1131
Anal. Chem. 1084,56,1131-1 135
LITERATURE CITED
A '13~
[ppml
20
10
0 0
02
04
06 MOLES Pb(N03)2 / MOLES LIGAND
Figure 6. Chemical shifts of the carbonyl carbon atoms and of the CH,CO protons of ligand 1 induced by the addition of Pb(NO& in CD,CN. The dashed lines represent values measured on a saturated solution.
amounts of Pb(N0J2 are added to a fixed amount of 1 in CD,CN, rather large 13Cand lH chemical shift changes of the ligand are induced up t o a molar ratio of near 1:l (Figure 6). The initial slopes of the curves in Figure 6 indicate, however, that the complexes formed are not exclusively a 1:1 stoichiometry. memA study has shown that brane electrodes of the type described here are attractive for the end-point detection in the titration of sulfate with aqueous solutions of lead ions. Registry No. 1,43133-06-8; 2,72469-41-1; 3,58726-79-4; 4, 72469-42-2; 5, 74267-27-9; 6, 89322-23-6; pbC1+, 19511-77-4; PbN03+, 12311-63-6; PbOH+, 12168-64-8; Pb(CHSCOO)+, 72954-17-7; Pb2+,14280-50-3; Pb, 7439-92-1.
(1) Petrgnek, J.; Ryba, 0. Anal. Chim. Acta 1074, 72, 375-380. (2) Masclni, M.;Pallozzi, F. Anal. Chim. Acta 1074, 73, 375-362. (3) Tamura, H.; Kimura, K.; Shono, T. J . Electroarlal. Chem. Interfacial Electrochem. 1080, 115, 115-121. (4) Fung, K. W.; Wong, K. H. J . Necfroanal. Chem. Interfaclal Electrochem . 1980, 11 1 , 359-368. (5) Ammann, D.; Morf, W. E.; Anker, P.; Meler, P. C.; Pretsch, E.: Simon, W. Ion-Sel. Electrode Rev. 1083, 5 , 3-92. (6) Senkyr, J.; Ammann, D.; Meier, P. C.; Morf, W. E.: Pretsch, E.: Simon, W. Anal. Chem. 1979, 51, 786-790. (7) Bertrand, P. A.; Choppin, G. R.; Rao, L. F.; Bunzii, J. G. Anal. Chem. 1083, 55, 364-367. (8) Schneider, J. K.; Hofstetter, P.; Pretsch, E.; Ammann, D.; Simon, W. Helv. Chlm. Acta t080, 63, 217-224. (9) Christensen, J. J.; Hill, J. 0.; Izatt, R. M. Science 1071, 174, 459-467. (IO) Oesch, U.; Ammann. D.; Pretsch, E.; Simon, W. Helv. Chim. Acta 1070, 62, 2073-2078. (11) Ammann, D.; Bisslg, R.; Guggi, M.; Pretsch, E.; Simon, W.; Borowitz, 1. J.; Weiss, L. Helv. Chlm. Acta 1075, 58, 1535-1548. (12) Pretsch, E.; Ammann, D.; Osswald, H. F.; Guggi, M.; Simon, W. Helv. Chlm. Acta 1080, 63, 191-196. (13) Kirsch, N. N. L.; Funck, R. J. J.; Pretsch, E.; Simon, W. Helv. Chim. Acta 1077, 60, 2326-2333. (14) Erne, D.; Morf, W. E.; Arvanitls, S.; Clmerman, 2.; Ammann, D.; Simon, W. He&. Chlm. Acta 1070, 62, 994-1006. (15) Sili6n, L. Q.; Marteii, A. E. "Stability Constants of Metal-Ion Complexes"; The Chemical Society, Burlington House: London, 1964; Speclai Publication No. 17. (16) Siil6n, L. G.; Marteii, A. E. "Stablilty Constants of Metal-Ion Complexes Supplement No 1"; The Chemical Society, Burilngton House: London, 1971; Speck1 Publication No. 25. (17) Guiibauit, 0. G.; Durst, R. A.; Frant, M. S.; Freiser, H.; Hansen, E. H.; Light, T. S.; Pungor, E.; Rechnitz, G.; Rice, N. M.; Rohm, T. J.; Simon, W.; Thomas, J. D. R. Pure Appl. Chem. 1078. 46, 127. (18) Morf, W. E. "The Principles of Ion-Selective Electrodes and of Membrane Transport. Studies In Analytical Chemistry 2"; Akad6mlal KiadB: Budapest (Elsevier: Amsterdam, New York), 198 1.
