124
Anal. chem. 1986, 58, 124-127
in this medium to obtain the AsOf concentration (see Figure 2). These calculations by difference imply an increasing error accumulation from As043-to AsOz-, as shown in Table VI, from which it can be also observed that the mixtures studied are from P(V):As(V):As(III) molar ratio of 1:l:l to 30:2:15, 5215, and 5152. Higher ratios and concentrationsthan those listed in Tables V and VI have not been used owing to the fact that the signal obtained in the IO3- medium (in which all three species react) falls out of the range in which reliable absorbance measurements can be made. As can be observed from the above-mentioned tables, the errors in the determination do not exceed 3% in any case for PO-: (and only in one of the mixtures for AsO4%),while for AsOf the percentage error amounts to 8.7. The average errors of these mixtures are 1.5%, 2.20%, and 4.33%, respectively. If sodium tartrate (concentration 1%)is added to the samples, silicate is tolerated up to 400-fold concentration that of each of the three analytes. Determination of AsO;, A s O ~ ~ and - , P043-in Real Samples. Prior to determining these anions in real samples, we have performed a study of interferenta by preparing a series of samples containing a fixed amount of As02-, and PO4+ (1% sodium tartrate to overcome the interference from Si032-)and variable concentrations of cationic and anionic species as shown in Table VII. The errors made (Table VIII) are similar to those found in Table VI for samples containing no foreign species. The determination of the ternary mixture of these anions in real samples has been carried out on five different natural waters from five towns and villages in the province of Cordoba which contained none of these anions and to which two different amounts of AsOz-, AsO~~-, and PO:(1% sodium tartrate) were added. The percent recoveries found are shown
in Table IX. These recoveries are between 95 and 103%, which shows the validity of the suggested method.
LITERATURE CITED Florence, T. M. Talanta 1982, 2 9 , 345-364. Pacey, G. E.; Bubnls, B. F. Int. Lab. 1984, 2 5 , 26-32. Florence, T. M.; Batley, G. E. CRC Crlt. Rev. Anal. Chem. 1980, 9 , 219. “Standard Method for the Examination of Water and Wastewater”, 4th ed.; American Public Health Association, American Water Works Associatlon and Water Pollution Control Federation: New York, 1975; p 192. Lemmo, N. V.; Faust, S. D.; Belton, T.; Tucker, R. J. Envlron. Sci. Health, Part A 1983, 18, 335-387. Toalev, D.; Petrov, J. Dokl. Bo&. Akad. Nauk 1981, 34, 1413-1416. Austenfeld, F. A.; Berghoff, R. L. Plant Soil 1982, 64, 267-271. Subramanian, K. S.; Leung, P. C.; Meranger, J. C. Int. J. Environ. Anal. Chem. 1982, 11. 121-130. Muenz, H.; Lorenzen, W. Z.Anal. Chem. 1984, 319. 395-398. Sugawara, K.; Kanamori, S. Bull Chem. SOC. Jpn. 1964, 37, 1358-1363. Shida, J.; Kaklzakl, S.; Horumi, Y.; Itoh, A.; Matsuo, T. S. Bull. Chem. SOC.Jpn. 1983, 56, 633-634. Morosanova, S. A.; Rozhmanova, N. B. Zh. Anal. Khim. 1981, 36, 1541-1 545. Johnson, D. L. Sci. Techno/. 1971, 5 , 411-414. Stauffer, R. S. Anal. Chem. 1983, 5 5 , 1205-1210. Pinaev, G. F.; Gornostaeva, L. V. Zh. Anal. Khlm. 1982, 37, 364-366. Johnson, D. L.; Pllson, M. E. 0. Anal. Chlm. Acta 1972, 5 6 , 289-299. Valc6rce1, M.; Luque de Castro, M. D. “Flow Injection Analysis: Principles and Appllcatlons”; Ellis Horwood: Chlchester, In press. Luque de Castro, M. D.; Valcircel. M. Analyst (London) 1984, 109, 413-419. Fogg, A. G.; Bsebsu, N. K. Analyst (London) 1981, 106, 1288-1295. Rels, 9. F.; Zagatto, E. A. G.; Jacintho, A. 0.; Krug, F. J.; Bergamin, F. H. Anal. Chim. Acta 1980, 179, 305. Van Staden, J. F. J. Assoc. Off. Anal. Chem. 1983, 66, 718. Hiral, Y.; Yoza, N.; Ohashi, Sh. Chem. Lett. 1980, 499. Johnson, K. S.; Petty, R. L. Anal. Chem. 1982, 5 4 , 1185. Hlrai, Y.; Yoza, N.; Ohashl, S. Bunsekl Kagaku 1981, 30,465. Hlral, Y.; Yoza, N.; Ohashi, S. Anal. Chlm. Acta 1980, 115, 269. Kuroda, R.; Ida, I.;Oguma, K. Mlkrochim. Acta 1984, I , 377-383.
