Anal. Cham. 1984, 56, 1021-1025
1021
Electrocatalytic Oxidation and Determination of Arsenic(III) a Glassy Carbon Electrode Modified with a Thin Film of Mixed-Valent Ruthenium(III, II) Cyanide
on
James A. Cox* and Pawel J. Kulesza
Department of Chemistry and Biochemistry, Southern Illinois University, Carbondale, Illinois 62901
performed in the presence of catalysts. For example, small amounts of Ru(III), Ru(IV), and Ru(VIII) catalyze the oxidation of As(III) by Ce(IV) (5,6). The oxidation of Ru(CN)64~ is a reversible, one-electron process (7) even when the complex is incorporated into a polymer film on an electrode surface (8). Moreover, a comparison of the formal potentials of the Ru(CN)63"4" and H3As04, H3As03 couples at pH 2 suggests that electrochemically generated Ru(CN)63~ may catalyze the electrochemical oxidation of As(III). Two methods were tested for immobilizing the catalytic couple at the electrode. By use of well-known methods (9-11), Ru(CN)64" was incorporated into a film of an anion-exchange polymer on the electrode substrate. The second method was based on reports by Neff et al. (12-14) and Itaya et al. (15-17) that describe the preparation and characterization of thin films of mixed valent hexacyanocomplexes of transition metals on electrodes. They specifically studied Prussian Blue, a mixed Fe(II)-Fe(III) complex with cyanide, and Ruthenium Purple, a ferric ruthenocyanide. From the reported properties, it appeared that a RulIID-RuiCN^3""’4" system may form a stable film that has the electrochemical characteristics required to catalyze the oxidation of As(III), a hypothesis that is verified herein. Glassy carbon is used as the base electrode over Pt or Au because As (III) can be oxidized at the latter two metals. The processes are highly irreversible and are very dependent upon the condition of the Pt and Au surfaces, probably because of catalysis of the As (III) oxidation by metal oxides (18-21). Because the quantity of the metal oxide is difficult to control, reproducible results at Pt and Au are not readily attained. With glassy carbon, the As (III) oxidation does not occur at the bare substrate, at least before the onset of oxygen evolution at about 1.4 V, so designing reproducible experiments in the presence of the ruthenium catalyst is easier. In addition, the role of the modifying film is more apparent when the electrolysis will not occur at the bare substrate in the accessible potential range.
An electrode prepared by dipping a glassy carbon substrate Into a fresh 2 mM RuCI3, 2 mM K4Ru(CN)e, 0.5 M NaCI solution at pH 2 and cycling the potential between 0.35 and 0.85 V vs. SCE for 25 min can be used for the oxidation of As(III). At the bare glassy carbon electrode As(III) was not oxidized prior to the discharge of the supporting electrolyte. The linear scan voltammetry peak current was directly proportional to the As(III) concentration over the range 2.0-0.005 mM. The effect of scan rate and rotating disk experiments were indicative of a process controlled by diffusion of the analyte In solution. The modified surface was stable for at least 10 weeks when stored In an electrolyte between experiments. Even after air-drying, the modified electrode yielded an oxidation peak current that was Identical with that on a freshly prepared electrode; the peak potential was shifted positive by 20-30 mV.