RECEIVED for review December 12,1983. Accepted February 1, 1984. This work was partly supported by the Schweizerischer Nationalfonds zur Forderung der wissenschaftlichen Forschung.
Differential Pulse Anodic Stripping Voltammetry of Cadmium(I I) with a Rotating Membrane-Covered Mercury Film Electrode Edward E. Stewart and Ronald B. Smart*
Department of Chemistry, West Virginia University, Morgantown, West Virginia 26506-6045
A rotatlng membrane-covered mercury fllm electrode (MCMFE) was constructed by placlng a dlalysls membrane over a glassy carbon rotatlng disk electrode and plating a thin mercury fllm onto the electrode surface through the membrane. Dlfferentlal pulse anodlc strlpplng voltammetry of cadmhm was used to evaluate the effects of pH, rotatlon rate, deposltlon time, and concentration on the MCMFE. The response was llnear from 4.0 X 10-o M Cd2+ to 1.07 X 10" M Cd2+ wlth a standard devlatlon of f9.80 X 10-lo M Cd2+for a 1.78 X 10" M Cd2+solutlon (RSD & l l . l % ) and a standard devlatlon of f6.44 X lo-' M Cd2+ for a 1.78 X lo-' M CdZ+ solution (RSD &3.64%). The llmlt of detection was estlmated to be 8.6 X 10-l' M. These results compared favorably to the bare mercury fllm electrode (MFE) for llnear scan anodlc stripping voltammetry.
It is now a well accepted fact that trace metals play a very important role in the ecology of aquatic organisms. Gaining an 0003-2700/84/0356-1131$01.50/0
adequate understanding of thisrole requires that the geochemical transport and biological interactions of trace metala be thoroughly investigated. However, in light of recent research it is apparent that a knowledge of total metal concentration is insufficient. Nielsen and Wium-Anderson ( I ) inferred that even though deep-sea water is rich in nutrients it is unsuitable for the growth of phytoplankton because it had a higher ratio of ionic to organically bound copper than surface waters. Black (2) also found that ionic copper was more toxic to aquatic organisms than organically bound copper. Furthermore, he concluded that the most stable copper complexes were the least toxic. Other authors (3-7) have also concluded that strongly bound metals are less toxic than uncomplexed metal. Electrochemical methods offer promise in solving the problems associated with chemical speciation. Differential pulse polarography (DPP), DC polarography, and ion-selective electrodes (ISE), while being quick, inexpensive, and nondestructive, seem to lack the required sensitivity. Because of its inherent sensitivity and nondestructive nature, anodic stripping voltammetry (ASV) appears to be the method of choice. One serious problem with voltammetry at mercury surfaces is the adsorption of humic substances and other naturally occurring 0 1984 American Chemlcal Society
1132
ANALYTICAL CHEMISTRY, VOL. 56, NO. 7, JUNE 1984
organic materials. Several investigators have found mercury surfaces especially prone to adsorption by organics (8-11). Buffle et al. (11) found that naturally occurring fulvic acids are adsorbed in the anionic form at mercury surfaces even at low pH. With the use of several polarographic techniques, they emphasized that adsorption of organics may cause shifts in peak potential and decreases in limiting or peak current when applied to natural water systems. They recommend (1)separation of the complexing agenh from the sample prior to analysis, (2) better understanding of the nature of the complexing agents, and (3) development of reproducible voltammetric electrodes that minimize the adsorption effects without loss of sensitivity. A t present, there is no simple solution to the problem of organic adsorption in natural water electrochemical determinations of trace metals. Benes and Steinnes (12) have suggested in situ dialysis as a method of determining the state of trace metals in natural waters. A unique solution to the measurement of ionic metal in natural water, which should eliminate or reduce adsorption problems, is the combination of the dialysis membrane with the mercury film electrode. This combination should give more accurate free metal measurements which could then be used to determine complexing capacity and metal organic conditional stability constants. There have been only a few reports of static membrane-covered mercury electrodes (13-1n and only Schimpf (16)attempted to utilize this combination for trace metal measurements. Two previous studies of rotating membrane-covered electrodes (18, 19) have not determined trace metals. In light of the analytical problems associated with sorption of surface active compounds in ASV, we have constructed a membrane-covered