While the electrode behaviour of the Ni(II)421 system in water was recently investigated and described in several papers I s, there is no detailed study, according to the authors' knowledge, of the Ni(II)~-system. There exist only a few observations on the presence of a prewave in the nickel(II)-iodide system in addition to the main aquo-complex wave 67. This results probably from the low stability of nickel(II)-iodide complexes which in aqueous solutions may possibly be formed in relatively concentrated not very stable solutions of iodides. Recently Eriksrud 8 investigated the electroreduction of nickel(II) in 1 M KI assuming no interaction of nickel(II) with iodides in the bulk of the solution. She found a significant increase of the cathodic reaction rate and explained it following the Parsons' model 9 by the interaction of an activated complex with the adsorbed iodides. However the formation of iodide complexes of nickel(II) in aqueous solutions is possible. Because of low stability of such complexes and shift of mercury oxidation potential to a value close to the formal potential of the Ni(II)lNi(Hg) couple the investigation of the electrode behaviour of the nickel(II)-iodide complexes is to some extent complicated. This is why to investigate the electrode processes of this system it was necessary to carry out the measurements under exactly specified conditions of iodide and nickel(II) concentrations, ionic strength and temperature.
These conditions and electrode processes occurring in this system are described in detail in the present paper.
EXPERIMENTAL
Reagents
Ni(C104)2 was prepared by neutralizing NiCO3 p.a. with HC104 p.a. and then purified by crystallization.
KI and Ca(NO3)2 p.a. were used without additional purification. The concentration of nickel(II) and calcium(II) in the solutions was determined by titration with standardized EDTA, and the concentration of iodides determined by weighing.
All solutions were prepared shortly before investigation using thrice distilled water. The second distillation of water was carried out from alkaline solution of KMnO4, and the third in an all-quartz still.
Mercury was chemically purified by prolonged shaking with acidified (HNO3) solution of Hg2(NO3) 2 and then distilled under reduced pressure.