spectrometers (ii) Liquid scientillation counters Liquid scintillation counting is a convenient and efficient means of detecting low levels of radiation from small amounts of samples The liquid scientillation solution consists of a primary, or a primary and secondary scintillator or phosphor dissolved in an organic solvent The radioactive sample is dissolved in the scintillation solution The radiation excites the organic solvent molecules, such as toluene, xylene, terphenyl, etc The excited molecules transfer their energy, mainly non-radiatively, to the primary scintillator causing emission of a fluorescent light pulse detected by the photomultiplier tube The newer models of liquid scintillation counters contain photomultiplier tubes that respond to the emissions of a primary scintillator PPO (2,5-diphenyloxazole), a primary INSTRUMENTATION: WATER AND WASTEWATER ANALYSIS scintillator, emits light pulses in the range 330 to 400 nm Blue sensitive photomultiplier tubes employed in older model counters necessitated the use of a secondary phosphor which absorbed the light pulse of the primary phosphor and emitted a light in the blue wavelength region POPOP, the 20 scintillator, 2,2Јp-phenylenebis(4-methyl-5-phenyl-oxazole), absorbs at about 360 nm and emits fluorescence light at 410 to 420 nm Therefore, PPO and POPOP are a good pair to provide an effective liquid scintillator for the older blue sensitive photomultiplier tubes A diagram of a scintillation counter is shown in Figure 41 (2) Signal processors and readout Radiation detection instruments are able to measure two characteristics; the number of particles or radiation events occurring in a unit time and the energy of each particle or event The counting of events is carried out in electronic countering devices Counting circuits convert voltage pulses to square waves, scale or reduce the number of input pulses, and count the frequencies of pulses using binary digital circuits A scaling circuit decreases the frequency of pulses arriving from the detector by a known fraction in order to accommodate the counting devices detection rate Binary digital circuits containing JK flip-flops accomplish this reduction A binary counter with four JK flip-flops produces one output pulse for every sixteen input pulses Arrangement of the binary circuits to provide a decade counting unit provides a decimal readout In ionization chambers, proportional counters, semiconductor detectors, and scintillation counters the size or height of the pulse is proportional to the energy of the radiation event Pulse height analyzers are coupled to the output of these detectors in order to form an energy dispersive instrument Pulse height analyzers employed to separate radiation 587 of differing energies are equivalent to monochrometers used to disperse uv, vis, and ir radiation Instruments are also available that carry out wavelength dispersion The radiation is dispersed using a crystal for x- and gamma rays A description of the operation of a pulse height analyzer is as follows: the radiation event is converted to a signal by the detector and amplified resulting in pulses as large as 10 volts A pulse height selector provides a narrow voltage window by rejecting voltage pulses between minimum and maximum values using electronic discriminator circuits This window can have a voltage range of 0.1 to 0.5 volts A pulse height analyzer can consist of one or several pulse height selectors One pulse height selector comprises a single channel pulse height analyzer The voltage range of 10 volts can be scanned, automatically or by hand, using a window of 0.1 volt yielding an energy dispersion spectrum Two to hundreds of channels comprise a multichannel analyzer Each channel with its own counting circuit is set for a specific voltage window Therefore, the total spectrum can be simultaneously counted and recorded Readout of counting rates can be displayed on solid state devices, printers, etc Spectra display on a potentiometric recorder or storage in a computer with subsequent printout is available (3) Applications Liquid scintillation counting is used for a variety of tasks: namely; measurement of the low energy beta emitters 3H, 14C, 32P and 35S; gamma and x-ray energy dispersive spectrometry; and beta-gamma coincidence scintillation counting for 131I Standard Methods2 has procedures for the analysis of cesium-134 and 137, and iodine-129 through 135, total radioactive strontium (89Sr and 90Sr) and 90Sr alone, tritium, radon222, uranium-234, 235 and 238, and total radium, radium226, radium-228, and total alpha and beta content of water Removable lead cao REFERENCES Lead shield Well Internal lead shielding Magnetic shielding Phototube Switch Crystal Position lock Lead shield Preamplifier Counter housing FIGURE 41 A well-type scintillation counter (Courtesy of TN Technologies, Inc.) © 2006 by Taylor & Francis Group, LLC Kieth, L.H., ed (1988), Principles of Environmental Sampling, American Chemical Society, Washington, D.C Amer Pub Health Assoc., (1995), Standard Methods for the Examination of Water and Wastewater, 19th Ed., Washington, D.C Mentink, A.F (1968), Specifications for an Integrated Water Quality Data Acquisition System, U.S Depart of Interior, FWQO, Washington, D.C.; STORET System Handbook (1973), U.S Environmental Protection Agency, Washington, D.C Ciaccio, L.L., Raul R Cardenas, Jr and J.S Jeris (1973), Automated and Instrumental Methods in Water Analysis in Water and Water Pollution Handbook, Vol 4, L.L Ciaccio, ed., Chap 27, Marcel Dekker Inc., New York American Society for Testing Materials (1996), Book of ASTM Standards, Vols 11.01 and 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Sons, New York 107 Birks, J.B (1964), The Theory and Practice of Scintillation Counting, Pergamon Press, New York 108 Kruger, P (1971) Principles of Activatino Analysis, John Wiley & Sons, New York LEONARD L CIACCIO Ramapo College ... of water and wastewater samples The Nature of Water- Related Samples and Sampling Considerations Nature of Water- Related Samples The category of water and wastewater samples can include water samples,... environmental water problems, monitoring of processes to treat wastewaters and drinking water, and the ecological monitoring of natural waters WATER AND WASTEWATER ANALYSIS In the last fifty years the... in water and wastewater characterization and instruments are available to measure specific and non-specific parameters Methodology The large variety of tests carried out on water and wastewater