Kaiser, J. (1998). “Sulfur isotope exchange and disproportionation of thiosulfate in chemical and microbial systems”. Master Thesis, Max Planck Institute for Marine Microbiology/University of Bremen. (Download)

In this thesis, a method for the synthesis of radioactive thiosulfate was established. The 35S label can be placed either at the outer (sulfane-)sulfur atom or at the inner (sulfonate-)sulfur atom. The reactants 35S0 and Na235SO3 are commercially available.

The chemical and radiochemical purities of the synthesised compounds was investigated by HPLC. Without special purification steps, it was between 92 and 96% for the molecule labelled at the outer sulfur atom. The radiochemical purity for the molecule labelled at the inner sulfur atom was only about 10%, since the reactant was signficantly contaminated by sulfate (ca. 75%) and a compound which could not be identified (ca. 13%).

The outer-labelled thiosulfate was used for isotope exchange experiments with hydrogen sulfide and polysulfide/hydrogen sulfide in anoxic, deionised water. Problems arose in the choice of a suitable reaction vessel, because the glass syringes with teflon gaskets originally designated for this purpose turned out to be unsuitable.

In addition to that, methods had to be developed for the separation of thiosulfate and sulfide and for the separation of the outer and inner sulfur atom in thiosulfate, in order to determine the activities of the individual fractions. The first separation was accomplished by precipitation with zinc salts, the second by decomposition with silver nitrate and subsequent dissolution of the silver sulfide precipitate in potassium cyanide solution.

No exchange was found for hydrogen sulfide only, neither at 20 °C nor at 80 °C. However, in the presence of polysulfide, a slow exchange was noted at 80 °C.

The exchange found experimentally was compared to theoretically expected values and measured data from a natural sediment. The degree of purely chemical exchange was too small to explain an isotope exchange of thiosulfate with other reduced sulfur compounds as it was postulated for some sediments.

In a hypersaline lake with high concentrations of reduced sulfur compounds, the role of thiosulfate in the sulfur cycle of the water column was investigated.