Polarography of Te (IV) Anions in Neutral Solutions in Presence of 2,2'- Dipyridyl and Fe(dipy)3 2+ - Complexes

M.B Dergacheva,V.N Statsyuk,L.A Fogel

doi:10.18321/ectj411

Abstract

The electroreduction of Te (IV) ions in neutral non-buffer solutions containing 2,2'-dipyridyl (4⋅10-5- 4⋅10-3 M) or tris-dipyridyl iron (II) complexes is studied by the polarographic method. NaF (0.01-0.5 M) or NaNO3 (0.1-1 M) are used as supporting electrolytes. The mechanism of electrochemical reactions of Te (IV) anions on mercury electrodes in the presence of the additives is discussed. The electroreduction of Te (IV) anions is shown to proceed through electron transfer and proton addition. The obtained results point to a considerable influence of electric double layer structure on electrochemical reactions of Te (IV) ions in the presence of inorganic and specifically adsorbed organic compounds in the electrolyte. It is shown, that 2,2'-dipyridyl does not form complexes with Te (IV) anions. Having been adsorbed on the surface of mercury electrode, 2,2'-dipyridyl complexes increase negative Ψ'-potential that results in a shift of Te (IV) electroreduction wave to more negative potentials and decrease in the current of Te (IV) wave and peak at -1.19 V. It is shown that 2,2'-dipyridyl molecules at ε &gt; 0, (ε – charge of an electrode) are adsorbed in plane orientation, and at ε &lt; 0, plane or vertical. Vertically adsorbed molecules cause a significant decrease in the double layer capacitance. At negative potentials orientation of 2,2'-dipyridyl molecules changes from plane to vertical with the increasing 2,2'-dipyridyl concentration. This change of orientation results in a typical maximum capacitance emerging in the potential range of -0.7 to -1.2 V. It is shown that the supporting electrolyte, 2,2'-dipyridyl and Fe(dipy)3 2+ have influence on the electroreduction of Te (IV) anions in neutral non-buffer solutions through a change in the Ψ'-potential of mercury electrode.

Highlights

It is known [1,2,3] that electroreduction of Te (IV) at mercury electrode involves two simultaneous processes occurring in the potential range of 0 to -1.3 V: Te (IV) + 6e → Te2- (1)The potential of maximum at E = -1.19 V corresponds to the potential of half-wave E1/2 in reaction (2)
It is shown that the supporting electrolyte, 2,2'-dipyridyl and Fe(dipy)32+ have influence on the electroreduction of Te (IV) anions in neutral non-buffer solutions through a change in the Ψ'-potential of mercury electrode
Classical polarograms of Te (IV) on Dropping mercury electrode (DME) using NaF (0.5-0.01 M) supporting electrolyte have been obtained to study the effect of electric double layer structure on Te (IV) reduction, and reasons of its change

Summary

Introduction

The potential of maximum at E = -1.19 V corresponds to the potential of half-wave E1/2 in reaction (2). The summed reduction current can be described by the reaction (3): Te2- – 2e → Te0 (2). Reaction (2) does not proceed at the potentials more negative than the potential of maximum The limiting diffusion current corresponds to four electron reduction of Te (IV) due to the reaction (4): Te (IV) + 2Te2- = 3Te0 (4). The adsorption nature of this maximum and its relation to the redaction of adsorbed Te (0) to telluride ions is shown in [6]. In previous and some current publications little attention has been paid to the form of existence of Te (IV) ions in solution and at the electrode surface. The influence of double electric layer structure on the electrochemical behavior of Te (IV) ions in the presence of inor-

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