Redox half-reactions that can be effected in homogeneous solution often either do not proceed electrochemically or occur with high overpotential. This is particularly true when a catalyst is required for the solution reaction. Modification of electrode surfaces provides an opportunity to immobilize catalysts and thereby extend electrochemical methodology to the study of such systems. In some cases a chemical catalyst, which undergoes a rapid electrophilic-nucleophilic reaction with the analyte to form a transient adduct, is immobilized. In other examples, the catalytic species is electrochemically generated and subsequently is involved in a redox reaction with the analyte; that is, the immobilized species is a redox mediator. In reality, the latter mechanisms are likely to also have some aspect of chemical catalysis. Otherwise, redox mediation is perhaps better classified as an induced reaction rather than catalysis. Although several of the reports of electrocatalysis at modified electrodes (1,2) have involved the model in which a redox mediator is immobilized on an electrode surface, practical analytical methods have not been developed with this mechanism. For example Stutts and Wightman (3) prepared surface-bound analogues of species that catalyze the oxidation of ascorbate in homogeneous solution. The resulting modified electrode also catalyzed the reaction; however, the reaction was slow. Ravichandran and Baldwin (4) incorporated N,Njy'JV''-tetramethyl-p-phenylenediamine directly into carbon paste and used the modified electrode to catalyze the oxidations of the reduced form of nicotinamide adenine dinucleotide and of ascorbic acid. The electrode was electrochemically well behaved, but a given surface in contact with solution had limited long term stability. The surface was easily restored, so the approach has promise as the basis for practical analytical methodology. The electrochemical oxidation of As(III) is amenable to investigation by the described model. The oxidation is highly irreversible at bare electrodes but can readily be chemically 0003-2700/84/0356-1021$01.50/0
EXPERIMENTAL SECTION The voltammetric experiments were performed with a PAR Model 170 electrochemistry system or a PAR Model 174A polarographic analyzer. All potentials were measured and reported vs. the saturated calomel reference electrode (SCE). Rotating disk voltammetry was performed with a Pine Instrument Co. (Grove City, PA) rotator. The current-potential curves were typically recorded at a 10 mV s"1 scan rate. All chemicals were ACS Reagent Grade and were used without further purification. The water was distilled and doubly deionized with Cole Parmer research grade cartridges. A standard stock solution of As(III) was prepared by dissolving sodium arsenite in the minimal volume (about 25 mL) of 1 M NaOH, neutralizing the solution with 1 M HC1, and diluting with distilled-deionized water.
A glassy carbon disk electrode from Princeton Applied Research (Princeton, NJ) was used for the rotating disk experiment. The cyclic voltammetry experiments were performed on a 5.5 mm2 epoxy-sealed glassy carbon disk substrate, also from PAR, that was modified as detailed below. Prior to modification, they were ©
1984 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 56, NO. 6, MAY 1984
cleaned by polishing with Fisher Gamal 0.1-gm alumina on a Gamal cloth with distilled water as the lubricant. In general, the electrode modification did not require very careful pretreatment of the glassy carbon substrates. For example, electrochemical conditioning in 1 M H2S04 electrolyte by cycling repetitively through the potential range from 0.2 to 1.1 V for a period of 5-10 min did not change the characteristics of the subsequently modified electrode. Unless otherwise stated, the electrode modification with mixed-valent Ru(III, II) cyanide microcrystalline deposits was accomplished by cycling the potential of a clean substrate between 0.35 and 0.85 V in a freshly prepared, deaerated solution of 2 mM RuCla, 2 mM K4Ru(CN)6, and 0.5 M NaCl at pH 2 for 25 min. The above potential limits should not be exceeded; otherwise, the deposit will contain Ru in oxidation states other than 2+ and 3+. Freshly prepared, deaerated solutions for the modification are strongly recommended. Slow decomposition and/or oxidation takes place in the RuC13, Ru(CN)64' mixture; formation of dark green- or gray-blue gels has been observed. Although stable catalytic surfaces can be obtained even from 4-week old mixtures, the films are thicker and yield significantly increased base line currents. Different thicknesses of the film can be obtained by varying the experimental parameters during the modification step. Longer cycling times in the fresh modification mixture result in thicker
films. The values of charges under voltammetric surface peaks were determined by integration of the current-time curves which result as the electrode potential was scanned well beyond the voltammetric peak potential where the current decreases to background levels (10,11).
E, V vs SCE
Figure Cyclic voltammogram of 2 mM NaAs02 In 0.5 M NaCl at pH 2 for (A) the glassy carbon/PVP, Ru(CN)63~,4“ electrode. Dotted line (B) was obtained in As(III)-free solution. The electrode was conditioned before the measurements were obtained by cycling glassy carbon/PVP for 20 min between 0.9 and 0.4 V at 50 mV s'1 in 2 mM K4Ru(CN)6, 0.5 M NaCl at pH 2. Scan rate for the displayed data was 50 mV s'1; electrode area was 5.5 mm2. 1.
