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

Abstract

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 ${\mathrm{NO}}_3^{-}$ would produce the electroactive species Cr(III)–NO₃–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 _s = 9.4 × 10⁻⁵ cm s⁻¹, while the constant for the chemical step has been estimated to be k _chem = 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 k ₁ = 1 × 10² s⁻¹ and k ₋₁ = 1.29 × 10⁴ 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 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.