Anomalous H/D isotopic effect in the kinetics of polarographic hydrogen evolution

Abstract

Abstract Electrolysis of acidic H 2 O+D 2 O mixtures has been performed using the well established mercury drop electrode. The experiment was done in order to study, for the first time in a chemical reaction, a possible cooperative behaviour of hydrogen species which is evident from neutron and Raman light scattering studies. The electrochemical–kinetic results show a volcano type of deviation of the rate constants, centered around [D]/[H+D]=0.8, from behaviour expected from nominal D and H concentrations. It can be fitted assuming an effective fractionation factor of l *=0.25, which deviates significantly from the established (bulk) fractionation factor of l =0.69. Basically, a different D + /H + -ratio is effective for electron transfer on the mercury drop surface than is present in bulk solution. Various strategies have been followed to explain the observed phenomena on a classical basis. The assumption that a reorganisation of the water ∣ mercury interface may generate the reduced fractionation factor is presently not sustained by convincing experimental or theoretical evidence. We therefore propose to consider limitations of the applicability of simple reversible statistical concepts to the electrochemical water splitting process being in line with ongoing research on quantum correlation with H/D systems.