2 >jA
RESULTS AND DISCUSSION means of immobilizing a redox mediator at an electrode surface is to incorporate it into a polymer film. Because of the anionic nature of our proposed mediator, Ru(CN)e3'4', it was preconcentrated from a K4Ru(CN)6 solution into a film of protonated poly(4-vinylpyridine), PVP, or of quaternized poly(4-vinylpyridine), qPVP, on glassy A
common
manner reported for hexacyanoferrates (9-11). Experiments with PVP coatings were performed in 0.5 M NaCl at pH 2. A nearly reversible cyclic voltammogram was obtained (Figure IB); the formal potential was estimated as 0.73 V vs. SCE. With repetitive scanning at 50 mV s'1 the peaks became broad and somewhat lower. The peak potential difference gradually increased. Eventually the electrode became passivated, presumably because of decomposition of the
.9
carbon in the
Ru(CN)63- (7). The electrode initially showed catalytic activity toward the oxidation of As(III). Whereas an oxidation current at a bare glassy carbon electrode in 2 mM As(III) at pH 2 (0.5 M NaCl) was not developed prior to breakdown of the supporting electrolyte, a peak at about 1.0 V was observed with an electrode freshly modified by incorporation of Ru(CN)64' in PVP (Figure 1A). The same results were obtained when the experiments were repeated with a qPVP-coated electrode in a 0.1 M phosphate buffer at pH 4.6 (the benzylated PVP acts as an anion exchange polymer in higher pH solutions than does PVP). Even in pH 10 solution, the oxidation of As(III) at 0.8 V was observed with the Ru(III, Ill-containing, qPVP-coated electrode. In all these cases, the above-mentioned passivation precluded the development of reproducible peaks by a convenient procedure; therefore, catalysis by the metalated polymer was not pursued. The second modification method was to adsorb a film of a mixed Ru(III), Ru(II) cyano complex on glassy carbon. The procedure was not as direct as that reported for Prussian Blue deposition (12,13,15,17). An important difference results from the fact that when RuC13 and K4Ru(CN)6 were mixed, a reaction in the bulk solution was not immediately observed. Only after about 2 h was the formation of a blue-green species apparent. The visible absorption spectrum of the solution
5
.7
E,
V
.3
.1
vs SCE
Steady-state cyclic voltammogram of the mixed-valent on glassy carbon In 0.5 M NaCl at pH 2. Scan rate was 50 mV s'1; geometric area of the electrode was 5.5 Figure 2.
Ru(III, II) cyanide deposit mm2.
comparable to that for Prussian Blue except that the low-frequency charge-transfer band (22) was slightly shifted toward a higher frequency. The same shift was reported when Ru(CN)e4' replaced Fe(CN)64- in Prussian Blue (22). Films that were prepared by dipping glassy carbon into the bluegreen solution gave distorted cyclic voltammograms indicative of a thick, highly resistive deposit. In the third modification procedure the glassy carbon electrode was cycled in a fresh mixture of ruthenium species as described in the Experimental Section. Because the resulting modified electrode gave well-developed cyclic voltammograms, this procedure was used for all subsequent work. Figure 2 shows the cyclic voltammogram in 0.5 M NaCl at pH 2 that was obtained with a glassy carbon electrode that was coated with a mixed-valent Ru(III, II) cyanide, mvRuCN, deposit. By comparison to a voltammogram of K4Ru(CN)6 at bare glassy carbon in the same electrolyte, the peaks at 0.76 V and 0.72 V were attributed to electrolysis of the surfacebound Ru(CN)63'4' couple. Analogously, the peaks at 0.22 V were attributed to the Ru3+,2+ couple. Integration of the peak for the oxidation of Ru(CN)64" yielded a charge of 1.1 µ which approximately corresponds to 2 X 10'10 mol cm'2 or about 2 monolayers. The procedure was reproducible. Consecutive preparations beginning with bare glassy carbon yielded charges of 1.1, 0.9,1.1, and 1.3 µ . The film was much thinner than the reported Prussian Blue deposits on glassy carbon. Since these species have fairly high was
ANALYTICAL CHEMISTRY, VOL. 56, NO. 6, MAY 1984
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Table I. Linear Scan Voltammetry Peak Characteristics for the Catalytic Oxidation of As(III) at Glassy Carbon Coated with a Mixed-Valent Ru(III, II) Cyanide Deposit0
mV
s
5
10
20 50 100 200 500 1000 0
E,
V
vs SCE
Figure 3. Linear scan voltammograms for (A) bare glassy carbon and
(B) the mvRuCN-coated glassy carbon electrode In 2 mM NaAs02, 0.5 M NaCI at pH 2. Curve C is a cyclic voltammogram of the mvRuCNcoated electrode in 0.5 M NaCI at pH 2. Scan rate was 50 mV s'1; geometric area of the electrode was 5.5 mm2.
