Studies on the Electrochemical Behavior of Arsenic on Solid Electrodes

STUDIES ON THE ELECTROCHEMICAL BEHAVIOR OF ARSENIC ON SOLID ELECTRODES

A. A. Kaplin, N. A. Veits, A. G. Stromberg, V. P. Gladyshev,
L. F. Trushina

(Presented by the scientific-methodological seminar
of the Department of Physical Chemistry)

The contradictory nature of the literature data on the electroreduction of arsenic on mercury electrodes [1—3], and the absence of such data for solid electrodes, considerably complicate the selection of conditions for determining arsenic by the method of film polarography with accumulation (FPA). In [4] the fundamental possibility of determining microquantities of arsenic by the FPA method was shown; in [5] a procedure was developed for determining arsenic by this method in hydrochloric acid.

The purpose of the present work is the further study of the electrochemical behavior of arsenic on solid electrodes under FPA conditions. As indicator electrodes we used graphite, platinum, and gold electrodes. The reference electrode was a saturated calomel electrode, and the supporting electrolyte was 1 M HCl. The dependence of the anodic current of arsenic on the pre-electrolysis potential passes through a maximum on all the electrodes used. The decrease in the magnitude of the anodic current of arsenic at sufficiently negative pre-electrolysis potentials is evidently associated with the formation of gaseous arsine. We studied arsine formation on graphite and mercury electrodes by a radiochemical method. The studies showed that arsine formation occurs at potentials more negative than $-0.7$ V. These data agree with the course of the curves of the dependence of the anodic current of arsenic on the pre-electrolysis potential, confirming our assumption that part of the arsenic reduced during pre-electrolysis is hydrogenated with formation of a gaseous product, thereby reducing the amount of arsenic on the electrode. The difference in the character of the curves on the electrodes is apparently associated with the different rate of electrochemical hydrogenation of arsenic on these substrates. The dependence of the anodic current of arsenic on the concentration of As(3+) ions in solution and on the pre-electrolysis time is linear up to a certain concentration limit $(2.4—3.2 \cdot 10^{-5}\ \text{g-ion/l})$, regardless of the type of electrode. This can be explained by passivation of the electrode due to an increase in resistance during formation of a film of elemental arsenic. The same order of magnitude of the coefficient $\gamma$ [6], calculated from this dependence and from monolayer filling, confirms the assumption of adsorption saturation of the surface.

In order to study the mechanism of the process, polarograms of the reduction of As(3+) on a platinum electrode were recorded. At

During the recording of the first polarogram, one peak is observed at: \(\varphi=+0.15\ \mathrm{V}\). On all subsequent polarograms, two peaks are recorded at \(\varphi=+0.25\ \mathrm{V}\) and \(\varphi=-0.2\ \mathrm{V}\). The difference in the potentials of the first peaks is apparently due to a decrease in the overvoltage of the reduction of As (3+) on a surface coated with platinum arsenide. We attribute the first cathodic peak to the reduction of trivalent arsenic to the element, with some depolarization due to the formation of the intermetallic compound \(\mathrm{PtAs}_2\). The limiting current value is reached at an As (3+) concentration equal to \(8\cdot10^{-6}\ \mathrm{g\text{-}ion/l}\). The peak at \(\varphi=-0.2\ \mathrm{V}\) is evidently associated with the formation of elemental arsenic on the surface of platinum coated with platinum arsenide. The limiting current is reached at an As (3+) concentration equal to \(1.6\cdot10^{-4}\ \mathrm{g\text{-}ion/l}\). On a graphite electrode, cathodic peaks of As (3+) were not recorded in the absence of other elements. In the presence of gold ions, two cathodic peaks are observed on a graphite electrode. Apparently, the mechanism of the process on gold is analogous to the process on platinum. The polarograms of reduction of As (3+) and As (0) on a platinum electrode, and of anodic oxidation of the electrolysis product on platinum and graphite electrodes, are analogous. This may indicate that the reduction of As (3+) proceeds through the stage of arsenic formation. However, it is necessary to take into account the possibility of oxidation of elemental arsenic by atmospheric oxygen with the formation of a film of arsenic trioxide. The observed shift of the potentials of the cathodic peaks of arsenic on the platinum electrode toward positive potentials with increasing concentration of hydrogen ions is evidently due to facilitation of discharge as a result of the transition of the arsenite ion into the ion \(\mathrm{AsO}^{+}\). The direction of the shift coincides with that expected on the basis of the Pourbaix diagram. It has been established that the solid phase on the electrode, which determines the presence of an anodic peak, is formed in the cathodic process characterized by a current maximum at \(\varphi=-0.2\ \mathrm{V}\). Pre-electrolysis at a more positive potential does not lead to the recording of an anodic current. This conclusion is confirmed by the fact that additions of the TBA ion decrease both the second cathodic peak and the anodic peak. The dependence of the anodic peak potentials on the concentration of \(\mathrm{H}^{+}\) ions indicates the participation of hydrogen ions in the electrode reaction. The order of the electrooxidation reaction of the electrolysis product with respect to hydrogen ions is 0.875. The order of the electroreduction and electrooxidation reactions with respect to chlorine ions is close to zero, which indicates that the complexes discharging at the electrode do not contain chlorine ions. We calculated the transfer coefficients of the arsenic discharge–ionization processes on graphite, platinum, and gold electrodes by various methods. It was shown that the kinetic parameters of arsenic electro-dissolution depend on the electrode material and on the nature of the elements discharging jointly with arsenic. \(\beta_{\mathrm{n}}\) for the process of arsenic electro-dissolution from a platinum electrode is 0.7; from gold, 0.96; from graphite during co-deposition with gold, 1.37; with copper, 1.92; with cadmium, 0.96; with lead, 0.69. Using the criteria proposed by Brainina [6], we showed that the process of electrooxidation of arsenic on a gold electrode is irreversible. The calculated value of the standard rate constant \((9.37\cdot10^{-8}\ \mathrm{cm/sec})\), under the assumption of a three-electron oxidation process, confirms this conclusion. The equilibrium potential of the system As (3+)—electrolysis product was measured. The measurement of \(\varphi_{\mathrm{p}}\) was carried out in a sealed cell under a stream of inert gas above the solution, since in the presence of oxygen spontaneous dissolution of the electrolysis product occurs.