mercury film electrode (MCMFE) and determined optimum experimental conditions. The effects of surface active compounds on the MCMFE have been described elsewhere (20). EXPERIMENTAL SECTION Apparatus. The glassy carbon (GC) electrode (DD20) and rotator (PIR) were manufactured by Pine Instruments, Inc. (Grove City, PA). An EG&G Princeton Applied Research Corp. (PARC, Princeton, NJ) Model 174A polarographic analyzer, a PARC Model 315 automated electroanalysis controller, and a Houston Omnigraphic Model 2000 X-Y recorder or Fisher Series 5000 strip chart recorder were used throughout. A Methrohm (SybronBrinkman, Des Plaines, IL) pH stat was used for all work at pH 6. An Orion (Cambridge, MA) Model 701A digital ion analyzer was used for all other pH measurements. A microprobe combination pH electrode (Fisher Scientific), an Ag/AgCl reference electrode (saturated KCl), a platinum wire counterelectrode, and a 50-mL PARC jacketed polarographic cell were used for all experiments. Reagents and Solutions. The 0.01 M supporting electrolyte was prepared from Reagent Grade KNOB(Fisher P-263). ACS Certified 1000 ppm atomic absorption standard (Fisher SO-(2-118) was diluted daily to prepare Cd2+solutions. KOH passed through Chelex-100 ion exchange resin and Ultrapure HN03 were used to make all pH adjustments. All solutions were prepared with double distilled water. All solutions were deaerated for 10 min with grade 5 ultrapure Nz prior to analysis. Working Electrodes. The GC rotating disk electrode was polished according to Freese (21) and Dube (22) in three steps. After the final polishing, the electrode surface was cleaned with a mild soap solution followed by rinses with dilute HN03 and distilled water. For membrane-covered electrode studies, 3 drops of 0.01 M KNO, were placed on the electrode surface. The membrane was then positioned on the electrode surface and held in place by a silicone rubber "0" ring. Care was taken not to overstretch the membrane. Membranes. The membranes were cut from Spectra/Por 6 cellulose dialysis tubing (Spectrum Medical Industries Inc., Los Angeles, CA) and had a 1000 molecular weight cutoff. The bags were soaked in 70 "C deionized water for 20 min, thoroughly rinsed with deionized water, and soaked in distilled deionized water for 48 h prior to their use. This procedure effectively removed the sodium benzoate preservative. Spectra/Por is most susceptible to attack by cellulytic microorganisms when thoroughly wet. In order to avoid this problem, the tubing was transferred to freshly boiled distilled water daily and fresh tubing was prepared weekly.
Electrode Plating, Preconditioning, and Storage. For all studies without a membrane, the GC electrode was rinsed with and distilled water and then plated at -0.500 V for dilute "03 300 s in 50 mL of deaerated 5.0 X lo4 M Hg(N03)2while rotating at 4900 rpm. Care was taken not to expose the mercury surface to air by keeping a large drop of the plating solution on the electrode surface during transfer to 50 mL of deaerated 0.01 M KNO,. To precondition the new electrode surface, three DPASV scans were run by the procedure outlined below using a 15-s quiet deposition time. The electrode was finally placed in the analyte solution, again allowing a drop of solution to adhere to the surface during transfer. For all studies with a MCMFE, the electrode was placed in 50 mL of deaerated 5.0 X lo-, M Hg(NO& immediately after the membrane was fitted and rotated at 1600 rpm for 10 min to allow membrane equilibration. The electrode was then plated for 480 s at -0.500 V. Preconditioning of the surface followed the same procedure as the MFE. The caution exercised with the MFE after preparation was not necessary with the MCMFE; however, the electrode membrane must never become dry. The MCMFE was stored by placing it in 50 mL of deaerated 0.01 M KNOBand polarizing it at 0.000 V with Nz continuously passed over the solution. MFE's were never stored, as they were easier to prepare and harder to preserve than the MCMFE. Procedure for DPASV. The instrumental settings used for all DPASV measurements were as follows: scan rate, 5 mV s-l; modulation amplitude, 100 mV; pulse repetition time, 0.5 s; conditioning potential, +0.100 V; deposition potential, -1.000 V; and final potential, +0.100 V. The rotation rate of the electrode was 1600 rpm throughout, except when it was the experimental parameter under investigation. A conditioning time of 1.5 min with rotation before and 15 s quiet deposition time after stirred deposition was used throughout. Stripping was done in a quiet solution. Solutions were 25.0 & 0.2 "C.
