It is known 1-4 that the Ni 2+ ion forms in CN-solutions the very stable complex, [Ni(CN)4] 2-, which is essentially the only complex present even at a total concentration of CN-lower than the stoichiometric value.
Under chemical or electrochemical reduction, this complex gives [Ni2(CN)614which is slowly reoxidized by Oz or water to ENi(CN)412-. If a very strong chemical reducing agent is used, such as metallic potassium in liquid ammonia, the zerovalent ENi(CN)414-obtained is relatively stable in the abser~ce of Oz or water.
In the earlier polarographic studiesS,% it was concluded that the Ni z+ complex is reduced with the uptake of one electron. According to HUME AND KOLTHOFF 7 the reduction is a two-electron process, the rate-controlling step being the electron transfer. From chronoamperometric measurements at constant current, DELAHAY AND MAMANTOV 8 concluded that the slow step is a preceding chemical reaction, while VL~EK 1,9 found that (from the dependence of the wave height and the half-wave potential on CN-concentration, drop-time, and temperature) the reduction proceeds through two fast one-electron steps and the formation of an intermediate which undergoes many equilibria with the other species present. His conclusions were also supported by measurements both with the Kalousek commutator and oscillography.
Recently 1Β°, a paper on ENi(CN)4~ ~-reduction at high CN-concentration was published. From the conflicting data there was evidence of the influence of ionic strength. On this basis, it seemed worthwhile to study the ionic strength effect ill order to gain a better knowledge of the mechanism of this reaction and to explain to some extent the contradictory results previously obtained.
In this paper, the results for the polarographic reduction of the nickel-cyanide complex (with different cations and concentrations of supporting electrolyte) are reported.
EXPERIMENTAL
All solutions were prepared from water twice-distilled over KMnOa. Salts, of reagent-grade quality, were recrystallized and, when possible, heated at a temperature a little below the melting or the decomposition temperature. LiCN and CsCN were prepared from KCN on Dowex 50. The polarograms were obtained manually using a three-electrode cell and electronic control of the potential. The chronoamperometric measurements with linearly changing potential were made with a stationary mercury electrode and an apparatus constructed in this Institute.