“Cap‐Pair” Effect as the Reason of the Catalytic Properties of Nicotinic Acid: Experimental and Theoretical Studies

Abstract

The influence of nicotinic acid (NC) on the kinetics and the mechanism of electroreduction of Zn2+ ions in the acetate buffer (pH=6.0) was investigated using electrochemical methods (EIS, CV, SWV and DC). It was shown that the anions of NC catalyze the electrode reaction (cap‐pair effect) by adsorbing on the surface of the mercury electrode. The catalytic activity of NC is due to its ability to form active NC−Zn2+ complexes on the electrode surface, facilitating the electron transfer process. However, no evidence of the formation of such complexes in the solution was found using classical molecular dynamics. Moreover, it was proved that the electroreduction of Zn2+ ions in the presence of NC is a two‐stage process. The first stage involves the transfer of the first electron, preceded by the partial loss of the hydration shell by the Zn2+ ions and formation of the active complex. Moreover, it was shown that in the range of lower concentrations, c≤1.10−2 mol.dm−3, the nicotinic acid shows weaker catalytic abilities than another form of vitamin B3 – nicotinamide. In the range of its higher concentrations, the nicotinic acid is a more effective catalyst for the electroreduction of Zn2+ ions.

Catalysis of Indium Ion Electroreduction in the Presence of Acetazolamide in Chlorates(VII) Solutions with Varied Water Activity

“…The number of water molecules in the In(III) hydration sphere is larger than in the case of In(II) or In(I) ion. It has been demonstrated that the adsorption of organic molecules increases due to the higher concentration of ClO 4 − ions which causes smaller hydration of a near‐electrode surface [19] . The competitive adsorption of acetazolamide, water and to a small extent, ClO 4 − in the double layer interface should be considered [25] .…”

“…As the concentration of the base electrolyte increases, the slope of the DC polarographic curves of the electroreduction of In(III) ions varies, confirming changes in the reversibility of the electrode process. [18,19] It should be also noted that in solutions with higher chlorates(VII) concentrations there is a significant increase in the limiting current and the waves become better defined. Such fluctuations in the limiting current are the evidence of changes in the viscosity of the base electrolyte solution.…”

“…It has been demonstrated that the adsorption of organic molecules increases due to the higher concentration of ClO 4 À ions which causes smaller hydration of a near-electrode surface. [19] The competitive adsorption of acetazolamide, water and to a small extent, ClO 4 À in the double layer interface should be considered. [25] The presence of active In-ACT complexes in the electrode mechanism in the electrolyte solutions with much lower water activity is not excluded either.…”