APPLICATION OF CATALYTIC STRIPPING VOLTAMMETRY FOR THE DETERMINATION OF ORGANIC SULFUR-COMPOUNDS AT A HANGING MERCURY DROP ELECTRODE - BEHAVIOR OF CYSTEINE, CYSTINE AND N-ACETYLCYSTEINE IN THE PRESENCE OF NICKEL ION

The differential-pulse cathodic stripping voltammetry of cysteine (CysH), cystine (Cys) and N-acetylcysteine (Ac-CysH) was studied at a hanging mercury drop electrode at pH 7 in the presence of nickel ion. 3-(N-Morpholino)propanesulfonic acid (MOPS) and phosphate-acetate buffer were used as supporting electrolytes. In the presence of CysH, after accumulation at potentials ranging between 0.0 and -0.4 V, the catalytic reduction of nickel ion gives a peak at -0.6 V versus the Ag-AgCl reference electrode. An additional effect of nickel ion is the suppression of the cathodic stripping peak due to mercury cysteinate reduction, thus permitting the simultaneous determination of another thiol with no catalytic activity (e.g., Ac-CysH). Consequently, CysH (or Cys) and Ac-CysH can be determined simultaneously or independently in the same sample. Cys is reduced at potentials preceding the catalytic peak, which is actually due to the CysH thus produced. Some differences between the behaviour of Cys and CysH are due to different accumulation mechanisms. The catalytic stripping voltammetry of CysH or Cys exhibits good sensitivity (detection limit about 1 nmol dm(-3) for 3 min accumulation). The stripping voltammetric method described appears to be the first involving adsorptive accumulation of a metal complex in which the organic ligand is determined catalytically. Further, this particular method affords some selectivity in the determination of sulfur compounds, which are normally determined by cathodic stripping voltammetry of their anodically accumulated mercury complexes.