Catalytic cathodic stripping voltammetry at a hanging mercury drop electrode of glutathione in the presence of nickel ion

Abstract

Glutathione (GSH) catalyses the reduction of nickel ion and this effect is strongly enhanced if GSH is previously concentrated as a mercury thiolate on a hanging mercury drop electrode. During the subsequent cathodic scan, GSH is released by the reduction of mercury ion at about –0.4 V. The catalytic reduction of Ni²⁺ gives a cathodic peak at –0.6 V versus Ag—AgCl (3 mol l^–1 KCl) reference electrode. By this means GSH can be determined by differential-pulse stripping voltammetry under the following conditions: pH = 7 (phosphate–acetate buffer); Ni²⁺, 1 mmol l^–1; deposition potential, –0.1 to –0.3 V; scan rate, 10 mV s^–1; pulse height, 50 mV; and pulse interval, 0.5 s. The detection limit is about 10 nmol l^–1, i.e., ten times higher than for the similar determination of cysteine, owing to the particular characteristics of nickel ion complexation by GSH. The height of the catalytic peak is directly proportional to GSH concentration between 10 and 120 nmol l^–1. Very small interferences are produced by Zn²⁺, NaCl, low concentrations of strong complexants and N-acyl derivatives of cysteine, Cu²⁺, cysteine and human serum albumin interfere slightly. A detailed discussion of the relationship between the structure of GSH and its behaviour in catalytic cathodic stripping voltammetry is presented.