Technical Notes Anal. Chem. 1995, 67, 1147-1151
Composite Graphite Ion Selective Electrode Array Potentiometry for the Detection of Mercury and Other Relevant Ions in Aquatic Systems Jo Anne Shatkin,t Halina Szejnwald BcOwn,t and Stuart LicM*sts* Environment, Technology and Society Program, and Department of Chemistty, Clark University, 950 Main Street, Worcester, Massachusetts 01610
Simultaneous detection of many ions in situ offers advantages in environmental monitoring, including the ability to distinguish a variety of complexed species. We demonstrate here a small mercury ion selective electrode responsive to changing halide concentrations, interpretable as mercury halide speciation in solution. We also present an array of similarly constructed small halidedetecting electrodes capable of Heredating halides and sulfide activities. These are a necessary h t step toward development of an ion selective microelectrode array for speciation detection of heavy metals in natural waters. Electrodes consist of a composite of a conductive epoxy and an insoluble salt of the species to be detected. Electrodes studied have an easily renewable surface of approximately 13 mm2. A simple rapid measure for regulated metals, capable of differentiating the in situ distribution of metal species, would be valuable as metal toxicity, transport, and fate characteristics vary with speciation. Under current EPA methodology, most regulated metals are analyzed by atomic absorption, requiring a separate protocol for each metal which does not differentiate metal valence or species.' Several mercury ion selective electrodes have previously been reported. These include pressed powder membrane-type elect r o d e ~and ~ , ~other^^-^ including solid state electrodes modiiled with mercury-sensitive material, resulting in linear response in the mercury range of 10-5-10-8 M after a preconcentrationstep. For an in situ sensor, the detection limit must be lower, as unbound mercury (Hgz') in natural environments is often present only in the part per billion range. Other common methods used +
Environment, Technology and Society Program.
* Department of Chemistry.
(1) USEPA Methodsfor Chemical Analysis of Water and Wastes; EPA Environmental Monitoring and Support Laboratory, EPA-600/479020; protocols 245.1 and 245.2, 1983. (2) RadiC, N. Sel. Electrode Rev. 1989,11, 177-89. (3) Najib, F. M.; Faizullah, A T.; Ahmed, F. Anal. Lett. 1981,14, 47-61. (4) Yamazato, M.; Fukuda, S.; Kato, M.; Yoshimori, T. Denki Kagaku 1973, 41, 789-794. (5) East, G. A; Da Silva, I. A Anal. Chim. Acta 1983,148,41-50. (6) Labuda, J.; Plaskoii, V. Anal. Chim. Acta 1990,228,259-263. (7) Wang, J.; Bonakdar, M. Talanta 1988,35, 277-280.
0003-2700/95/0367-1147$9.00/0 0 1995 American Chemical Society
to measure mercury include cold vapor atomic absorption,' GCcold vapor atomic fluorescence,8 graphite furnace and gold film sensor detection.'O In principle, an ion selective electrode can be used to quantify and correct for interferences by that ion on another electrode. This approach has been used to correct for the presence of alkali cation interferences in alkaline pH measurements" and for determination of several trace ions at physiological concentrations.12 Arrays of microelectrode sensors have been previously fabricated for several applications, including gas sensing and human blood ~hemistry.'~J~ The application of the array concept, using detection of ions at several electrodeswith known selectivity coefficients for both the ion of interest and its interferents, allows simultaneous and accurate detection where many interferent ions may be present. In this study, we present several conductive epoxy composite ion selective electrodes. We demonstrate a small mercury electrode responsive to changing halide concentrations, interpretable as mercury halide speciation in solution. As a first step toward an ion selective microelectrode array for speciation detection in natural waters, we present here an array of similarly constructed small halide-detecting conductive epoxy composite electrodes capable of differentiating halides and sulfide activities. These represent a step in the eventual development of a multichannel heavy metal potentiometric sensing array. The conductive material used to fabricate the electrodes has been previously demonstrated to make a good base material for amperometric and biological sensor^.^^-^^ The epoxy is durable, strong, and fairly uniform, providing an easily renewable surface by sanding and polishing.
w9
(8) Bloom, N.; Fitzgerald, W. F. Anal. Chim. Acta 1988,208,151-151. Keller, B. J.; Peden, M. E.Anal. Chem. 1984,56, 2617-2618. Ping, L.; Dasgupta, P. K. Anal. Chem. 1989,61, 1230-1235. Licht, S. Anal. Chem. 1985,57, 514-519. Otto,M.; Thomas, J. D. R Anal. Chem. 1985,57, 2647-2651. Forster, R J.; Regan, F.; Diamond, D. Anal. Chem. 1981,63,876-882.
(9) (10) (11) (12) (13) (14)
Carey, W. P.; Beebe, K. R; Kowalski, B. R Anal. Chem. 1987,59, 15291534. (15) Wang, J.; Vamghese, K. Anal. Chem. 1990,62,318-320. (16) Wang, J.; Golden, T.; Varughese, K.; El-Rayes, I. Anal. Chem. 1989,61, 508-512. (17) Wang, J. Anal. Chem. 1981,53, 2280-2283.
Analytical Chemistty, Vol. 67, No. 6, March 15, 1995 1147
External
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Figure 1. Schematic representation of a 50 electrode array. The expanded section represents the actual array used. Each electrode is electrically separately addressable, allowing simultaneous detection of many environmentally relevant ions, correction for interelectrode interferences, and subsequent speciation into major complexes.
