Reaction mechanism and partial charge transfer of Tl|Tl+-systems studied by impedance spectroscopy (SSSS method). Part 1. The Tl(Hg)|Tl+-reaction in the presence of SCN−

Abstract

The Tl(Hg)|Tl+reaction in 3 M KSCN is investigated in order to elucidate the reaction mechanism. A set of 14 impedance spectra (measurements from 0.16 to 160 kHz) is simultaneously fitted to the theoretical relations of two- and three-step reactions with partial charge transfer. The introduction of special models for the adsorption and the reaction rates allows one to diminish the number of parameters. A detailed analysis of the errors of the numerical estimation is given. The Tl(Hg)|Tl+ reaction proceeds in two effective steps with a partially charged intermediate (l1=0.78±0.02). Both the partially charged Tl- and SCN−-species seem to be in direct contact with Hg. The adsorption follows a Langmuir isotherm, for the chosen concentrations of the redox components (cO=0.25…5 mM, cR=0.2…500 mM) the coverage degree is 0.40<θ<0.98. The rate constants of both steps are greater than 10 cm s−1.