Soil and Environmental Analysis: Modern Instrumental Techniques - Chapter 3 pps

3 Electroanalytical Methods in Environmental Chemical Analysis Iain L. Marr The University of Aberdeen, Aberdeen, Scotland I. BACKGROUND Of the many electrical parameters that can be measured, only three need be considered here as of practical importance—potential, current, and conductivity—thus opening the way to three techniques that really have something to offer in the area of environmental analysis, where samples are always complex and determinants of interest usually are present at very low concentrations. Electrical charge, measured in the technique known as coulometry, does indeed have important analytical applications (especially in the Karl Fischer determination of water), but not often in the field of environmental analysis, and therefore will not be discussed further. The potential of an electrode may be related to the concentration of a particular species if a number of conditions are met—the ‘‘clever’’ chemistry of electrode membrane manufacture makes it possible to construct probes with useful sensitivity and selectivity to individual species in solution. While the tendency for a chemical reaction to proceed is measured, no current is actually allowed to flow. Potentiometry using ion selective electrodes is the technique in question, and using the glass electrode for the measurement of pH is one special (and the best known) example. A great deal more information can be obtained by electroanalytical methods if one parameter is varied and a second measured—so-called two- dimensional measurement. In this case a signal pattern rather than one measured value is used to identify, as well as quantify, one or more species in a solution. The current can be monitored as the potential across a cell is TM Copyright n 2004 by Marcel Dekker, Inc. All Rights Reserved. scanned: the approach is termed amperometry. When the current is controlled by diffusive processes in solution, the technique is termed polarography, and while few people will have any time for a dropping mercury electrode these days, the modern variants of anodic and cathodic stripping voltammetry have much to offer for the direct determination of very low concentrations of metal ions in natural waters, even in seawater. Since the electrical conductivity of an aqueous solution depends on the concentration of ions dissolved in it, this parameter serves as a useful indicator of salinity, adequate for rough measurements in the field and for more precise determinations if care is taken with calibration and temperature compensation. While a current is allowed to flow in this technique, the potential is very small and is reversed at millisecond intervals so that there is no net chemical reaction on the electrode surface. Finally, we should consider the factors that continue to make electroanalytical techniques attractive for routine measurements. They are generally simple, their limitations are well understood, and, above all, they provide an electrical signal that is very easy to transmit via telemetering systems, to store electronically, and to process in a computer. Thus they make possible a number of useful determinations at low concentrations in real samples. II. POTENTIOMETRY The Nernst law relates the potential of an electrochemical cell (E, in mV) to the activities (a i , a j ) of a given species in the two halves of the cell, illustrated in Fig. 1. E ¼ R ÁT n ÁF ln a 1 a 2 ð1Þ This equation can be converted to use logarithms to the base 10, in which case the constant term is multiplied by 2.3 to become 0.059/n volts at room temperature, where n is the number of electrons involved in the electro- chemical oxidation/reduction of the species. In dilute solutions, activities can be taken as equal to concentrations, so the equation takes on a simple practical significance. Instruments such as pH meters that have a temperature compensation circuit simply change the slope factor RT/nF and do not take into account any changes in potential due to the chemistry, such as changes in solubilities. 