The reduction wave of nickel(II) at the dropping mercury electrode is irreversible and drawn-out in sodium nitrate or sodium perchlorate solutions. When pyridine 1-4, o-phenylenediamine 5-9, ethylenediamine 1Β°, thiourea t~, chloride ion 12'~3, thiocyanate ion 14-16, etc. are present in aqueous solutions, nickel(II) exhibits a catalytic polarographic wave which arises at less negative potentials than those for the usual hydrated nickel(II) reduction wave.
Nickel(II) in aqueous thiocyanate solution produces a well-defined reduction wave in the potential region between -0.6 and -1.1 V vs. SCE. The polarographic behavior of the nickel(II)-thiocyanate system is unusual; on the direct current (d.c.) polarogram of the system a minimum appears at relatively negative potentials, and on the alternating current (a.c.) polarogram a negative admittance is observed in the potential region where the minimum appears on the d.c. polarogram ~v'~8.
The present authors found that nickel(II) in aqueous thiocyanate solutions containing quaternary ammonium salts gave a new catalytic polarographic wave having the shape of a "maximum". This catalytic wave, which is not due to stirring and also does not have the characteristics of the Brdi6ka catalytic wave, occurs in the potential region where the reduction wave of nickel(II) obtained in thiocyanate solution reaches its limiting plateau. Of the many quaternary ammonium salts investigated, tetraethylammonium perchlorate was the most suitable. In the present paper, the characteristics of this maximum wave and the conditions for its occurrence are reported. The most plausible mechanism to explain the catalytic effect is proposed. EXPERIMENTAL Ni(H20)2(C104) 2 and tetraethylammonium perchlorate (Et4NC104) were prepared using the same procedures as given in a previous paper 19. All the other chemicals were of analytical reagent grade. Redistilled water was used and all solutions were deaerated with high-purity nitrogen. D.c. polarograms and current-time (i-t) curves of individual drops at constant potential were obtained with the potentiostat previously described 2Β° and made from operational amplifiers. Polarograms were recorded with a Yokogawa X-Y recorder, PRO-11A type and i-t curves with a Riken Denshi SP-G2 type recorder. In the following section and in the figures, "current" means the maximum current observed at the drop time. A.c. polarograms were obtained with a pen-recording Yanagimoto