We investigated the influence of a number of elements on the magnitude of the anodic current, the peak potential, and the half-peak width of arsenic. Introduction into the sol-

... solution of copper, platinum, palladium, and gold ions leads to a significant increase in the anodic current of arsenic. The joint discharge of arsenic and cadmium or lead on a graphite electrode leads to distortion of the peak shape. We have studied the copper—arsenic system in greater detail. A different character of the dependence of the anodic current of arsenic on the concentration of copper ions on graphite and gold electrodes was found. If, on a graphite electrode in the presence of comparable concentrations of copper, a significant growth of the arsenic peaks is observed, then on a gold electrode the arsenic peak decreases by a factor of 1.5. It may be supposed that the increase in the sensitivity of arsenic determination in the presence of a number of elements is associated with facilitation of arsenic discharge. On the other hand, at the stage of anodic oxidation of arsenic, the presence of a second component on the electrode surface may lead to a change in the activity of arsenic in the deposit and to a corresponding change in the sensitivity of its determination. In the copper—arsenic system, at a constant concentration of one of the components in solution and an increasing concentration of the second, a decrease in the anodic peaks of both elements is observed (when a gold electrode is used as the indicator electrode), in comparison with the anodic peaks of these elements in the absence of the second component. At the same time, the appearance of additional anodic peaks in the intermediate region of potentials is noted. The dependence of the anodic current on the mole fraction of one of the components in the deposit can be described by a parametric relation, which in our case will have the form: \(I = B_0 N \exp(-BN^n)\),

where the quantities \(B\) and \(n\) characterize the nature and magnitude of the interaction of the components.

We calculated the values of \(B\) and \(n\) for a constant copper concentration equal to \(4 \cdot 10^{-5}\) g-ion/l and an arsenic concentration varying in the range \(8 \cdot 10^{-6}\)—\(5 \cdot 10^{-5}\) g-ion/l. For copper \(B = 1.225\), \(n = 2\); for arsenic \(B = 0.132\), \(n = 1\). These data indicate the presence of a strong interaction in the system, associated with the formation of compounds between copper and arsenic. We calculated the ratio of copper and arsenic in this compound. It corresponds to the composition \(\mathrm{CuAs}\).

REFERENCES

1. V. P. Gladyshev, A. Dzhumashev, M. T. Kozlovskii. Doklady AN SSSR, 199, 5, 1971.
2. E. G. Vasil’eva, S. I. Zhpanov, T. A. Kryukova. “Electrochemistry,” 4, 24, 1968.
3. I. P. Arnold, R. M. Johnson. Talanta, 16, 1191, 1969.
4. L. F. Trushina, A. A. Kaplin. ZhAKh, 25, 1616, 1970.
5. L. F. Trushina, A. A. Kaplin, N. A. Veits. In: Methods for the Analysis of Chemical Reagents and Preparations. Moscow, IREA, issue 20, 1971.
6. Kh. Z. Brainina. Stripping Voltammetry of Solid Phases. Moscow, 1972.