112 Marr TM Copyright n 2004 by Marcel Dekker, Inc. All Rights Reserved. Figure 1 indicates that we require a means of connecting our potential- measuring device, such as a digital voltmeter, to the two solutions. The two electrodes, shown as silver wires in Fig. 1, serve this purpose, and when coated with silver chloride and kept in a potassium chloride solution of constant concentration, they maintain constant potentials. Any difference in the two potentials is then taken up as a small constant included in Eq. (1). There are a number of practical points which arise from this simple idea: The membrane separating the two halves of the cell should be ideal and respond only to the ion of interest (here the hydrated proton, H þ aq ). The success of modern ion-selective electrodes derives from the ingenuity of chemists in developing a range of membrane systems that come close to meeting this requirement (Vesely et al., 1978). No current should flow through this cell, as this would entail flow of the ions through the membrane. The digital voltmeter must therefore have a high input impedance, taking no more than a few pA of current from the cell. Sufficient ions should be available in the two solutions to enable the membrane to respond to them. Measurements in very dilute waters are, for this reason, rather difficult and will certainly entail a longer equilibration time, say 1–2 minutes, till a stable potential is obtained. Two electrodes are always required for potentiometry, even when it appears, at first sight, that the measurement can be made with only one, as is the case with combination pH electrodes (see below). The internal reference electrode is usually not accessible to the user, but the external one must be maintained by topping up with the electrolyte from time to time. A saturated calomel electrode (see below) is frequently used as a separate external reference electrode, and if the presence of traces of chloride is undesirable, then the sulfate version can be used instead. Figure 1 Schematic diagram of a Nernstian concentration cell with membrane. Electroanalytical Methods 113 TM Copyright n 2004 by Marcel Dekker, Inc. All Rights Reserved. A. The Glass Electrode—Measurement of pH Much research has gone into the development of special glasses for the glass electrode, but the key to modern electrodes lies in the substitution of lithium in the glass for sodium, to avoid the electrode responding better to the sodium ions at high pH than to the very dilute hydrogen ions. Few users these days ever think of the ‘‘alkali error’’ associated with electrodes fabricated from Macinnes and Dole’s soda glass in 1930, and we can expect a working range from pH 1 to pH 13, with deviations becoming significant only outside this range, using the lithia–lime glass developed by Cary and Baxter in 1949. Everything you could possibly need to know about the glass electrode has been summarized excellently by Galster (1991). A modern combination pH electrode is shown in Fig. 2, and a schematic diagram of the glass membrane in Fig. 3. The body may be glass or glass sheathed in plastic for greater robustness. The bulb is sometimes surrounded by a plastic protecting shield to minimize the chances of breaking it by rough contact against sample containers. However, this impairs the accessibility of the electrode surface to the ions in the solution and calls for either good stirring of the solution or longer waiting time until the reading is taken. Further, great care is then necessary to ensure that the electrode is thoroughly washed, e.g., with a jet of distilled water, between samples. Glass electrodes will give excellent service for a working life of one to two years if looked after. A few important points should be remembered: The glass membrane is thin (ca. 50 mm) and is easily scratched or broken, especially if used to stir crystals when making up solutions. The membrane will change irreversibly if allowed to dry out, though if caught in time, an overnight soak in 1 M hydrochloric acid might rejuvenate the very thin hydrated gel coating. Figure 2 Combination glass electrode. 