Semi-automatic catalytic titration method for the analysis of cystine-cysteine mixtures and its application to urine samples

Abstract

A semi-automatic photometric catalytic titration method for the analysis of cystine-cysteine mixtures is proposed. The procedure is based on the inhibitory effect of mercury(II) on the oxidation of 4,4′-dihydroxybenzophenone thiosemicarbazone (used as titrant reagent) by hydrogen peroxide in the presence of copper(II) as catalyst. The pseudo-induction period of the titration curve, which is directly related to the mercury concentration, decreases owing to the complexation of cystine-cysteine with mercury(II). The difference between the volume of 4,4′-dihydroxybenzophenone thiosemicarbazone added in the absence and presence of amino acid for the same initial amount of mercury is related to the concentration of the amino acid in the sample. Based on this principle, two sets of reaction conditions, in the absence and presence of hydroxylamine, allowed both cystine and cysteine to be determined. The results obtained demonstrate the suitability of the proposed method for the determination of these amino acids in urine samples.