KINETICS AND THERMODYNAMICS OF THIOGLYCOL ADSORPTION ON MERCURY ULTRAMICROELECTRODES

A method for determination of the thermodynamic and kinetic parameters of electrosorption from fast sweep rate cyclic voltammograms was deve loped and tested using a simulation program. The method was then used for determination of the thioglycol adsorption kinetics on Hg in KOH a nd H2SO4 aqueous solutions. Cyclic voltammetric experiments were carri ed out with a hemispherical Hg ultramicroelectrode (5 mu m radius) at sweep rates ranging from 1000 to 100000 V s(-1). The experimental data fit relatively well to a theoretical model based on the Frumkin adsor ption isotherm, despite the fact that the current model assumes indepe ndence of the electrosorption valency and the interaction parameter of the potential and the electrode coverage, and that it neglects possib le reorientation of adsorbed molecules on the surface with a change in electrode coverage and/or potential. It was found that the electrosor ption valency for thioglycol in both solutions studied is similar and close to unity; the standard energies of adsorption are 2.93 and -46.6 kj mol(-1) in 1 M H2SO4 and 1 M KOH respectively; the interaction par ameters are -3670 J mol(-1) (in 1 MH(2)SO(4)) and 2450 J mol(-1) (in 1 M KOH); the rates of adsorption and desorption steps extrapolated to the zero charge potential are k(a)(0) = 4.7 X 10(5) s(-1), k(d)(0) = 9 .8 x 10(6) s(-1) (in 1 MH(2)SO(4)) and k(a)(0) = 1.7 x 10(12) s(-1), k (d)(0) = 2.12 x 10(4) s(-1) (in 1 M KOH). The interaction parameter fo r the activated complex in both studied solutions is 3000 J mol(-1) la rger than the corresponding interaction parameter of the product. The results are discussed in terms of participation of protons in the adso rption-desorption process and the effect of potential on the orientati on of adsorbed molecules.