114 Marr TM Copyright n 2004 by Marcel Dekker, Inc. All Rights Reserved. The potential should stabilize in 15–30 seconds after immersion in a sample solution. Slower response may indicate that the electrode is nearing the end of its useful life. At the same time the Nernstian response is also beginning to be lost. For this reason, pH meters that permit the meter plus electrode to be checked and adjusted in two different buffers are always to be preferred. However, apparently slow response may be due to the behavior of the sample: outgassing of CO 2 from some water samples may cause the pH to drift upwards because the pH really is changing. A pH electrode should always be checked against two buffers—even an electrode with a hole in it can be made to read pH 4, but will read that same value in all solutions. The 0.05 M potassium hydrogen phthalate (10.2 g L À1 ) buffer with pH ¼4.00 and the 0.05 M sodium tetraborate buffer (19.1 g L À1 ) buffer with pH ¼9.20, both at 20  C, are reliable and easy to prepare one-component buffers. Make them up fresh each week, as they are likely to deteriorate owing to bacterial growth and to absorption of CO 2 from the air, respectively. B. ISFET pH Sensors Transistors are three-electrode electronic devices in which a small current flowing between two of the electrodes (emitter to collector) is controlled by a second, very small, current flowing between the emitter and a third, intermediate, electrode called the base. In a field-effect transistor (FET), the controlling electrode responds to potential, normally generated by the adjacent electronic circuitry, but sometimes by the external signal to be measured, for example a potential generated by a chemical electrode. Thus pH meters now use a metal oxide semiconductor FET (MOSFET) operational amplifier to measure the potential of the glass electrode without taking any current from it. The extension of the MOSFET concept has been to coat the metal oxide layer with a chemically selective coating – the ‘‘membrane’’ of a chemical electrode – and to allow a carefully Figure 3 Schematic diagram of the glass membrane in a pH electrode. Shaded area: dry glass membrane electrode for fluoride. Electroanalytical Methods 115 TM Copyright n 2004 by Marcel Dekker, Inc. All Rights Reserved. chosen chemical system to control the transistor current. Such a device has been termed a CHEMFET, a chemically sensitive field-effect transistor, but now is more usually given the name ISFET, ion-selective field-effect transistor (Bergveld, 1972). A silicon nitride coating, for example, deposited on the metal oxide gate results in a device that has near ideal response to hydrogen ions, with a working range of pH 1–13, and that is a great deal tougher than any glass electrode. Mettler-Toledo and Thermo-Russell market ISFET pH electrodes for demanding environmental applications, but they can be used only with appropriate ISFET meters and not with conventional pH meters. C. Single-Crystal Lanthanum Fluoride Frant and Ross’s announcement in 1966 that doping LaF 3 with a little EuF 2 resulted in a single crystal with sufficient electrical conductivity to be used as an electrode membrane, and one that would respond ideally to fluoride ions in solution, represented a major breakthrough in the area of ion-selective electrodes, much valued because of the difficulty at that time of determining this anion by any other route (Frant and Ross, 1966). The construction is shown in Fig. 4. One crucial practical problem was how to cement the LaF 3 single crystal to the plastic body and at the same time to guarantee perfect electrical insulation. Users should be warned that single crystals are brittle and will shatter if dropped on a hard surface. The behavior of this electrode is worth discussing because it illustrates problems common to most other electrodes, and also ways of overcoming these problems. Hydrofluoric acid, HF, is a weak acid, with pK a ¼3.5, and the electrode responds to the hydrated fluoride ion. Therefore all solutions must be adjusted to a pH of 5 or greater, where the acid is effectively fully dissociated. An acetate buffer is therefore added to all solutions, standards and samples alike. Figure 4 Single-crystal membrane electrode for fluoride. 