RECEIVED for review April 9, 1985. Accepted July 19, 1985.
Determination of Trace-Level Chromium(V1) in the Presence of Chromium(I11) and Iron(III) by Flow Injection Amperometry Kenneth W. Pratt* and William F. Koch
Center for Analytical Chemistry, National Bureau of Standards, Gaithersburg, Maryland 20899
Chromlum(V1) Is determined by flow InJectionamperometry at Au and Iodized Pd electrodes without prlor chromatographic or other separation. Dissolved 0, and Cr( I I I ) do not Interfere. Use of H,PO, as the supportlng electrolyte suppresses the interference f r m Fe( I I I). Chloride Ion Interferes in the detemlnatlon at Au electrodes but not at Pd electrodes. Decay In sensltlvlty of the electrodes wlth tlme has been eliminated by contlnuous precondltionlng of the electrode wlth a pulsed-potentlal wave form In place of constant-potentlal amperometry. The detectlon limit for Cr(V1) is 5 ng/mL.
The toxicity of Cr depends on its oxidation state, Cr(V1) being significantly more toxic than Cr(II1) (1-3). Hence, oxidation-state-specific determinations of Cr are of particular interest. Element-specific techniques, such as atomic absorption spectrometry, require a preliminary chemical separation of Cr(V1) from Cr(II1) for the selective determination
of Cr(V1). This separation is generally achieved by liquidliquid extraction (4,5) or ion exchange (6-9), requiring additional sample preparation prior to the actual determination. Amperometric (electrochemical) determination of Cr(V1) inherently discriminates against Cr(II1) without preliminary chemical separation. However, amperometric techniques are not element-specific, and other species that are reduced at the potential used for reduction of Cr(V1) interfere with its determination. One particularly significant interference in environmentalsamples is Fe(II1). Previous workers have used polymer-modified electrodes (10,11) or liquid chromatographic separation (12) to eliminate the interference of Fe(II1) in amperometric determinations of Cr(V1). Here we report an alternative procedure: the use of H3P04as the supporting electrolyte for the trace-level amperometric determination of Cr(VI), at Au or Pd electrodes. This procedure suppresses the interference from Fe(II1) since the complex species Fe(P04)36and Fe(HP04)33-(13-15) are formed and are not reduced at the potential used for the amperometric detection
This article not subject to U S . Copyright. Published 1985 by the American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 58, NO. 1, JANUARY 1986
of Cr(V1). Hence, prior separation of Cr(V1) from Fe(II1) is not required. Direct determination of Cr(V1) by flow injection analysis (FIA) with high sample throughput is feasible, due to the use of a solid working electrode and the absence of a preliminary separation of Cr(VI). The technique is not subject to interference from dissolved oxygen, and hence, deaeration of the supporting electrolyte is not required. Supporting electrolytes that complex Fe(II1) have been used previously in polarographic determinations to suppress interferences from Fe(II1). Verbeck et al. (16) and Cox et al. (17)used NaF as the supporting electrolyte in determinations of U(V1) and Cr(VI), respectively. Tartrate at pH 9 and thiocyanate, both of which complex Fe(III), have also been recommended (18). Recently, Harzdorf and Janser (19)used a phosphate buffer for the polarographic determination of Cr(V1) at pH 10-12. Interfering cationic species were separated by coprecipitation