Low Frequency-square wave voltammetry of weak acids at platinum Microelectrodes

The reduction process of monoprotic weak acids (lactic and acetic acid, and dihydrogen phosphate ion) and mixtures of a strong and a weak acid (perchloric and one of the above weak acids) have been investigated by square-wave voltammetry (SWV) at a platinum microelectrode 12.5 μm radius. The SW measurements have been performed at low frequencies (i.e., 3 Hz), where the heterogeneous electron transfer process, involving hydrogen evolution (2H + + 2e = H2), is essentially reversible, and non-planar diffusion conditions prevail in the mass transport. Under these conditions, the forward and the reverse current-potential profiles are sigmoidal, whereas the net current-potential profile is peak shaped. The influence of dissociation constant and analytical concentration of the weak acid on the features of the square-wave voltammograms has been examined in detail. In general, both these characteristics of the weak acids affect the square-wave parameters peak potential (Ep), width at half height (W1/2) and net peak current (ip) in a complex manner. Their dependence on concentration is due to the 2:1 stoichiometry of the heterogeneous electron transfer, while the dissociation constant contribution arises because of the CE mechanism involved in the overall electrode process. The kinetic of the homogeneous reaction has been investigated by the use of the forward current plateau (achieved by the end of the sweep) along with a theoretical equation. Mixtures of a strong and a weak acid can provide one or two waves, depending on the acid strength and on the analytical concentration of each acid. Peak potential differences of about 80 mV are needed to discern two peaks. Calibration plots, examined for all the weak acids, have shown that linearity may exist only over relatively small concentration ranges.