resistivities (13), a procedure that yielded a very thin film was important for applications to electrocatalysis. The charges under the four peaks shown in Figure 2 are about equal. The peak heights are directly proportional to scan rate up to 500 mV s'1, which is expected for surface processes.
Although the mvRuCN film is similar to Prussian Blue and Ruthenium Purple, several important differences are apparent. First, the deposition is by potential cycling in an essentially unreacted mixture of RuC13 and K4Ru(CN)6. The importance of this step may be the formation of Ru(CN)e3~. This species is somewhat unstable; it decomposes to Ru(CN)3-5H20, a dark green, spáringly soluble salt (7). A species with a 1:3 ratio of Fe:CN has been reported to adsorb on Pt (23). Therefore, the mvRuCN deposit may be more complicated than a simple complex of Ru3+-Ru(CN)63"4'. In addition, the deposition of the film may not be only by adsorption but also may be Contributed to by precipitate formation. It is also significant that Ru(CN)64' is not electroactive in the Ruthenium Purple film (14, 16) but seemingly is nearly Nernstian by cyclic voltammetry at 50 mV s'1 in mvRuCN on glassy carbon. Finally, unlike the Prussian Blue case (13), variation of the cation of the supporting electrolyte in the set of Na+, K+, NH4+, Li+; and Rb+ did not change the voltammetric behavior of the mvRuCN film. The deposit apparently does not have the zeolite-like structure of Prussian Blue. The mvRuCN coating on glassy carbon was very stable. The Figure 2 behavior was sustained for at least 4 h during which the potential was continuously cycled at 50 mV s'1 between 0.85 and 0.10 V. When the potential is scanned out to 1.0 V, the film is not destroyed. An additional anodic peak occurs at 0.94 V; the corresponding cathodic peak is at 0.90 V. Théy probably result from the reversible oxidation of Ru(III) in the film to Ru(IV) (24, 25). The onset of a background process, probably a catalyzed 02 or Cl2 evolution (16, 26), is observed beyond 1.0 V. The working range was independent of pH in the range from 1 to 4. That mvRuCN catalyzes the oxidation of As(III) is evident from Figure 3. At bare glassy carbon, As (III) is not oxidized prior to the discharge of the supporting electrolyte (Figure 3A). The potential of the As(III) oxidation peak at mvRuCN (Figure 3B) relative to that for the oxidation of Ru(CN)64' (Figure 2) indicates that electrochemically generated Ru(CNj^'in the film oxidizes As(IlI). The fact that the presence of As(ÍII) attenuates the Ru(CN)63' reduction peak supports this hypothesis. Evén with mvRuCN films that yield charges of only 0.5 µ (i.é., only about 0.4 monolayer) per peak on experiments in the supporting electrolyte, the catalysis occurs. The As(III)
as
1
¿pa,
mA
14.1 19.9 27.9
44.0 67.0 87.3 138 190
mV"1'2 6.31 6.29 6.24 6.22 6.20 6.17 6.17 6.00
The conditions are the ¡pa, anodic peak current. in Figure 2 except the scan rate, v, is varied.