RESULTS AND DISCUSSION Response Characteristics. Electrode rotation rates (0-8100 rpm) were varied to determine if the amount of cadmium plated during the deposition step was proportional to u1J2as predicted by the Levich equation i(t)dep = 0.62nFAD2J3y-'J6~1/2C
(1)
At the available rotator settings the Levich equation was not followed. A stripping current plateau between 900 and 4900 rpm indicated transport of electroactive species during electrodeposition was limited by diffusion through the membrane. Gough and Leypoldt (19) and Chien et al. (18)observed similar behavior for membrane-covered electrodes. A decrease in i, at rotation rates above 4900 rpm was probable due to solution cavitation during deposition. Stirred deposition time (0-300 s) was varied for different Cd2+concentrations at pH 5.00 f 0.02 to determine if i, was a linear function of deposition time. For 8.90 X lo-' M and 1.78 X M solutions stirred deposition time was never allowed to exceed 50 s, because preliminary studies indicated that changes in peak morphology occurred when the amount of CdO per unit area exceeded a certain value. Attainment of this value was empirically determined when stripping currents exceeded 15 PA. Peak current was clearly a linear function of deposition time. To further characterize the behavior of the MCMFE, the slopes of the i, vs. deposition time experiments were plotted vs. cadmium concentration. The slope of this plot then provided an excellent way to represent the sensitivity of the electrode. The sensitivity was calculated as 3.10 X lo4/.LAM-' s-l 3.86 X lo6 PA M-l, with the second term representing the sensitivity of 15 s quiet deposition. Since this slope was a linear function of concentration, it was reasonable to assume that i, was also a linear function of Cd2+concentration. Calibration curves for Cd2+concentrations of 1.78 X lo-' M to 1.78 X M were prepared at pH 5.00 0.02. The stirred deposition times utilized and recommended for the
-
+
*
ANALYTICAL CHEMISTRY, VOL. 56, NO. 7, JUNE 1984
-730
P I/
-
-710-
>
n
\
W
-690 -
O
-670-
-6 50 "
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
1.0
*. O
/ 0 0
3,O
2.0
4.0
(0) increasing pH
ex-
corresponding Cd2+concentrations are as follows: (1) 300 S, M; (2) 15 s, lo-' M; (3) 0 s, IO4 M. The data all yielded correlation coefficients above 0.9990; however some asymmetry in the stripping peaks was observed above 3.56 X lo4 M Cd2+ at 15 s quiet deposition. Linearity of response was maintained up to 1.07 X M Cd2+,but at higher concentrations i, began to decrease. The limit of detection was estimated according to the method of Florence (23). The average peak current (n = 8) at pH 5.00 f 0.02 and 600 s stirred deposition was 0.62 f 0.048 PA. The concentration in the blanks was determined as 4.0 x IO4 M by standard additions and linear regression analysis. The minimum detectable difference between a mean sample and a mean blank reading (i, - ib) was calculated from
i, - i b = 2Sb1l2
5.0
8.0
7.0
6.0
PH
PH
Flgure 1. Peak current vs. pH for the MFE: periment; (0)decreasing pH experiment.
1133
Figure 2. Peak potential vs. pH for the MFE: (0)increasing pH experiment; (0)decreasing pH experiment.
6 -
4,
.-n 4-
2-
0
(2)
where Sbis the standard deviation of the blank, PA. The limit of detection was then estimated by multiplication of the inverse of the sensitivity (pH 5.00 f 0.02 and 600 s stirred deposition) by the minimum detectable current difference. This estimate was 8.6 X M which compared favorably to that obtained for Pb2+by Florence (23) using a MFE and linear scan ASV. The precision and accuracy of this method were also determined at pH 5.00 f 0.02. Eight samples (1.78 X M Cd2+)were measured by using a 300-s stirred deposition time. The calculated result was 1.73 X M f 0.93 X lo4 M Cd2+ (RSD 11.1%,2.8% error). Nine samples (1.78 X lo-' M Cd2+) were measured with a 15-s stirred deposition time, and the calculated value was 1.77 X lo-' M f 6.44 X lo4 M Cd2+(RSD 3.64%, 0.60% error). Background corrected currents were used for all calculations by subtracting the i, of the blank from all Cd2+anal@ peaks. These values were comparable to those obtained by Florence (23) for Pb2+using linear scan ASV. Effect of pH on MFE. The influence of pH on i, and E, has been investigated for both the MFE and the MCMFE. At the MFE, i, gradually decreased as the pH was lowered from 4.0 to 1.5, which was probable due to an increase in hydrogen evolution. This was also accompanied by an increase in background current between -1.000 V and -0.800 V. Florence, as cited in ref 24, has recommended that Cd2+be analyzed at pH >3 at a MFE to avoid hydrogen evolution. E, was relatively constant between pH 2.0 and 4.8 but shifted to slightly more negative values below pH 2.0. Between pH 4.0 and 4.8 there was a maximum and relatively constant value for .,i The peak current decreased from pH 4.8 to a minimum at pH 6.4 and was accompanied by large negative shifts in E,,, These results are summarized in Figures 1 and 2 and are due to either surface adsorption of some cadmium-hydroxy species or the formation of nonadsorbed electroinactive species at the electrode surface after the stripping step.
