The kinetics of the hydrogen evolution reaction on zinc in EDTA solutions in the pH 3–10 range

Polarization curves of the hydrogen evolution reaction on zinc were obtained in Ethylenediaminetetraacetic acid (EDTA) solutions of different total molar concentrations (0.05, 0.10, 0.15 and 0.20 mol dm−3), the pH values of which were systematically varied (pH 3.0–10.0). The Tafel slopes of the cathodic polarization curves are −120 to −145 mV dec−1 at lower currents densities (approximately 10−5–5×10−4 A cm−2), while the higher current densities (approximately 5×10−4–10−4 A cm−2) the slopes are in the range −190 to −370 mV dec−1. The apparent cathodic reaction orders with respect to the hydrogen ions and EDTA, which were obtained from the cathodic polarization curves at the lower current densities, are z−(H+)≅1/2 and z−(H4Y)≅1, respectively. On the basis of these results, a mechanism of the hydrogen evolution reaction on zinc in the EDTA solutions examined, where protonated EDTA species (the protonated EDTA anions and the acid itself) participate directly as reactants is proposed. The unusual polarization behavior of the zinc electrode in the higher current density range is discussed.