same
oxidation peak current is virtually equal to that in Figure 3B. Because such small coverages give a strong catalytic effect, a more specific chemical reactivity than simple outer-sphere electron transfer is probably involved in the overall process (27, 28). The catalytic activity is probably due to a combination of chemical catalysis with redox mediation. It has been pointed out that some of the best electrocatalysts react
through inner-sphere pathways (28), i.e., with the participation of the ligand-bridged transition states. The reaction that is catalyzed probably involves the formation of the As(IV) intermediate since the oxidation of As(IV) to As(V) is considered to be rapid (29-31). It is not surprising that As (III) is not oxidized at bare glassy carbon. The situation is comparable to the observations at Pt electrodes. When that surface is free of oxides, the process does not occur, but when lower oxides of Pt are present, they react with As (III) in a manner that yields a net oxidation of As(III) to As(V) at 0.9 V in 1 M H2S04 (20). At bare glassy carbon, active oxides are not present (32). From the Figure 3A behavior it is also apparent that -alumina particles, which can be catalytic (2), thát are present in trace amounts on the glassy carbon surface from the electrode pretreatment, do not have a significant role in the As(III) oxidation. The effect of scan rate, , on the peak potential and current for the oxidation of As(III) at a mvRuCN-coated glassy carbon electrode is summarized in Table I. At scan rates of 5-500 mV s'1, the current function, ipir1/2, is independent of i>; at 1000 mV s'1, the function decreases by only 5%. The oxidation is therefore diffusion controlled which attests to a rapid (not rate limiting) reaction between Ru(CN)63' in the film and As (III). Moreover, charge propagation through the thin film does not limit the net reaction. In these experiments the electrode was held at 0.35 V for 90 s prior to each scan. The conclusion that the As(III) oxidation is diffusion controlled was verified by measurements at a rotating glassy carbon disk electrode that was coated with an estimated 2 monolayers of mvRuCN. The limiting current was directly proportional to the square root of the rate of rotation, 1/2, over the range 400 to 3600 rpm. With 2.0 mM As(III) in 0.1 M NaCI at pH 2, the values of the function ¡.'of1'''2, in units of AiA/(rpm)1/2, for various values were the following: 34.5, at 400 rpm; 34.7, at 900 rpm; 34.6, at 1600 rpm; 34.0, at 2500 rpm; and 33.8, at 3600 rpm. The results of the experiments at the modified rotating disk are in marked contrast to the expectations at electrodes in which the catalyst or mediator is immobilized in a polymer layer. In the latter casé it can be expected that the rate-limiting step will be a combination of diffusion in solution with the rate of the catalytic redox reaction, the rate of electron propagation in the film, and/or the rate of analyte diffusion in the film (33-35). As a result, the most useful mechanism for analytical applications, rate limitation by diffusion of the analyte in solution, may not be attainable at such electrodes
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ANALYTICAL CHEMISTRY, VOL. 56, NO. 6, MAY 1984
Effect of the Supporting Electrolyte on the Catalytic Oxidation of As(III) at the mvRuCN-Coated Glassy Carbon Electrode Table II.
electrolyte0 LiCl NaCl KC1
RbCl CaCl2 NH4C1
NaC104 NaH2P04 Na2S04
NaN03
Spa, V
¡pa, mA
0.800 0.794 0.795 0.792 0.796 0.806 0.802 0.785 0.804 0.806
43 44 42 44 41 44 47
40 44 47
v
Epa/j,
l
40 38
40 39 43 44 40 40 44 42
Concentrations, 0.5 M; all solutions adjusted to pH 2 with the conjugate acid of the electrolyte anion; Epa and Epa-2. peak and half-peak potentials, respectively. Other conditions are the same as in Table I. 0
(36). With the mvRuCN-coated electrode, the outer surface of the deposit acts as the electrolysis plane, and this ideal case is realized.
