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

ORTIZ, PATRICIA I.; CUÉLLAR, MARIELA; GARAY, FERNANDO; VETTORELO, SABRINA N.

JOURNAL OF SOLID STATE ELECTROCHEMISTRY (PRINT)

SPRINGER

Año: 2016 vol. 20 p. 3279 - 3286

1432-8488

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 NO3- would produce the electroactive species Cr(III)?NO3?DTPA and this last species would release NO2- 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 NO3- to NO2-. Furthermore, it was found that the electrochemical reaction is quite irreversible, with a constant of ks = 9.4 × 10−5 cm s−1, while the constant for the chemical step has been estimated to be kchem = 1.3 × 104 s−1. Considering that the equilibrium constant is K = 0.01, it is possible to estimate the kinetic constants of the chemical reaction as k1 = 1 × 102 s−1 and k−1 = 1.29 × 104 s−1. These values of k1 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 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.