The electrolytic hydrogen–deuterium separation factor and reaction mechanism

Abstract

The theory of the electrolytic separation factor (S) for H and D is examined for various rate-determining mechanisms in the cathodic hydrogen evolution reaction. Isotopic dif­ferences of all relevant thermodynamic quantities and partition functions for the reacting entities are taken into account in the calculation of rates of pairs of isotopically analogous reactions. Values ofScharacteristic of various mechanisms and conditions are obtained and it is shown that the value arising if electrochemical desorption of adsorbed H or D is rate determining, is 6.1, whilst that for simple discharge of hydroxonium ions is 3.0. The experimental data forSare readily divisible into two groups having, respectively, values close to these two figures. The values ofSpredicted theoretically for selected metals, assuming the rate-determining mechanism indicated by independent experimental criteria, are in agreement with the values ofSmeasured experimentally for these metals.Sis examined as a function of D2O content of the solution in limiting cases. The ratio of rates of D and H production from solutions in pure D2O or H2O, respectively, is characteristic of reaction mechanism. Anomalies in the theory of proton discharge and isotope separation from alkaline solution are discussed.