The catalysis of the As (III) oxidation by the mvRuCN film not influenced significantly by the supporting electrolyte (Table II). Each tabulated experiment was performed at pH 2; however, with the 0.5 M NaCl system it was demonstrated that the variation of the pH over the range 1-5 did not change the peak current; the peak potential varies by only 30 mV. The results were also unchanged by variation of the NaCl concentration from 0.1 to 1.0 M at pH 2. Even though the As(III) oxidation is irreversible, the peak shape is comparable to that for a reversible, two-electron case. For example, the average Epa Epa/2 value in Table II is 41 mV; for an uncomplicated, reversible, two-electron process, this parameter is 28 mV. As mentioned in the Experimental Section, the use of longer cycling times in the modification medium (Ru3+, Ru(CN)e4_, NaCl at pH 2) results in thicker films on the glassy carbon surface. For example, when the substrate is cycled for 150 min instead of the usual 25 min, the mvRuCN deposit contains sufficient RuiCNle4" to yield a charge of about 5 µ under the anodic linear scan voltammetric peak. The film is therefore about five times thicker, i.e., about 10 monolayers, than the usual value. In a 2 mM As(III) solution, this electrode produces an As(III) oxidation peak current at 50 mV s~4 that is 13% lower than the value in Figure 3B, and the peak is broader (Epa £pa/2 is 50 mV). The increased resistivity of the film undoubtedly causes these changes. In many cases a thicker deposit of an electrocatalyst could be advantageous because of an increase in catalytic activity (37); for example, a three-dimensional arrangement of the catalytic sites may be more effective than the two-dimensional geometry of monolayers (27). The thinnest stable film that gives current limitation by diffusion of the analyte in solution is optimal for analytical purposes because of minimization of the deleterious effects of resistance on the peak development. The catalysis that is demonstrated in Figure 3 is not immediately observed at a bare glassy carbon electrode with either 2 mM K4Ru(CN)e or RuC13 simply added to a solution of 2 mM As(III) in 0.5 M NaCl at pH 2. However, if the glassy carbon electrode is cycled between 0.9 and 0.4 V for 4 h at 50 mV s'1 in 2 mM K4Ru(CN)6, 0.5 M NaCl at pH 2, the previously mentioned blue-green precipitate becomes visually apparent on the electrode surface, and the oxidation of As(III) can be again observed. This precipitate has been described as a Ru analogue of Berlin Green (7), ferric ferricyanide, i.e., the oxidized form of Prussian Blue. The Ru3+ that is required for the precipitate formation (and, hence, the catalysis) oriwas
-
-
ginates from the decomposition of electrochemically generated Ru(CN)63_. These results suggest that the catalysis is related
specifically to the microcrystalline Ru precipitate rather than a direct reaction between Ru(CN)63~ and As (III). Such a species may also be present in the polymer layer used in obtaining the data in Figure 1. The analytical experiments were performed by linear scan voltammetry at 50 mV s"1 in pH 2, 0.5 M NaCl with the 2 monolayer film of mvRuCN on glassy carbon. The electrode was conditioned by electrolysis at 0.35 V for 120 s between scans. The analyte solution was not deaerated. The analytical current was the difference between the peak current at about 0.79 V and the background current measured at the mvRuCN-coated electrode in the supporting electrolyte at the
potential. The background-corrected peak current was directly proportional to the As(III) concentration over the range 5.0 µ to 2.0 mM. A linear least-squares curve fit over that range (eight points) yielded the following: slope, 22.0 ± 0.1 mA/M; correlation coefficient, 0.9999; standard error of the current estimate, 0.22 µ ; standard error of the estimate of concentration, 9.9 µ . The detection limit using the criterion of the concentration that yields a current of twice the uncertainty of the background current was 3.5 µ As(III). Above 4 mM As(III) the working curve has a negative deviation from linearity. The use of differential pulse voltammetry (scan rate, 2 mV s-1; pulse amplitude, 100 mV; pulse duration, 2 s) increased the sensitivity (slope of the Working curve) by 8-fold, but because of a commensurate increase in the background uncertainty, the detection limit was not improved. Species that are electroactive at the mvRuCN-coated electrode near 0.8 V will interfere. For example, the presence of 0.1 mM thiocyanate, nitrite, ascorbic acid, or iodide with 0.1 mM As(III) causes a positive interference. A 10-fold excess of bromide relative to As(III) causes a statistical increase in current. Important species that do not alter the sensitivity are As(V), in 100-fold excess, gelatin, at 60 mg/L, and dissame
solved oxygen.