EXPERIMENTAL SECTION
Preparationof Electrodes. Mercury graphite composite ion selective electrodes (MCGISE) were prepared with mercuric suEde (10% by weight) and mercury metal (HgO), 1%by weight, mixed with a two-part conductive epoxy, Dylon Grade RX,which is cast under vacuum into a 1mL tuberculin disposable syringe to form an electrode body. A copper wire is inserted via the syringe needle mount prior to the epoxy addition. Removal of the mold after a 12 h room-temperature drying period leaves a cylindrical electrode base. The electrode surface was manuallypolished first with a silicone carbide glass cutting wheel Wale Apparatus Co. Sl2CrSR grain size 120 soft, type bond rubber) and then with alumina slurry (successively 5 pm, 1pm, and then 0.3 pm) on a polishing cloth. The sides of the polished electrodes are coated with Clear GE Silicone I1 sealant for waterproohg. Once dry (24 h), the electrode surface is electrochemically polished in aqueous 0.1 M Na& to remove contaminants by sweeping the potential from -0.5 V at 10 mV/s to +3.00 V using a conventionalthree-electrode potentiostatic contigwation: a PINE D E 4 potentiostat with a platinum counter electrode and a Corning saturated calomel reference electrode (equipped with a KNO3 salt bridge). The current at 3.00 V of applied potential was recorded as an indication of electrode resistance. Additionally, the resistance across the entire electrode was determined at several locations on the surface as a measure of the uniformity of the electrode. Acceptable resistances for these electrodes are in the kiloohm range, while highly variable or high resistances in the megaohm range indicate air bubbles from casting that lead to poor electrode performance. Approximately 30% of these electrodes did not meet these standards. Electrodes were stored soaking in deionized water until use. The silver halide electrodeswere prepared in a similar manner, using an ion selective salt and epoxy, using a glass tube (4 mm i.d.) that was not removed but leaving an insulating shield for the 1148 Analytical Chemistry, Vol. 67, No. 6, March 15, 7995
electrode body. Glass tubes were not used for mercury electrodes because of wetting that developed between the epoxy and glass wall during presoaking. These electrodes were only polished manually. Solutions for analysis were prepared with analytical grade reagents. All glassware in cells was acid washed. Generally, electrodes were polished prior to experiments. Electrodes and cell were connected to a high-input impedance voltmeter (1013 8) for 12 h prior to an experiment and overnight between experiments. Electrode potential was recorded by a computerinterfaced Orion 920 ion analyzer at 2-3 times per minute. RESULTS AND DISCUSSION Composite Array Methodology. Using a conductive epoxy,
we have studied several sensors as a first step toward the creation of an array of microelectrodes, each with a separately addressable potentiometric ion selective sensor. Figure 1 is a schematic of such an array of 50 electrodes. As an initial step toward its development, we have designed individual sensors for mercury and silver and for halides that commonly interfere with their detection at low levels. Four electrodes as shown in the expanded section of Figure 1 were tested in a two by two electrode array. Mercury Speciationand a Mercury Sensor. Mercury forms many species in the presence of chlorides and bromides and solutions of varying pH. The linearity of response of our prepared mercury composite graphite electrode (MCGISE) was measured in three sets of solutions, and in each solution the concentration of free mercury(II) was calculated by the EPA's MmteqA2 model and computer program.18 Stable mercuric compounds with a wide range of formation constants are desirable for demonstration of the utility of a mercury sensor. At low Hg2+concentration,a series (18) U S . Environmental Protection Agency, Center for Exposure Assessment Modeling. MinteqM/PRODEFAZ, A Geochemical Assessment Model for Environmental Systems: Version 3, Athens, GA, 1991.
Table 1. Prediction of the Unbound Mercuric Ion Concentration in a Saturated Mercuric Halide Solution [CII, [Brl, M equilibrated
salt added
nominal
KBr KBr KBr
0.03 0.01 0.003 0.8 0.16 0.04 0.007
KCl KCl KCl KCI
0.0066 0.0025 0.0008 0.685 0.152 0.039 0.0068
fi-ee [Hg2+l
log @Hgz+
3.35 10-15 1.52 10-14 1.14 x 1.02 x 10-12 1.04 x IO-" 1.1 x 10-10 3.16 10-9
-14.77 - 14.00 -13.05 -11.99 -10.98 -9.95 -8.50
of standards are made with mercury bromide salts, while at midrange and highest concentrations, mercury nitrate and chloride salts may be respectively employed. Hg2+ solutions more concentrated than M were made by dilution of 0.1 M H g @IO&. More dilute mercuric ion concentrations were determined in accordance with KH~x, = [Hg2+l[X-12 using -log K H ~=x 13.16 ~ for C1- and 17.43 for Br-, respectively. The solutions were prepared by addition of KCl for lo-" M < [Hg2+]< M or KBr for [Hg2+]< lo-" M. Few stable mercuric compounds have solubility characteristics in the region between the halides and nitrates, but a linear trend from low to high concentrationranges was found. For the concentrated range ([Hg2+l > M), mercuric nitrate in an acidified solution was used. Solutions were made by dilution of a 0.1 M Hg(NO3)2 solution in 0.1 M HN03. For determining the detection and quantitation limits in the trace metal range, sparingly soluble mercuric bromide and mercuric chloride solutions were made. The concentration of free mercuric ion in these solutions is low due to complex speciation equilibria which allow the formation of at least nine distinct ionic and covalent complexes involving mercury. In a saturated solution of mercuric chloride, the concentration of unbound mercuric ions (Hg2+)is roughly 30 parts per billion, also present is mercury in bound species, including HgClOH, HgC12, etc. For lo-" M < [Hg2+] < M, the chloride concentration was set by KCl addition to limit the unbound mercuric ion (Hg2+) concentration. For solutions containing
