The electrolytic H/D separation factor-agreement of the experiment with the hypothetical maximum value

Abstract

A fresh approach to separation factor studies is described based on (a) a search for conditions giving maximum experimental values of S (optimisation), (b) the use of an isotopic ratio (H:D=20:80) which simplifies theoretical interpretation and minimises experimental error. Two methods are developed for obtaining H/D electrolytic separation factors under clearly defined experimental conditions. The first consists in continuously maintaining a fresh cathode surface by depositing cathode material onto it while hydrogen evolution is in progress (the scavenging technique). The second consists in displacing other surface impurities by the deliberate use of bath additives which adsorb onto the cathode so that the surface is maintained in a definite, reproducible state (the displacement technique). When both methods are combined in hot alkaline electrolytic baths having iron electrodes, certain of the additives investigated (EDTA and urea) thereafter give maximal, reproducible experimental separation factors Such baths are said to be optimised and optimisation persists even when the baths are subsequently operated at low temperatures. The optimised results so obtained are of considerable theoretical interest sinceo1.The position of their Arrhenius plot is shown to identify the rate-step unequivocally as a slow discharge.2.The slope of this plot is indistinguishable from that for the hypothetical maximum separation factor Smax. The position of their Arrhenius plot is shown to identify the rate-step unequivocally as a slow discharge. The slope of this plot is indistinguishable from that for the hypothetical maximum separation factor Smax. This identity of both position and slope is the first example to be found of clearcut agreement between experiment and theory for Arrhenius plots of electrolytic separation factors. However it presents difficulties of interpretation since the theoretical maximum value requires a transition-state consisting of free, gaseous, atomic hydrogen—a condition unlikely to exist at the cathode. Suggested models to account for: (a) this identity of experimental and hypothetical values and (b) the mechanism of optimisation, are discussed. In addition to its theoretical significance, the discovery of the optimising effect of certain impurities may account for lack of reproducibility in previously reported values of S.