1.0
2.0
3.0
4.0
5.0
6.0
7.0
PH
Figure 3. Peak current vs. pH for the MCMFE: (0)increasing pH experiment: (0)decreasing pH experiment.
-710
-650
I
1.0
2.0
3.0
4.0
5.0
6.0
I
7.0
PH
Figure 4. Peak potential vs. pH for the MCMFE: (0)increasing pH experiment; (0)decreasing pH experiment.
Dryssen and Lume (25)showed that CdOH', Cd(OH)y, and Cd(OH)42-could exist in aqueous solution although only in very small amounts, while Biedermann and Ciavatta (26) reported the existence of CdOH+, Cd20H3+,and Cd4(0H)44+ in concentrated cadmium ion solutions. While these species represent only a very small fraction of the cadmium ions in the bulk solution, they could exist in appreciable amounts near the electrode surface. During the stripping step the cadmium ion concentration is very high in this region. If the adsorption of cadmium-hydroxy complex(es) were the cause of the decrease in i, and shift in E,, these complex(es) must be nonreducible or nonlabile during the time between the end of one potential pulse and the application of the next. If they were labile, enhanced DPASV stripping currents would be observed because adsorbed species would make many more repetitive contributions to the measured
1134
ANALYTICAL CHEMISTRY, VOL. 56, NO. 7, JUNE 1984
Potential
Figure 5. Effect of pH
on stripping voltammograms for the MCMFE.
current than would species that could diffuse away from the electrode surface. The formation of nonadsorbed electroinactive cadmium-hydroxy species would also lead to lower observed currents. The increase in i, observed between pH 6.4 and 8.0 was not accompanied by any further change in E,. The onset of formation of a reducible or labile complex at pH 6.4 could account for the gradual increase in .,i Assuming that formation of cadmium-hydroxy complex(es) and/or subsequent adsorption caused large negative shifts in E, and that between pH 6.4 and 8.0 the percentage of labile (possibly) adsorbed complex(es) was increasing while that of the nonlabile complex(es) was decreasing, an increase in i, without further shifts in E, would seem reasonable. The effect of pH on i, and E, was the same whether the pH was approached from higher or lower H+ concentration which indicated any adsorption was chemically reversible. Further indication of the occurrence of adsorbed species was the appearance of a small peak 150 mV more negative than the cadmium ion stripping peak which began to appear between pH 6.5 and 7.0 and gradually increased in height. Other authors (25,27,28) have observed the onset of such a doublet for copper ion and zinc ion solutions at moderate to high pH values. They speculated that these peaks were due to adsorption of metal-hydroxy or metal-carbonate species. Lewin and Rowel1 (29) observed a similar pH dependence for cadmium ion by using linear scan ASV in acetate buffer; however, no increase in i, above 6.4 was observed. This would not be expected even for labile complex formation during stripping using linear scan ASV. Once oxidized, the species would not make the repetitive current contributions it would in DPASV. Zirino and Healy (24), Sinko and Dolezal (28), and Schonberger and Pickering (27),also using buffered solutions, observed a different pH dependence. They found i, to be relatively constant between pH 3.5 and 6.0 with some depression a t higher pH values. Unbuffered solutions were used in our study to avoid possible complexation of Cd2+. The pH at the electrode surface may not have been the same as that of the bulk solution, and different processes, vis-a-vis those in buffered solutions, could have occurred. Effect of pH on MCMFE. The pH dependence of i, at a MCMFE was considerably different than the MFE. Between pH 1.5 and 4.0, i, increased up to pH 6.4 accompanied by steady negative shifts in E,,. In fact, the sensitivity of the MCMFE exceeded that of the MFE above pH 5.6 which was probably due to the formation of some species during stripping which limited diffusion out of the membrane. This would