with added Al(III), rather than by complexation. The present work extends this concept to the determination of Cr(V1) a t solid electrodes. The choice of H3P04as the complexant for Fe(II1) is dictated by the low pH value, 0-1, required for the optimum reduction of Cr(V1) a t Au (201, Pt (12),and Pd electrodes. Other possible ligands for Fe(III), such as fluoride, tartrate, and EDTA, exist as the protonated forms at this pH and do not complex Fe(II1). In addition, Cr(VI) oxidizes many potential organic ligands in acid solution. H3P04 complexes Fe(II1) in acid solution (13-15) and is not oxidizable. EXPERIMENTAL SECTION Apparatus. Determinations of Cr(V1)were performed with a flow-injection system (21) using Au and Pd vibrating wire electrodes (VWEs). The system functioned by gravity flow at a volume flow rate of 0.6 mL/min. The volume of injected sample was 0.439 mL, and the volume of the flow-through cell for the VWE was 60 pL. Vibrational parameters for the VWEs are noted in the figures. Preliminary investigations of the voltammetric behavior of Cr(V1) and Fe(II1) were conducted with the VWEs in a 20-mL electrochemical “batch” cell of standard design. Experiments were performed with the VWE in a three-electrode configuration using a commercial potentiostat (Model 174A, Princeton Applied Research Co., Princeton, NJ). Cyclic voltammograms were recorded with this instrument by connecting a triangular wave-form generator to the external sweep input of the potentiostat. All electrode potentials were measured and reported in volts vs. the saturated calomel reference electrode (SCE). The counter electrode was a Pt wire for the Pd VWE and a Au wire for the Au VWE. The Pd electrode,prior to use, was immersed under open-circuit conditions in a 0.1 mol/L KI solution and then rinsed with water. Iodine was adsorbed at the Pd electrode in this pretreatment in a manner analogous to that observed at Pt electrodes (12). Previousstudies (22,23)have establishedthat the adsorbed species at Pt electrodes is neutral I atoms. Amperometricmeasurements using continuous preconditioning of the electrode were obtained with the PAR 174A in the normal pulse mode. This use of continuous preconditioning of the electrode, followed by sampling measurements of the electrode current after a fixed delay, is a simplified version of triple-pulse amperometry (24),in which the negative conditioning and measurement potentials are the same. The instrument was modified for a longer pulse time (233 ms in place of 57 ms) by changing the 0.5-pF capacitor (C214) in monostable I1 of this signal processing board (25) to a 2.7-pF unit. The overall period of the pulse wave form was reduced from 500 to 250 ms by replacing the 100-kQ resistor (R66) in the clock circuit with a 50-kQunit. The resulting pulse wave form consisted of a 17-ms pulse at the preconditioning potential, E,, followed by a 216-ms delay at the measurement potential, E,, and a 17-msperiod, also at E,, for sampling of the electrode current. The succeeding cycle began, with the next preconditioningpulse, directly after the conclusion of the sampling interval. The increased delay between application of the pulse potential and sampling of the electrode current permitted the
125
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E (V vs. SCE)
Flgure 1. Voltammetrlc response to Cr(V1) and Fe(II1) in 0.5 moVL H 2 S 0 4 (Au VWE, 400 Hz, 0.3 mm,-,): -e, H2S0., alone; ---, 0.20 mmol/L Cr(V1); -, 0.20 mmol/L Cr(V1) 0.5 mmol/L Fe(II1).
