Determination of total ionic polysulfides by differential pulse polarography

Ionic polysulfides occur in various natural environments and waste waters from industrial processes; however, their direct determination in samples containing additional sulfur species has remained a challenge. Our goal was to develop a polarographic technique that could be used to directly determine total ionic polysulfides in the presence of other sulfur species such as polythionates and sulfide. Ionic polysulfides in an aqueous carbonate buffer (pH 9.5) were measured using differential pulse polarography (DPP) in combination with a dropping mercury electrode. An adsorption phenomenon associated with polysulfide or a product of its cathodic reaction with the Hg electrode created a characteristic current minimum at -1000mV with a magnitude proportional to the concentration of S(0) existing in the form of polysulfidic sulfur. Sulfide and thiosulfate did not interfere and polythionates caused only a decrease in the magnitude of the minimum. The specificity of the technique and the lack of significant interferences make it an attractive alternative for the measurements of samples with polysulfidic sulfur concentrations between 10-5 and 10-3M.