Exemplifying performance of kinetics-sensitive double-step voltcoulometry: redox reactions of protons in unsupported acids

An alternative electroanalytical technique to the widely used differential pulse voltammetry (DPV) is presented in connection with monitoring proton redox reactions in strong acids (HCl, H2SO4, HNO3) and in l-ascorbic acid (AA) in the absence of any supporting electrolyte. Contrary to the DPV method, the current flowing through the working electrode in response to a double-step change of the applied potential is first integrated and subsequently processed by a three-channel correlator. Expressing the faradaic transient charge as Q(t)∝tβ, the ratio Rβ=[Q(t1)−2Q(5t1)+Q(9t1)/(Ilimt1), where t1 is the delay of the first sampling event with respect to the trailing edge of the potential double step and Ilim is the limiting current of the corresponding steady-state voltammetric wave, it is calculated and then compared to the values found experimentally. The sensitivity to the kinetics represented as dRβ/dβ has an optimum around β=0.5, a value consistent with the Cottrell equation. The experimental data point to a crucial role of CO2 (H2CO3) dissolved in the acid solution, envisaged as the reversed sign of the measured charge. After deaerating the solution by argon the sign became positive, nevertheless the experimental Rβ values were systematically higher than the predicted ones. The reaction of the protons of AA at negative potentials seems to be of the EC type when proceeding from negative to more positive potentials. Moreover, there is a dominant voltammetric wave of AA at positive potentials coming from an irreversible reaction, accompanied by a relatively weak peak of the correlated charge.