Electrochemical determination of thiosulfate in seawater in the presence of elemental sulfur and sulfide

Linear sweep voltammetry (LSV) has been used for the direct determination 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 supporting electrolyte and in natural seawater samples. The determination is based on anodic depolarization reaction between sulfur and the mercury electrode. Investigations were carried out over wide concentration ranges, from 10(-8) to 10(-4) M of sulfide and 10(-6) to 10(-4) M of thiosulfate in the solution. In seawater conditions (pH 8.3) thiosulfate displays a voltammetric peak at about -0.15 V, and sulfide and/or elemental sulfur between -0.6 and -0.7 V (vs. Ag/AgCl). Based on the appearance of the voltammetric peaks of thiosulfate and sulfide at different potentials, and using different experimental conditions, i.e., accumulation 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 determination in seawater can be used if the concentrations of sulfide are lower than 5x10(-5) M. Higher concentrations of sulfide have to be decreased by acidifying and degassing the sample to prevent interferences. The method was applied for the analysis of sulfur species in marine samples from anoxic environment.