Strong toxicity and carcinogenesis of arsenic has been the reason to develop many analytical techniques for controlling and determining its content in environmental samples, mainly in natural and drinking water [1]. Considering simplicity, low-cost, velocity and sensitivity of electrochemical methods, they have found wide application to arsenic determination. That was possible due to the electrode processes of arsenic compounds and therefore better knowledge of their mechanism is significant. Most of the studies were conducted on the large size electrodes: mercury, platinum [2] or gold [3,4], not many works were carried out on the microelectrodes [1]. It seems that the mechanism of the electrode processes occurring on the gold electrodes has not been completely explained in the literature yet. Cyclic voltametry studies carried out on the gold microelectrode show, that As(III) ions adsorb on the electrode surface. This phenomenon was not considered by other researchers, concerning electrode processes taking place on the gold. Our studies indicate, that As (III) adsorbed on the electrode surface is involved in the following reaction: The product of this process, As(0), is also adsorbed on the electrode surface. The relation between the charge of As(III) adsorption/desorption peaks and (i) deposition potential, (ii) deposition time and (iii) As(III) concentration were measured. The monolayer charges and adsorption isotherms were determined for three electrolytes: 0.5 M H₂SO₄, 0.04 M borax and 0.1 M NaOH. The analysis indicates that adsorption of As(III) can be described by Frumkin isotherm, and also pH of the solution strongly affects the molecular interaction in the monolayer. The molecular interaction parameter changes from positive value (g = 0.9) in acidic conditions, to negative value (g = -1) in strong basic solution. The analysis of kinetics of investigated processes was carried out in range pH from 0 to 13. The standard rate constants of reaction were determined for different values of pH. The obtained values from 1.4·10^-6 s^-1 to 9.2·10^-9 s^-1 indicate the irreversibility of that process. The rate of the process decreases with pH increase.

References:

1. P.Salaün, B. Planer-Friedrich and C.M.G. van den Berg, Anal. Chim. Acta 585(2007)312,
2. T.D. Cabelka, D.S. Austin and D.C. Johnson, J. Electrochem. Soc. 131(1984)1595,
3. Z. Jia, A.O. Simm, X. Dai and R.G. Compton, J. Electroanal. Chem. 587(2006)247,
4. T.E. Dinan, W.F. Jou and H.Y. Cheh, J. Electrochem. Soc. 136(1989)3284.

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1. Detection of Surface-active Compounds at Au and Pt Ultramicroelectrodes