Polarographic Behaviour of the Hexa-ammine-chromium(III) Ion: Role of Hydrogen Ions
HEXA-AMMINECHROMIUM(III) ion in acetate buffer solutions is reduced at the dropping mercury electrode, and gives two polarographic waves. The first wave, which is well defined, is considered to be a one-electron reduction of Cr(III) to Cr(II)1. Its half-wave potential at 25° C was found to be −0.970 V versus saturated calomel electrode in 0.1 M acetate buffer solution which contained 0.9 M potassium chloride and 0.005 per cent gelatin (Fig. 1, curve a). The limiting current of this wave was diffusion-controlled and proportional to the concentration of the complex ion up to 5 mM. Both the half-wave potential and the diffusion current remained constant when the pH was varied from 4.5 to 5.45. No change was observed when potassium chloride was replaced by ammonium chloride at the constant ionic strength. The plot of log ī/(īd − ī) versus E, where ī means the average current, īd the average diffusion current, and E, the potential of the dropping mercury electrode, gave a straight line with a reciprocal slope of 64 mV, whereas the examinations with Kalousek's circuit and alternating current polarograph showed that this wave has nearly irreversible character.
Mechanism of electroreduction of cobalt(II) in thiocyanate solutions at mercury electrodes
Abstract The process of electroreduction of cobalt(II) in thiocyanate solutions at mercury electrodes has been investigated by cyclic voltammetric, chronoamperometric and polarographic methods. The influences of pH, the concentrations of Co(II) and SCN − , and the reduction products of SCN − , CN − and S 2− on the reduction waves are described. The polarographic pre-wave is an autocatalytic in nature. A mechanism involving an initial reduction of Co(II)—SCN − at a mercury electrode followed by the chemical reduction of thiocyanate ion with the electroreduced metallic cobalt, and taking into account cyanide, sulfide, and hydroxide ions, the latter being produced by the hydrolysis of cyanide ion, is presented. Cobalt sulfide adsorbed at the electrode surface stimulates further reduction of Co(II)—CN − and —SCN − complexes, and depresses the interfering influence of Co(OH) 2 , which is reductively desorbed from the electrode surface with giving rise to an additional peak near −1.08 V vs. SCE.
A New Compound of Univalent Cobalt
POLAROGRAPHIC behaviour of the ions Co(dipy)33+ and Co(dipy)32+ shows that they are reversibly reduced at the dropping mercury electrode to a compound of univalent cobalt which does not decompose in neutral aqueous or ethanolic solutions.