The effect of additives on the practical electrolytic separation of hydrogen and deuterium. II. Separation factor at Hg in aqueous and non-aqueous media

Studies of the effects of additives on the electrolytic H/D separation factor,S_H/D, previously reported in Part I [2] for Pt and Fe electrodes, are extended to the case of Hg for which the mechanism of cathodic H₂ evolution is well established. The behaviour of urea and guanidine was investigated. In the case of urea, the separation factor at Hg is enhanced but decreases with increasing current density or cathodic potential. The dependence ofS_H/D on current density can be correlated with the diminishing surface excess of urea with increasing negative potential. Experiments with proton sources other than H₂O in liquid water were investigated by making measurements ofS_H/D from solutions of H₂O/HOD, CH₃COOH/CH₃COOD and CF₃COOH/CF₃COOD in CH₃CN with NaClO₄ as electrolyte. An interesting inverse H/D isotope effect arises with trifluoroacetic acid (TFA) as proton source; the effect is interpreted in terms of competitive reactions of H and D with TFA rather than unusual bonding and vibrational characteristics of the TFA activated complex. The results suggest that useful isotopic separations can be made by use of selective competitive reactions. A kinetic analysis of the inverse isotope effect is given.