116 Marr TM Copyright n 2004 by Marcel Dekker, Inc. All Rights Reserved. As mentioned earlier, in high salt concentrations, ion activities deviate significantly from analytical concentrations and calibrations based on concentration, even for low fluoride concentration, and become inaccurate. The answer is to add a high concentration of sodium perchlorate to all solutions, to maintain a constant electrolyte strength for all measurements. Certain metal ions, notably aluminum and iron(III), form very stable fluoride complexes and will effectively mask free fluoride in e.g. a river water sample, so that very low fluoride concentrations will be reported if the measured potential is converted to fluoride concen tration. The answer here is to add a strong complexing agent, EDTA or CHDTA, to mask the metal ions. The fluoride electrode is therefore used with TISAB (total ionic strength adjuster buffer) being added to all solutions, making possible a working range of 0.1–100 mg L À1 of fluoride (Frant and Ross, 1968). As the Nernst equation is operative, the potential, in mV, is plotted against the log 10 of the concentration, either in mg L À1 or as molarity (Fig. 5). D. Silver Halide in Silicone Rubber Membranes The concept of using a sparingly solubl e metal salt as the responsive component of a membrane lies behind the design of many types of electrode, and the silver halides are the obvious choices for making a halide ion selective electrode. The problem of making a ‘‘membrane’’ that was both mechanically strong and electrically conducting was solved by Pungor et al. (1966) by compressing finely powdered silver halide with a small amount of Figure 5 Typical calibration for determination of fluoride with a LaF 3 electrode. Electroanalytical Methods 117 TM Copyright n 2004 by Marcel Dekker, Inc. All Rights Reserved. silicone rubber as binder. Nernstian response was obtained over useful working ranges for chloride, bromide, and iodide, but there is a degree of response to other halides best described by the interion response factor: E cell ¼E const þ2:3 RT nF ln½a i þK i,j ða j Þ For good performance and little interference, the K values should be small, ideally 0.001 or less. General problems with ISEs and how to characterize their performance have been discussed by Moody and Thomas (1972). The sulfide electrode presents some difficulties in use, as free sulfide ion is obtained only at very high pH, where oxidation of the ion is facilitated. The standard procedure is to use a high pH buffer (1 M NaOH, ca. pH 14) with an oxygen scavenger such as cresol, but this usually is a messy solution that covers the electrode in oxidation products. It is also no solution to the problem of measuring sulfide directly in sediments, where depth profiles are of interest in investigations of the microbial mat and the pore water composition changes in composition with depth. However, as the main interference is a pH effect, it can be countered by measuring the pH and correcting the sulfide electrode potential directly, with a series of calibration graphs covering the pH range of interest (Fig. 6). E. Liquid Ion Exchanger Membranes for Anions Liquid ion exchangers can be held on porous glass or ceramic supports to serve as membranes for ion-selective electrodes but are nowadays more commonly mixed in with a polymer to give a solid plastic membrane, enabling a large variety of chemistries to be utilized. Long-chain quaternary amines are dissolved in a viscous solvent as their ion pairs, e.g., cetyltrimethylammonium cation with nitrate or perchlorate, offering good selectivity, especially for larger single-charged anions. Generally one makes the assumption that interfering ions will not be present in the test sample, as for example perchlorate when an electrode is being used to monitor nitrate Figure 6 Sulfide calibrations recorded at different pH values. 118 Marr TM Copyright n 2004 by Marcel Dekker, Inc. All Rights Reserved. in river water. These electrodes require attention, with regular replacement of the liquid, but are nevertheless useful for environmental monitoring purposes, covering as they do the concentration range of interest. F. Plastic Membranes for Cations Early investigations in the 1960s showed that metal ions immobilized in a PVC (polyvinyl chloride) matrix, as sparingly soluble salts, could behave as selective membranes. These offer the advantage of simplicity of replacement compared with the liquid ion exchanger membranes. Successful calcium electrodes, for example, were made by incorporating the calcium salt of didecylphosphoric acid along with neutral dioctylphenylphosphonate as modifier, in PVC (Crags et al., 1974). Much research has subsequently gone into designing highly specific ligands for a range of metals, in which the dimensions of the chelate formed when the long-chain arms wrap around the metal