Voltammetric studies in molten salts

Abstract

A study has been made of the eleotrodeposition of silver onto platinum from aqueous solutions and from molten alkali nitrates using the linear sweep technique and measurement of step-function transients. The nature and role of the platinum oxide film and its effect upon the rate of now phase formation on it have been studied. The investigations included copper deposition on platinum and silver deposition on vitreous carbon from aqueous media, both of which are invariably preceded by three dimensional nucleation and growth controlled by hemispherical diffusion of the depositing ions. Electron micrographs chow a radial growth of silver nuclei on platinum in support of the assumption that nucleation is an essential precursor of the appearance of maorosteps. A detailed study of the early stages of the silver deposition on vitreous carbon in NaNO_ KNO eutectic at 2500C has been made using galvanostatic, potentiostatic and potentiostatic-galvanostatic-potentiostatic pulse trains. The second potentiostatic pulse was used to estimate the nuclear numberdensity at the galvanostatic potential maximum. The maximum in thegalvanostatic potential-time response was used as a basis for the determination of the charge transfer rate constant for the bulk silver silver ion reaction in nitrate melt and found to be in satisfactory agreement with previously published values. A further study encompasses the deposition of other metals such as Pb(Il), Cd(II), Co(II), Ni(II) and Cu(II) onto platinum from molten nitrates which in these cases behaves as a reactive solvent. The results are found to be complicated by a rapid reduction of the nitrate melt by the depositing metals. Thus no evidence of the birth of a new phase could be obtained. The corresponding ionic diffusion coefficient was evaluated by each voltammetric technique and the values obtained compared to those reported in the literature. A novel feature of the electrochemical investigations in LiCl-KCleutectic at ~ 4500C was the successful use of vitreous carbon substrate for the study of nucleation and crystal growth processes of Cu(I) and Co(II). The relation between the potentiostatic current-time transients and the nuclear density for these systems has been established. Attempts to, use other electrode materials to study the deposition of silver from alkali molten chlorides were made using electrodes of platinum, tungsten, copper and nickel but no reproducible results for the last three electrodes were obtained.

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