Effect of the electrical double layer and pyridine adsorption on the catalytic polarographic current of nickel(II)-pyridine complexes

Abstract

The effect of the electrical double layer (concentration and nature of supporting electrolyte), pH, temperature and adsorption of pyridine on the Ni2+ catalytic polarographic pre-wave in the presence of pyridine has been studied. Two concurrent reactions are shown to proceed on the electrode forming complexes [NiPy]ads2+ (from Ni2+ and Pyads), and [NiPy2]ads2+ (from [NiPy]2+ and Pyads), which are reduced at close potentials. Both complexes are formed in reactions with mercury-adsorbed pyridine, in accordance with the concept by Mark and co-workers of the character of organic amine involvement in similar processes. The kinetic equation, checked by employing the ψ1-potential on the basis of Gouy-Chapman theory, is valid at the supporting electrolyte concentrations C≤0.05. When C>0.05, the calculated catalytic currents are lower than the experimental catalytical currents. The effect of the solution pH is in accordance with the kinetic equation only at large C-values (C=1.0 M). The retardation of the catalytic process is observed at C=0.02 M and low pH-values (<6.0). The causes of discrepancy between the experimental and theoretical data are discussed. The effect of the nature of the cation on the catalytic current (Cs+<Na+<Li+) is in a qualitative agreement with changes of ψ1-potential. Rate constants were established for heterogeneous chemical reactions of formation of [NiPy]ads2+ and [NiPy2]ads2+ in the absence of electrical field effects.