ion approach the ideal for the particular metal ion. G. Electrodes for the Alkali Metals Glass electrodes were originally explored for determination of the alkali metals, especially sodium and potassium, particularly with medical applica- tions in mind. However, though a glass electrode for sodium has long been marketed and is useful in that sodium is usually present, at least in physio- logical fluids, at one hundred times the concentration of potassium, so that potassium does not cause a significant interference, the complementary problem of the determination of potassium seemed insoluble. It was only when Simon and his team (1970) showed that complexes between certain antibiotics and potassium were so much more stable than the corresponding ones with sodium, that a really selective electrode could be manufactured. Valinomycin, a cyclic 6-membered peptide that displays a selectivity constant with a factor of three to four thousand in favor of potassium against sodium, has formed the basis of a range of successful commercial electrodes. The combination of reagents is formulated into a plastic membrane. H. Gas-Sensing Electrodes Electrodes are commercially available for a few gases—ammonia and carbon dioxide in particular. In fact, they do not respond in the way that the ion-selective electrodes do but are pH electrodes covered with special coatings, often of silicone rubber, that offer selective permeability to the gas in question. Thus ammonia arriving at the glass electrode surface causes a rise in pH, whereas carbon dioxide causes a lowering. The working ranges are much smaller than those of the true ion-selective electrodes. Electroanalytical Methods 119 TM Copyright n 2004 by Marcel Dekker, Inc. All Rights Reserved. I. General Comments on ISEs Because the response of an electrode to the determinant is logarithmic, establishing the limit of detection is a little more difficult than for linear response systems. Midgley (1984) has discussed this matter, showing how the intersection of the sloping line and the low-level constant potential can help. Technical data in Table 1 show that a wide range of electrodes is available, for many common ions, covering concentrations of interest in environmental work as well as in many medical applications. The advantages of such probes include The simplicity of a direct reading device requiring little or no chemical sample treatment A wide working range, typically three orders of magnitude Reasonable tolerance to other ions in many environmental samples Suitability for continuous monitoring using data loggers to collect measurements The possibility of being made with very small dimensions for exploring concentration profiles, e.g., in tissue or in sediment J. Redox Electrodes Noble metal electrodes respond to the redox potential of their environment without actually dissolving or corroding, a fact frequently made use of for assessing the state of the chemistry in, e.g., sediment pore water. A 1-mm diameter platinum wire is sealed into an insulating sheath and can then be pushed into a wet sediment with little risk of breakage. Redox electrodes are Table 1 Examples of Ion-Selective Electrodes Species Type Range (M) Species Type Range (M) Ammonia gas 5Â10 À7 to 1 Iodide solid-state 5 Â10 À8 to 1 Bromide solid-state 5 Â10 À6 to 1 Lead solid-state 10 À6 to 0.1 Cadmium solid-state 10 À7 to 0.1 Nitrate plastic 7 Â10 À6 to 1 Calcium plastic 5 Â10 À7 to 1 Nitrite plastic 4 Â10 À6 to 10 À2 Carbon dioxide gas 10 À4 to 10 À2 Perchlorate plastic 7 Â10 À6 to 1 Chloride solid-state 5Â10 À5 to 1 Potassium plastic 10 À6 to 1 Copper solid-state 10 À8 to 0.1 Sulfide solid-state 10 À7 to 1 Cyanide solid-state 8 Â10 À6 to 10 À2 Sodium glass 10 À6 to 1 Fluoride solid-state crystal 10 À6 to 1 Thiocyanate solid-state 5 Â10 À6 to 1 Source: Data from Thermo-Orion (2001) and from Metrohm (1999). 120 Marr TM Copyright n 2004 by Marcel Dekker, Inc. All Rights Reserved. [...]... Limit of detection Alizarin Red S, pH 7.1 pH 2.8 with HCl Dimethylglyoxime, pH 8 .3 Catechol, pH 7.5 1-Nitroso-2-naphthol, pH 6.9 Dimethylglyoxime, pH 8 .3 pH 2.8 with HCl 8-Hydroxyquinoline, pH 6.9 Catechol, pH 6.9 Ammonium pyrrolidone dithio-carbamate, pH 7 .3 À1. 13 À0.7 À1. 13 À0.27 À0. 