A steady state voltammetric procedure for the determination of hydrogen ions and total acid concentration in mixtures of a strong and a weak monoprotic acid

A linear sweep voltammetric (LSV) method for the simultaneous determination of free hydrogen ions and total concentration of the acids forming aqueous mixtures of a weak and a strong acid is proposed. The method is based on recording LSVs in the solutions investigated by a platinum microelectrode, and on the mathematical analysis of the voltammetric responses. In particular, the first derivative of the waves allows establishing the steady-state limiting currents due to hydrogen ions. Using the latter values, the overall LSV currents and a theoretical equation, which is valid for weak acids having dissociation constants higher than about 10-6, both hydrogen ions and total acid concentration can be evaluated. The validity and degree of accuracy of the proposed procedure is preliminarily assessed by digital simulation for mixtures of a strong acid and acetic acid (HAc). In general, an agreement between expected and evaluated (by the proposed method) concentration values within 2% is found. The proposed method is then tested in synthetic mixtures made by HClO4 and HAc at different concentrations. An application to some vinegar samples is also presented to individuate if these matrices were adulterated by strong mineral acids. For these experimental cases, the concentrations values evaluated with the proposed method and those determined by the classical titration procedure agree within 4%.