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)-H2O-DTPA and would produce the electroactive species Cr(III)-NO3-DTPA and this last species would release 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 to . Furthermore, it was found that the electrochemical reaction is quite irreversible, with a constant of k (s) = 9.4 x 10(-5) cm s(-1), while the constant for the chemical step has been estimated to be k (chem) = 1.3 x 10(4) s(-1). Considering that the equilibrium constant is K = 0.01, it is possible to estimate the kinetic constants of the chemical reaction as k (1) = 1 x 10(2) s(-1) and k (-1) = 1.29 x 10(4) s(-1). These values of k (1) and k (-1) 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 EA degrees' ae -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.