53 À1.01 À0.5 À0.68 À0.69 30 10 10 5 60 10 30 50 15 À1.15 10 ng LÀ1 ng LÀ1 ng LÀ1 ng LÀ1 ng LÀ1 ng LÀ1 ng LÀ1 ng LÀ1 ng... ASV by addition of a complexing agent Talanta 38 : 735 – 739 Metrohm Ltd 1999 Metrosensor Electrodes Catalogue Metrohm Ltd., CH-9101 Herisau, Switzerland Midgley, D 1984 Limits of detection of ion-selective electrodes Anal Proc 21:284–287 Moody, G.J and Thomas, J.D.R 1972 Development and publication of work with selective ion-sensitive electrodes Talanta 19:6 23 639 Neeb, R 1961 Anodische amalgamvoltammetrie:... Biomed Eng 19 :34 0 35 4 Bolton, P.W., Currie, J.C and Tervet, D.J 1978 An index to improve water quality classification Water Pollut Control 271–284 Crags, A., Moody, G.J and Thomas, J.D.R., 1974 PVC matrix membrane ionselective electrodes J Chem Ed 51:541–544 Cresser, M.S., Killham, K and Edwards, T 19 93 Soil Chemistry and Its Applications Cambridge University Press, Cambridge Frant, M.S and Ross, J W... Table 2 Eh Values for Important Redox Reactions in Soils and Sediments Reaction Eh (mV) at 25 C pH 5 O2 to H2O NO3À to NO2À MnO2 to Mn2þ Fe(OH )3 to Fe2þ SO42À to S2À CO2 to CH4 H2O to H2 pH 7 930 530 640 170 À70 À120 À295 820 420 410 À180 À220 À240 À4 13 Note: Potentials are with respect to the standard hydrogen electrode Source: Cresser et al., 19 93 TM Copyright n 2004 by Marcel Dekker, Inc All Rights... Science 207: 135 5– 135 6 Simon, W., Wuhrmann, H.-R., Vasak, M., Pioda, A.R and Stefanac, Z 1970 Ionselective sensors Angew Chem Int Ed Eng 9:445–455 Standing Committee of Analysts 1978 Measurement of Electrical Conductivity and Laboratory Determination of the pH Value of Natural, Treated and Waste Waters HMSO, London Standing Committee of Analysts 1987a Determination of Twelve Trace Metals in Marine and Other... for 30 minutes will usually solve this problem UV irradiation will also break down any organic complexing ligands, releasing the metals into the (acidified) TM Copyright n 2004 by Marcel Dekker, Inc All Rights Reserved Determination of Some Trace Metals in Seawater by Stripping Voltammetry Element Technique Al Cd þ Pb Co þ Ni Cu Fe Ni Pb U V DP-CSV DP-ASV CSV DP-CSV CSV CSV DP-ASV CSV DP-CSV Zn DP-CSV... retain an important role for electroanalytical techniques in environmental analysis REFERENCES Ansa-Asari, O., Marr, I.L and Cresser, M.S 1999 Evaluation of cycling patterns of dissolved oxygen in a tropical lake as an indicator of biodegradable organic pollution Sci Total Environ 231 :145–158 Bergveld, P 1972 Development, operation and application of the ion-selective fieldeffect transistor as a tool for... 154:15 53 1555 Frant, M.S and Ross, J W 1968 Use of a total ionic strength adjustment buffer for electrode determination of fluoride in water supplies Anal Chem 40:1169–1171 Galster, H 1991 pH Measurement Verlag Chemie, Weinheim Glud, R.N., Klimant, I., Holst, G., Kohls, O., Meyer, V., Kuhl, M and Gundersen, ¨ J.-K 1999 Adaptation, test and in situ measurements with O2 microoptrodes on benthic landers Deep-Sea... Marine and Other Waters by Voltammetry or AAS HMSO, London Standing Committee of Analysts 1987b Dissolved Oxygen in Natural and Wastewaters HMSO, London Thermo-Orion 2001 Laboratory Products Catalogue Thermo-Orion, Beverly, MA, U.S.A Vesely, J., Weiss, D and Stulik, K 1978 Analysis with Ion-Selective Electrodes Ellis Horwood, Chichester, England TM Copyright n 2004 by Marcel Dekker, Inc All Rights Reserved... test and in situ measurements with O2 microoptrodes on benthic landers Deep-Sea Research I 46:171–1 83 Grasshoff, K., Kremling, K and Ehrhardt, M 1999 Methods of Seawater Analysis 3d ed Wiley-VCH, Weinheim TM Copyright n 2004 by Marcel Dekker, Inc All Rights Reserved Electroanalytical Methods 135 ` Kemula, W and Kublik, Z 1958 Application de la goute pendante de mercure a la ´ ´ determination de minimes . mgL À1 Fe CSV 1-Nitroso-2-naphthol, pH 6.9 À0. 53 60 ng L À1 2 mgL À1 Ni CSV Dimethylglyoxime, pH 8 .3 À1.01 10 ngL À1 60 mgL À1 Pb DP-ASV pH 2.8 with HCl À0.5 30 ngL À1 50 mgL À1 U CSV 8-Hydroxyquinoline,. limit Al DP-CSV Alizarin Red S, pH 7.1 À1. 13 30 ng L À1 10 mgL À1 Cd þPb DP-ASV pH 2.8 with HCl À0.7 10 ngL À1 50 mgL À1 Co þNi CSV Dimethylglyoxime, pH 8 .3 À1. 13 10 ng L À1 60 mgL À1 Cu DP-CSV Catechol,. with a working range of pH 1– 13, and that is a great deal tougher than any glass electrode. Mettler-Toledo and Thermo-Russell market ISFET pH electrodes for demanding environmental applications, but