On the source of oscillatory instabilities in the electroreduction of thiocyanate complexes of nickel(II) at mercury electrodes

zThr mechanistic aspects of the electroreduction of thiocyanate complexes of nickel(II) at mercury electrodes, responsible for the formation of the negative polarization resistance and oscillatory instabilities, were studied. Of two possible sources of negative resistance-adsorption of an inhibitory layer of NiS and the desorption of catalytic SCN- fr om mercury surface at negative potentials. the latter was found to predominate under the conditions studied. Nickel(II) sulphide was suggested as one of the causes for the limited lifetime of oscillations only. and not as a sourer of actually observed instabilities. An inner-sphere bridge mechanism of the electroreduction of thiocyanate complexes of nickel(II) was proposed, as this pathway is considerably faster than the irreversible outer-sphere reduction. occurring at more negative potentials. The proposed mechanism was confirmed quantitatively by its digital simulation and compared with that suggested earlier for In(III)-SCN- complexes. An example of current oscillations is presented. (C) 1999 Elsevier Science S.A. All rights reserved.