Theoretical and experimental study of the catalytic cathodic stripping square-wave voltammetry of chromium species.

The electrocatalytic mechanism of Cr(III) reduction in the presence of diethylenetriaminepentaacetic acid (DTPA) and nitrate ions is studied theoretically and experimentally by using stripping square-wave voltammetry (SWV). Experimental curves are in excellent agreement with theoretical profiles corresponding to a catalytic reaction of second kind. This type of mechanism is equivalent to a CE mechanism, where the chemical reaction produces the electroactive species. Accordingly, the reaction of Cr(III)-HO-DTPA and $$ {\mathrm{NO}}_3^{-} $$ would produce the electroactive species Cr(III)-NO-DTPA and this last species would release $$ {\mathrm{NO}}_2^{-} $$ to the solution during the electrochemical step. In this regard, the complex of Cr(III)-DTPA would work as the catalyzer that allows the reduction of $$ {\mathrm{NO}}_3^{-} $$ to $$ {\mathrm{NO}}_2^{-} $$ . Furthermore, it was found that the electrochemical reaction is quite irreversible, with a constant of k = 9.4 × 10 cm s, while the constant for the chemical step has been estimated to be k = 1.3 × 10 s. Considering that the equilibrium constant is K = 0.01, it is possible to estimate the kinetic constants of the chemical reaction as k = 1 × 10 s and k = 1.29 × 10 s. These values of k and k indicate that the exchange of water molecules by nitrate is fast and that the equilibrium favors the complex with water. Also, a value for the formal potential E°' ≈ −1.1 V was obtained. The model used for simulating experimental curves does not consider the adsorption of reactants yet. Accordingly, weak adsorption of reagents should be expected. [ABSTRACT FROM AUTHOR]