ELECTROCHEMICAL DETERMINATION OF THIOSULFATE IN SEAWATER IN THE PRESENCE OF ELEMENTAL SULFUR AND SULFIDE

Linear sweep voltammetry (LSV) has been used for the direct determinat ion of thiosulfate in the mixture with other sulfur species (sulfide, elemental sulfur) in 0.5 M NaCl/3x10(-2) M NaHCO3, pH 8.3 as supportin g electrolyte and in natural seawater samples. The determination is ba sed on anodic depolarization reaction between sulfur and the mercury e lectrode. Investigations were carried out over wide concentration rang es, from 10(-8) to 10(-4) M of sulfide and 10(-6) to 10(-4) M of thios ulfate in the solution. In seawater conditions (pH 8.3) thiosulfate di splays a voltammetric peak at about -0.15 V, and sulfide and/or elemen tal sulfur between -0.6 and -0.7 V (vs. Ag/AgCl). Based on the appeara nce of the voltammetric peaks of thiosulfate and sulfide at different potentials, and using different experimental conditions, i.e., accumul ation at different starting potentials [E=0 or E=-0.2 V (vs. Ag/AgCl)] , a method for the direct determination of thiosulfate and sulfide in the mixture has been proposed. The direct method of thiosulfate determ ination in seawater can be used if the concentrations of sulfide are l ower than 5x10(-5) M. Higher concentrations of sulfide have to be decr eased by acidifying and degassing the sample to prevent interferences. The method was applied for the analysis of sulfur species in marine s amples from anoxic environment.