+
capacitive charging and surface faradaic current to decay to a negligible value prior to measurement of the electrode current. Halving the period of the pulse wave form doubled the number of data points collected per unit time. Current-voltage and current-time curves were obtained on an X-Y recorder and a strip chart recorder, respectively. Linearity for each instrument was f0.1%, with full-scale response time of less than 0.5 s. A digital voltmeter with =kO.l% accuracy was used for all voltage measurements. All solutions were prepared from reagent grade chemicals and doubly distilled, deionized water. Solutions were not deaerated, except as noted. Chemicals were used as received, without further purification. Stock solutions (0.01-0.02 mol/L) of Cr(VI),Cr(III), and Fe(II1) were prepared from KZCrzO7,CrC13.6Hz0, and NH4Fe(S04)z.12Hz0, respectively. The stock solution of Fe(II1) was prepared by direct dissolution in two media: 0.5 mol/L HN03, for use in determination at the Au VWE where C1- would not be present, and 1.8 mol/L H3PO4 + 0.2 mol/L HC1, for determinations at the Pd VWE. This procedure prevented formation of insoluble Fe2(S04)3 and consequent incomplete dissolution of the NH4Fe(S04)z.12Hz0. RESULTS AND DISCUSSION Voltammetry at Au Electrodes. Experimental voltammograms are shown in Figures 1and 2 for Cr(V1) and Fe(II1) a t a Au VWE in HzS04 and H3P04,respectively. In H2S04 (Figure 1)a wave corresponding to the reduction of Cr(V1) is evident, with a half-wave potential ( E l j z )of +0.62 V. Cr(V1) is reduced a t a mass-transport limited rate at potentials negative of +0.5 V. Fe(II1) is reduced at potentials negative of +0.5 V ( E l p = +0.33 V) and thus interferes with the determination of Cr(V1) in H2SO~.In H3P04 (Figure 2), E1j2 for the reduction of Cr(V1) is +0.63 V, and Cr(V1) is again reduced at a mass-transport limited rate at potentials negative of +0.5 V. However, the reduction of Fe(II1) in H3P04does not occur at potentials positive of +0.25 V. Hence, the amperometric determination of Cr(V1) without interference from Fe(II1) is feasible a t potentials from +0.5 to +0.3 V at Au electrodes in 2 mol/L H3P04. The voltammetric features evident in both Figures 1 and 2 at potentids positive of +0.7 V are related to the oxidation and reduction of the Au surface (26). This surface oxidation and reduction of Au occur at similar potentials in 0.5 mol/L HzS04 and 2.0 mol/L H3P04. The rise in current at potentials negative of +0.1 V is due to the reduction of dissolved O2 in the supporting electrolyte. The presence of O2 thus does not
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ANALYTICAL CHEMISTRY, VOL. 58,
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140 120 100 80 I OlA)
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E (V vs. SCE) Figure 2. Voltammetric response to Cr(V1) and Fe(II1) in 2.0 mol/L H,PO, (Au VWE, 400 Hz, 0.3 mm,-,): --, H3P0, alone: ---, 0.20 mmoi/L Cr(V1); -, 0.20 mmol/L Cr(V1) 0.50 mmoi/L Fe(II1).
+
interfere with the amperometric determination of Cr(V1) at potentials from +0.3 to +0.5 V. Voltammograms obtained in deaerated 2.0 mol/L H3P04 indicated an E l p value of +0.09 V for the reduction of Fe(II1). This negative shift of 240 mV for the reduction of Fe(II1) results from the formation of the Fe(II1)-phosphate complex in H3P04, as noted above. Amperometry at Au Electrodes. Initial results for the amperometric determination of Cr(V1) by flow injection analysis, using a constant applied potential of +0.4 V, showed an unacceptable downward drift in the sensitivity of the VWE. Peak heights for five successive injections of 0.4 mmol/L Cr(V1) decreased ca. 10% from the first to the last peak. Substitution of the wave form described in the Experimental Section for the constant applied potential alleviated this problem. Peak heights obtained at the Au VWE using a preconditioning potential, E,, of +1.3 V and a measurement potential, E,,,, of +0.4 V were constant within 1% . Continuous preconditioning of the VWE in this manner only affected the overall sensitivity of the VWE, without altering the relative behavior of Cr(V1) and Fe(II1). The decrease in sensitivity for the reduction of Cr(V1) at constant potential results from fouling of the Au electrode by CrOOH or Crz03 (10, 27). Presumably, this adsorbed Cr(II1) species is desorbed from the electrode during the preconditioning pulse, E, = +1.3 V, concurrently with the oxidation of the Au surface at this potential. Results are shown in Figure 3 for the determination of Cr(V1) in 2.0 mol/L H3P04at the Au VWE using continuous preconditioning as described above. Routine determinations of Cr(V1) at the 100 ng/mL level are readily performed, with detection limits for Cr(V1) on the order of 5 ng/mL for a 2:l signal-to-noise ratio. Also shown in Figure 3 is the response of the Au VWE to 56 pg/mL Fe(II1) in 2.0 mol/L H3P04. The response of the Au VWE to Fe(II1) at this level is equivalent to the response to Cr(V1) at the detection limit of 5 ng/mL, indicating virtual immunity to the presence of Fe(1II). The reproducibility of response to four repeated injections of Cr(V1) was f0.5% at a level of 10 pg/mL and f 2 % at a level of 100 ng/mL. The most serious interference in the determination of Cr(VI) at the Au VWE was C1-. Voltammetric studies indicated that C1- at a level of 5 mmol/L shifted the value of Ellzfor the reduction of Cr(V1) from +0.62 to +0.40 V. At a C1-
t
Inject
Inject
Flgure 3. Amperometric response of Au VWE in FIA to injected Cr(V1) and Fe(I 11) in 2.0 mol/L H,PO, periodic preconditioning: E, = 1.3 V, E m = +0.4 V, 400 Hz, 0.3 mm,-,; (A) 104 ng/mL Cr(V1) and (B) 56 pg/mL Fe(II1).