The stability of the film and the catalytic efficiency in the presence of heavy metal ions that have been reported to form
sparingly soluble precipitates with Ru(CN)64' or Ru(CN)63' were examined. The presence of 10 µ Cu(II), Cr(III), Pb(II), or Fe(III) with 0.1 mM As (III) in the pH 2, 0.5 M NaCl electrolyte did not affect the As(III) oxidation peak current or potential. When these metals were at concentrations of 1 mM or greater, the catalytic activity of the deposit decreased. For example, 1.0 mM Pb(II) lowered the analytical current for 0.1 mM As(III) by 18% and shifted the peak potential by 80 mV toward more positive values. Such concentrations of cationic Fe(III) poisoned the surface and precluded analytical measurements. Repetitive cycling for 5 min between 0.35 and 0.85 V in pH 2, 0.5 M NaCl restored the electrode catalytic activity. With Fe(III) as Fe(CN)63', neither interference nor poisoning was observed. The mvRuCN coating was extraordinarily stable. When not in use it was stored at open circuit in pH 2, 0.5 M NaCl. There was no significant change in catalytic activity during the 10-week period, after which it was intentionally destroyed. Ten weekly measurements on a 0.1 mM As(III) solution under Figure 3 conditions gave an average current of 9.4 ± 0.2 µ ; the range was 0.6 µ . Experiments in the above solution were performed on a series of freshly prepared mvRuCN films on a selected glassy carbon substrate. Each time the glassy carbon was pretreated as described in the Experimental Section. The following currents were obtained: 9.4,9.9,9.4,8.7,9.6, and 8.9 µ . Each of these points is the average of three trials. These data demonstrated that both the electrode preparation method and
ANALYTICAL CHEMISTRY, VOL. 56, NO. 6, MAY 1984
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1025
(6) Worthington, J. B.; Pardue, H. L. Anal. Chem. 1970, 42, 1157-1164. (7) DeFord, D. D.; Davidson, A. W. J. Am. Chem. Soc. 1951, 73,
1469-1474.
(8) Bruce, J. A.; Wrlghton, M. A. J. Am. Chem. Soc. 1982, 104, 74-82. (9) Oyama, N.; Shlmomura, T; Shigehara, K.; Anson, F. C. J. Electroanal. Chem. 1980, 112, 271-280. (10) Oyama, N.; Anson, F. C. J. Electrochem. Soc. 1980, 127, 640-647. (11) Oyama, N.; Anson, F. C. J. Electrochem. Soc. 1980, 127, 247-250. (12) Neff, V. D. J. Electrochem. Soc. 1978, 125, 886-887. (13) Éllls, D.; Eckhoff, M.; Neff, V. D. J. Phys. Chem. 1981, 85,
1225-1231. (14) Rajan, K. P.; Neff, V. D. J. Phys. Chem. 1982, 86, 4361-4368. (15) Itaya, K.; Shlbayama, K.; Akahoshi, H.; Toshima, S. J. Appl. Phys. 1982, 53, 804-805. (16) Itaya, K.; Ataka, T.; Toshima, S. J. Am. Chem. Soc. 1982, 104,
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Figure 4. Controlled potential electrolysis working curve at the mvRuCN-coated glassy carbon electrode: electrolysis potential, 0.8 V; sampling time, 3 min; geometric area of the electrode, 5.5 mm2; electrolyte, 0.5 M NaCI at pH 2.
the catalytic activity are quite reproducible. Some applications, such as in an electrochemical detector in a flow system, would require that the mvRuCN film be used in the controlled potential electrolysis mode. Figure 4 shows a working curve obtained under these conditions. The reproducibility was about 5% under the Figure 4 conditions; five replicate measurements on a 1.0 X 10"6 M As(III) solution yielded 58 ± 3 nA. Here, the modified electrode was simply immersed in an unstirred solution. With a flow system, a better signal-to-noise ratio would probably be attained, so the mvRuCN-coated glassy carbon electrode may be well suited to a variety of analytical situations. Further, even when catalysis is not required, this modified electrode may have value. The surface seems to possess better long-term stability than typical solid electrodes, such as bare glassy carbon.
Registry No. Ru(CN)6^, 54692-27-2; Ru(CN)64-, 21029-33-4; As, 7440-38-2; C, 7440-44-0; PVP, 25232-41-1.
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Received for review December 8,1983. Accepted February 6, 1984. This work was supported by the National Science Foundation under Grant CHE-8215371.
