Mechanism of the electroreduction of Ni (II) ions on mercury electrodes catalyzed by pyridine and its derivatives: nicotinamide, <Emphasis Type="Italic">N</Emphasis>,<Emphasis Type="Italic">N</Emphasis>-diethylnicotinamide and nicotine: concept of parallel heterogeneous catalytic reactions

The theory of the polarographic catalytic currents (mechanism CE) has been developed for the system: Ni²⁺-L-X^p− where L: pyridine (Py), nicotinamide (NA), N,N-diethylnicotinamide (DEN), nicotine (NC) and X^p−: NO⁻ ₃, AcO⁻, HPO²⁻ ₄ . The theory is based on the kinetic parallel heterogeneous catalytic reactions:

(1a)

(1b)

with the use of Langmuir’s adsorbed isotherm. The kinetic equations obtained for average and instantaneous currents allowed to determine the Langmuir’s parameters (NA<DEN<NC), kinetic parameters and the contribution of reactions 1a and b to the summary catalytic currents. The k _h^″ value rises with the increase of the NiX^2−p stability constant. The correlation k _h^″≫k _h^′ was explained by the additional effect of the field electrode through X^p−. These effects base the reaction 1b instead of accepted early alternative reaction of the ligand exchange. In spite of the fact that k _h^″≫k _h^′, the contribution of the reaction 1a in the summary catalytic current attained more than 60% (Py, DEN) due to the influence of the ψ₀ potential. Dedicated to Professor Zbigniew Galus on the occasion of his 70th birthday and in recognition of his many contributions to electrochemistry