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 developed and tested using a simulation program. The method was then used for determination of the thioglycol adsorption kinetics on Hg in KOH and H2SO4 aqueous solutions. Cyclic voltammetric experiments were carried 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 adsorption isotherm, despite the fact that the current model assumes independence of the electrosorption valency and the interaction parameter of the potential and the electrode coverage, and that it neglects possible reorientation of adsorbed molecules on the surface with a change in electrode coverage and/or potential. It was found that the electrosorption 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 parameters 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 for the activated complex in both studied solutions is 3000 J mol(-1) larger than the corresponding interaction parameter of the product. The results are discussed in terms of participation of protons in the adsorption-desorption process and the effect of potential on the orientation of adsorbed molecules.