+
concentration of 0.2 mol/L, Ellzfor the reduction of Cr(V1) was +0.25 V. This negative shift precludes the determination of Cr(V1) at Au electrodes in the presence of both Fe(II1) and C1-, since interference of Fe(II1) occurs at the more negative potential required for reduction of Cr(V1) in the presence of C1-. An additional effect noted in the presence of C1- was an apparent cathodic response of the Au VWE to C1-, when continuous preconditioningwas used. This cathodic response was attributed to the reduction of AuC14-ions formed during the anodic preconditioning pulse. Voltammetry at Pd Electrodes. Voltammetric studies conducted with Pd VWEs with adsorbed I indicated that the effect of C1- on the reduction of Cr(V1) was significantly less than that noted at Au or Pt VWEs. At the Pd VWE, the Ell2 value for the reduction of Cr(V1) in 2.0 mol/L H3P04was +0.69 V. In 1.8 mol/L H3P04 0.1 mol/L HC1, the reduction of Cr(V1) reached a mass-transport limited value at +0.35 V. Determination of Ellz at the Pd VWE in the presence of C1was not possible, due to an anodic contributionto the electrode current at potentials positive of +0.35 V. This anodic partial current resulted from oxidation of the Pd electrode to form PdC142-and was not observed at potentials negative of +0.3 V. Determination of trace-level Cr(V1) at the Pd VWE was thus possible at potentials negative of +0.3 V without a significant anodic background current. The effects and behavior of adsorbed I on Pd were similar to those previously noted at Pt electrodes (12). In the presence of the adsorbed I, oxidation and reduction of the Pd surface was inhibited at potentials negative of +0.8 V. Dissolved O2 was not reduced at the electrode at potentials positive of +0.1 V, and the double-layer charging current was reduced by a factor of at least 3. Adsorbed I was not desorbed between 0.0 and +0.8 V in 0.5 mol/L HzS04 or 2.0 mol/L H3P04or between 0.0 and +0.4 V in 1.0 mol/L HC1. Adsorption of I on the Pd VWE shifted the Ellz for reduction of Cr(V1) from +0.54 to +0.69 V, indicating electrocatalysis of the reduction of Cr(V1) by the adsorbed I. The results of the voltammetricstudies indicate that Cr(V1) can be determined at a potential of +0.3 V using a Pd electrode with adsorbed I. Dissolved 02,Fe(III), and C1- do not interfere in the determination of Cr(V1) under these conditions. Amperometry at Pd Electrodes. Results are shown in Figure 4 for the determination of 100 ng/mL Cr(V1) in 1.8 mol/L + 0.2 mol/L HCl at the Pd VWE with adsorbed I. Also shown are the responses of the electrode to 100 ng/mL Cr(V1) + 56 pg/mL Fe(II1) and to 56 pg/mL Fe(III), in the absence of Cr(V1). These results were obtained by using continuous preconditioning as described above. Values of E,
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ANALYTICAL CHEMISTRY, VOL. 58, NO. 1, JANUARY 1986 A
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Flgure 4. Amperometric response of W VWE in FIA to injected Cr(V1) and Fe(II1)in 1.8 mol/L H,PO, 0.2 mol/L HCI, perlodic precondltioning: E, = 0.0 V, E, = 4-0.3V, 340 Hz, 0.5 mm& (A) 100 ng/mL Cr(VI), (B) 100 ng/mL Cr(V1) + 56 pg/mL Fe(III), (C) 56 pg/mL Fe(I1I).