make the analyte available for many more contributions to the measured current. The pH dependence of i, was again the same whether approached from higher or lower pH, below pH 4.0. A small peak 150 mV more negative than the cadmium stripping peak also appeared between pH 6.5 and 7.0 and gradually increased in height as shown in Figure 5. The initial i, after adjustment above pH 4.0, differed depending on whether the preceding scan had been conducted at higher or lower pH. DPASV (15 s quiet deposition) at pH 7.20 yielded no Cd2+peak on repeated runs if Cd2+was present in the solution and membrane (from preceding scans) and base was added to elevate the pH. However, if a blank was adjusted to pH 7.20 and run, and then Cd2+was added, successive scans yielded increasingly larger i, values. The pH-dependent formation of a species capable of interfering with transport across the membrane must be involved and might be influenced by adsorption-desorption within the membrane. Further support for this was seen in i, vs. pH studies done by decreasing the pH from an initial value of 7.20 as shown in Figure 3. Upon acidification, i, only gradually decreased to a steady value (dotted line) and the approach was a function of pH. At higher pH values, more repetitive scans were required to attain a constant i, and larger deviations between the first few scans at a given pH were observed. Above pH 5.0, the first scan after pH lowering was higher than the preceding steady value. This might be due to the dissociation of some cadmium-hydroxy species which, because transport was still restricted, was not able to rediffuse out of the membrane before the next scan was initiated. The lower the pH the less cadmium-hydroxy species present in the membrane, and consequently the closer the initial scan was to the steady value. Increasing the pH from 5.0 elevated the i, relative to the steady value. Once the pH exceeded 5.75 the response decreased with addition of base. Experimental parameters such as amount of time allowed after a scan for reequilibration and rate of addition of base influenced the response in a general manner. Initially depressed peak currents, upon successive scanning, approached the steady values obtained when pH was decreased. This might also be due to the formation of a cadmium-hydroxy complex or precipitate in the membrane. Constant i, can be obtained as high as pH 6.5. Standard additions at pH 6.00 f 0.01, while yielding accurate and reproducible results for initial Cd2+concentrations, did so only when the third scan after addition of Cd2+was used. Regardless of the direction of pH adjustment, E, was more negative at higher pH. Since transport through the membrane was limited as pH increased, the concentration of cadmium ion at the electrode surface would be greater during stripping at higher pH, and the probability of formation of cadmiumhydroxy complexes would be increased. Assuming the degree of cadmium-hydroxy complex formation and/or subsequent adsorption was concentration dependent, we would expect the negative shift in E,. At present, it is recommended that analysis should not be conducted above pH 6.0. The amount of Cd2+plated into and stripped out of the amalgam appears to be the critical factor with this electrode. We are currently investigating the MCMFE for use in in situ environmental analysis as well as for the determination of metal binding constants of humic substances where adsorption of high molecular weight organic compounds on mercury electrodes could cause serious interferences.
ACKNOWLEDGMENT We acknowledge the work of Robert Shrout, Carl Wise, Robert Victor, and David Gover for their professional expertise in the maintenance and construction of the equipment used in this study.
