The electrolytic hydrogen–deuterium separation factor and reaction mechanism

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 of S characteristic 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 for S are readily divisible into two groups having, respectively, values close to these two figures. The values of S predicted theoretically for selected metals, assuming the rate-determining mechanism indicated by independent experimental criteria, are in agreement with the values of S measured experimentally for these metals. S is examined as a function of D 2 O content of the solution in limiting cases. The ratio of rates of D and H production from solutions in pure D 2 O or H 2 O, respectively, is characteristic of reaction mechanism. Anomalies in the theory of proton discharge and isotope separation from alkaline solution are discussed.