+
Vibrating A
A
t 20nA
Flgure 5. Effect of large excesses of Cr(II1) and Fe(II1) on the amperometric response of Pd VWE to trace-level Cr(V1). Condklons are as in Figure 4: (A) 100 ng/mL Cr(VI), (B) 100 ng/mL Cr(V1)+ 105 pg/mL Cr(III), (C) 100 ng/mL Cr(V1) 4- 105 pg/mL Cr(II1) 4- 56 pg/mL Fe(II1).
and E,,, were 0.0 and +0.3 V, respectively. The presence of a 560-fold excess of Fe(II1) increases the apparent peak height for Cr(V1) by only 7%. This compares to a 4% increase obtained for 10-fold excess of Fe(II1) at the dropping Hg electrode in NaF supporting electrolyte (16). Response to solution (C) containingFe(II1) in the absence of Cr(VI) is 22% of the response to the Cr(V1) reference solution. The slight response to Fe(II1) reflects the more negative potential used at the Pd VWE for the determination of Cr(V1) in the presence of C1-. This potential, +0.3 V, is located at the foot of the wave for reduction of Fe(II1) in H3P04+ HCl at the Pd electrode. The reproducibility of response to repeated injections of Cr(VI), as seen in Figure 4,is approximately f 2 % and is unaffected by the presence of Fe(II1) in the injected samples. With continuous preconditioning, the sensitivity of the Pd VWE with adsorbed I decreases by less than 10% per hour. This contrasts with a 10% decrease in 2 min noted for constant-potential amperometry at +0.3 V. Figure 5 illustrates the immunity of the direct determination of Cr(V1) to interference from Cr(II1) and from Cr(II1) + Fe(III), both present in large excess. Results are shown for the determination of 100 ng/mL Cr(V1) (solution A), 100 ng/mL Cr(V1) + 105 pg/mL Cr(II1) (solution B), and 100 ng/mL Cr(V1) + 105 pg/mL Cr(II1) 56 pg/mL Fe(JI1) (solution C). Conditions for the determination are the same as those used in Figure 4. The presence of a 1050-fold excess of Cr(II1) has no significaQt effect on the response of the Pd electrode to Cr(V1); peak heights obtained for solution B are within the 2% range obtained for Cr(V1) alone. The simultaneous presence of a 1050-foldexcess of Cr(II1) and a 560-fold excess of Fe(II1) increases the apparent response to Cr(V1) by 2% to 4%. Also illustrated in Figure 5 is the increase in sensitivity resulting from the use of the VWE. Vibration of the electrode
+
127
increases the sensitivity for the determination of Cr(V1) by a factor of 10. The detection limit for Cr(V1) at the Pd VWE is the same as that obtained at the Au VWE, 5 ng/mL for a signal-to-noise ratio of 2. The distortion of the peak obtained for solution C at the stationary electrode is a result of the preconditioning wave form. During the preconditioning pulse (E = 0.0 V), Fe(II1) is reduced to Fe(I1) at the Pd electrode. This Fe(I1) reacts with Cr(V1) in the diffusion layer surrounding the electrode, reducing the concentration of Cr(V1) and decreasing the current during the sampling interval. At the VWE, electrogenerated Fe(I1) is rapidly removed from the vicinity of the electrode by the vigorous convection induced by the vibration of the electrode. Hence, this effect is virtually absent at the VWE. In conclusion, Cr(V1) may be determined amperometrically at trace levels in the presence of excess Fe(II1) by using H3P04 as the supporting electrolyte. Interference from a 500-fold excess of Fe(II1) is less than 2% at a potential of +0.4 V at Au electrodes and 7% at a potential of +0.3 V at Pd electrodes. Chloride ion interferes in the determination at Au electrodes but not at Pd electrodes. Decay in the sensitivity of the electrodes with time is virtually eliminated by continuous preconditioning of the electrode. Registry No. Cr, 7440-47-3;Fe, 7439-89-6; H,P04, 7664-38-2; Au, 7440-57-5; Pd, 7440-05-3;iodine, 7553-56-2.