Anal. Chem. 1984,56, 1135-1137
Registry No. Cadmium, 7440-43-9;mercury, 7439-97-6;carbon,
7440-44-0. LITERATURE CITED (1) Nielsen, E.; Wium-Anderson, W. Mar. Blol. (Berlin) 1970, 6, 93. (2) Black, J. Ph.D. Dissertation, The University of Michigan, Ann Arbor, MI, 1974. (3) Anderson, D. M.; Morel, F. M. Limnol. Oceanogr. 1978, 23,283. (4) Gachter, R. K.: Lum-Shue-Chan; Chou, Y. K. Schweiz. Z. Hydro/. 1973, 35, 252. (5) Ailen, H. E.; Hall, R. H.; Brisbin, T. D. Environ. Sci. Techno/. 1980, 14, 441. (6) Sunda, W.; Guiiiard, R. R. J . Mar. Res. 1978, 3 4 , 511. (7) Andrew, R. W.; Beisinger, K. E.; Glass, G. E. Wafer Res. 1977, 1 1 , 309. (8) Brezonic, P. L.; Brauner, P. A.; Stumm, W. Wafer Res. 1978, 10, 805. (9) Batley, 0. E.;Florence, T. M. J . Electroanal. Chem. 1978, 72, 121. (10) Jacobsen, E.; Lindseth, H. Anal. Chim. Acta 1977, 72, 123. (11) Buffle, J.; Cominoll, A,; Greter, F. L.; Haerdl, W. 4th International SAC Conference Proc. Anal. Div. Chem. SOC. 1978, Vol. 15. (12) Benes, P.; Steinnes, E. Water Res. 1974, 6,947. (13) Bersier, P.; Bersier, J.; Hugli, F. Helv. Chim. Acta 1980, 43, 478. (14) Berge, H.; Kunkel, S. Anal. Chlm. Acta 1971, 54, 221. (15) Bowers, R . C.; Wilson, A. M. J . Am. Chem. SOC. 1958, 80, 2968. (16) Schimpff, W. K. Ph.D. Dissertation, The Universlty of Mlchlgan, Ann Arbor, MI, 1971. (17) Pungor, E.; Feher, 2s.J . Nectroanal. Chem. 1977, 75, 241. (18) Chien. Y. W.; Oisen, C . L.; Sokoioski, T. 0. J . Pharm Scl. 1973, 62, 435.
.
1135
(19) Gough, D. A.; Leypoldt, J. K. Anal. Chern. 1979, 51,439. (20) Stewart, E. E. M.S. Thesis, West Virginia University, Morgantown, WV, 1983. (21) Freese, J. W. M.S. Thesis, West Virginla University, Morgantown, WV, 1981. (22) Dube, G. B. Ph.D. Dissertation, The Pennsylvania State University, State College, PA, 1979. (23) Florence, T. M. J . Elecfroanal. Chem. 1970, 27, 273. (24) Zirlno, A.; Heaiy, M. L. Environ. Scl. Techno/. 1972, 6 , 243. (25) Dryssen, D.; Lumme, P. Acta Chem. Scand. 1982, 16, 1785. (26) Biedermann, 0.; Ciavatta, L. Acta Chem. Scand. 1982, 16, 2221. (27) Schonberger, E. A.; Pickering, W. F. Talanta 1980, 27, 11. (28) Sinko, I.; Dolezal, J. J . Electroanal. Chem. 1970, 25, 299. (29) Lewin, V. H.; Rowell, M. J. Effluenf Wafer Treat. J . 1973, 13, 273.
RECEIVED for review September 12,1983. Accepted February 1, 1984. Although the information described in this article has been funded partially by the US.Environmental Protection Agency under assistance agreement R810540-01-0to R.B.S.,it has not been subjected to the Agency’s required peer and administrative review and therefore does not necessarily reflect the views of the Agency and no official endorsement should be inferred. Acknowledgment is also made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research.
Voltammetric pH Measurements with Surface-Modified Electrodes and a Voltammetric Internal Reference Israel Rubinstein Department of Plastics Research, Weizmann Institute of Science, Rehouot 76100, Israel
It Is demonstrated that by comblnlng surface-modified electrodes with voitammetrlc technlques, it Is possible to construct pH probes conslstlng exclusively of metal wlre electrodes, which are natural candidates for mlnlaturlzatlon. Changes In the pH translate to shlfts in voltammetric peak potentials. Two modes of operation are presented, one that employs a pH-sensitive reference electrode and another that employs a pH-dependent electroactlve specles confined to the worklng electrode. In the latter mode, a second, pH-independent electroactlve species Is also bound to the working electrode, to provide a voltammetric-type reference potential.
Numerous methods have been suggested and used to date for pH measurement, mostly potentiometric (1,2).Of these, the pH-sensitive glass electrode has by far been the most widely used, due to its high selectivity, reliability, wide pH range, and convenience of use. Other types of pH electrodes, e.g., the antimony-antimonous oxide (1,3,4) or other innovative metal oxide electrodes ( 5 , 6 ) ,have also been studied. The need to miniaturize pH probes has been recognized for some time, primarily for biological applications and also in corrosion studies (7,8). p H electrodes for either extra- or intracellular measurements were constructed and tested, both in vitro and in vivo (9-16). Here, again, glass pH microelectrodes are much preferred over any other type. Innovative single-barreled or double-barreled glass electrodes with tip diameter as small as
