Galvanostatic double-pulse method with polarographic generation of reactant in situ

Abstract

Modified procedure in the galvanostatic double-pulse (g.d.p.) method is presented. In this procedure the electrolyte only requires to contain initially either the oxidant or the reductant so that the rates of fast electron transfer reactions on a dropping mercury electrode (DME) may be measured. The redox species lacking in the bulk of electrolyte is generated on the DME surface by the polarographic electrolysis at a controlled potential. A g.d.p. is applied between the DME and the counter electrode immediately after the polarographic electrolysis is interrupted. The real moment of termination of the polarographic electrolysis is detected by generation of a sharp voltage spike from the output of the potentiostat; the spike activates the g.d.p. generator. The duration between the termination of the polarographic electrolysis and the application of the g.d.p is successfully reduced to 0.1 μs; the procedure of the analysis of experimental results may thus substantially be identical with that in the conventional g.d.p. method. A rigorous analysis is made for the modified g.d.p. method and a potential-time characteristic is derived. The assessment of the success of the instrumentation as well as the theoretical treatment is made in measuring the electron transfer rate for the trioxalatoferriate/trioxalatoferrate redox system. The observed formal standard rate constant and the transfer coefficient are 1.15±0.05 cm s −1 and 0.78±0.05, respectively, at 25°C. They agree satisfactorily with the results obtained by the conventional g.d.p. method.