LITERATURE CITED (1) Mertz, W. Phys. Rev. 1969, 49, 163. (2) Moore, J. W.; Ramamoorthy, S. "Heavy Metals In Natural Waters"; Springer-Verlag: New York, 1984; pp 58-76. (3) Fanot. S. D.; Aly, 0.M. "Chemistry of Natural Waters"; Ann Arbor Science Pub.: Ann Arbor, MI, 1981; pp 376-393. (4) Gllbert, T. R.; Clay, A. R. Anal. Chlm. Acta 1973, 67,289. (5) Hiiro, K.; Owa, T.; Takaoka, M.; Tanaka, T.; Kawahara, A. Bunseki Kagaku 1976, 25, 122. (6) Cresser, M. S.; Hargitt, R. Anal. Chlm. Acta 1976, 87,196. (7) Pankow, J. F.; Janauer, G. E. Anal. Chim. Acta 1974, 69,97. (6) Isozakl, A.; Kumagal, K.; Utsumi, S. Anal. Chim. Acta 1983, 753,15. (9) Miyazaki, A.; Barnes, R. M. Anal. Chem. 1981, 53,364. (10) Cox, J. A.; Kulesza, P. J. J . Electroanal. Chem. 1983, 759,337. ( 1 1 ) Cox, J. A.; Kulesza, P. J. Anal. Chim. Acta 1983, 754, 71. (12) Larochelle, J. H.; Johnson, D. C. Anal. Chem. 1978, 50,240. (13) Nicholls, D. "Comprehensive Inorganic Chemistry"; Bailar, J. C., Eme-
(17)
leus, H. J., Nyholm, R., Trotman-Dickerson, A. F., Eds.; Pergamon Press: Oxford, 1973; Vol. 3, p 1047. Salmon, J. E. J . Chem. SOC. 1952, 2316. Salmon, J. E. J . Chem. SOC. 1953, 2644. Verbeck, A. A.; Moelwyn-Hughes, J. T.; Verdier, E. T. Anal. Chim. Acta 1960, 22, 570. Cox, J. A.; West, J. L.; Kulesza, P. J. Anakst (London) 1984, 709,
(19) (20) (21) (22) (23) (24)
brief C-2. Harzdorf, C.; Janser, G. Anal. Chim. Acta 1984, 765,201. Lindstrom, T. R.; Johnson, D. C. Anal. Chem. 1981, 5 3 , 1855. Pratt, K. W.; Johnson, D. C. Anal. Chim. Acta 1983, 748, 87. Lane, R. F.; Hubbard, A. T. J . Phys. Chem. 1975, 79,808. Johnson, D. C. J . Nectrochem. SOC. 1972, 779, 331. Hughes, S.; Meschi, P. L.; Johnson, D. C. Anal. Chim. Acta 1981,
(14) (15) (16)
927. (18) Princeton Applied Research Corp., Princeton, NJ, 1976, application
132, 1. (25) Operation and Service Manual, Model 174A Polarographic Analyzer, Princeton Applied Research Corp., Princeton, NJ, 1979: pp V111-7+ VIII-13. (26) Belanger, G.; Vijh, A. K. In "Oxides and Oxide Films"; Vijh, A. K., Ed.; Marcel Dekker: New York, 1977; Vol. V, Chapter 1 . (27) Heumann, T.; Panesar, H. S. J . Electrochem. SOC. 1963, 770,628.
RECEIVED for review June 13,1985. Accepted August 19,1985. Certain commercial equipment, instruments, or materials are identified in this report to specify adequately the experimental procedure. Such identification does not imply recommendation or endorsement by the National Bureau of Standards, nor does it imply that the materials or equipment identified are necessarily the best available for the purpose. This work was presented in part at the 188th National Meeting of the American Chemical Society, Philadelphia, PA, Aug 1984.
