Chemically treated silver electrodes for the determination of cysteine

This paper describes the use of Ag/Ag2S and Ag/AgI ion-selective electrodes by means of chemical pretreatment of a silver wire or tube for the determination of cysteine (RSH) in batch and flow injection analysis (FIA). Batch experiments were made with a Ag/Ag2S wire electrode in an acidic medium (0.1 mol/l HClO4) in water solutions. Equilibrium potentials measured by the serial dilution of Ag+ and serial addition of cysteine at constant ionic-strength and pH are shown in dependence on log c(Ag+) and log c(cys) and for cysteine indicate constant linear response within the concentration range between 3.1 x 10(-5) and 2.0 x 10(-2) mol/l at the slope of 56.7+/-1.0 mV {p(cys)}(-1). These results suggest the formation of an RSAg layer on the exposed electrode surface. In the FIA a carrier stream containing 1.0 x 10(-5) mol/l Ag+ + 0.1 mol/l HClO4 was used and the Ag/Ag2S electrode was then characterised by a constant linear response within the concentration range between 5.0 x 10(-5) and 5.0 x 10(-3) mol/l at the slope of 53.4+/-1.0 mV {p(cys)}(-1). Measurements of cysteine with Ag/AgI electrode are enabled by the reduction of iodine in the carrier stream. At optimum FIA parameters (flow rate, tube length, sample volume and the composition of the carrier stream were optimised) the electrode is characterised by a constant linear response within the concentration range between 5.0 x 10(-5) and 1.0 x 10(-2) Mol/l at the slope of 59.9 1.0 mV {p(cys)}(-1).