Determination of thiosulphate and sulphur

Further oxidation of elemental sulphur wiU lead to the formation of thiosulphate, polythionates, sulphite and finally sulphate  

It has also been suggested that the first product of biological sulphur oxidation is sulphite Goldhaber and Kaplan, 1974), which reacts abiologically with the original sulphur to produce thiosulphate  

It is still unknown how elemental sulphur is activated for further oxidation. The organisms largely responsible for the oxidation of sulphur belong to the genera Chromatium and Chlorobium. Tuttle and Jannasch (1972) suggested that heterotrophic oxidation might be 

Since thiosulphate is an intermediate oxidation product of sulphide in an alkahne or neutral medium, sulphide interferes with the determination of thiosulphate and, thus, any traces of sulphide should be removed by the precipitation of zinc sulphide with zinc acetate before the iodometric determination of thiosulphate. The solubility constant for ZnS has been given by Dyrssen and KremUng (1990). After precipitation of the zinc sulphide in a stoppered bottle full to the brim, a measured amount of the supernatant seawater sample is pipetted into a known amount of iodine. The latter oxidizes the thiosulphate to tetrathionate. The surplus iodine is then titrated by adding a thiosulphate solution of known concentration. 

Ammonia will not be oxidized to nitrite before the reaction of iodine with the thiosulphate in the sample has taken place. Nitrite will also react with iodine therefore, the surplus iodine must be determined rapidly in order to avoid formation of additional nitrite by oxidation of ammonia. The reactions are as follows  

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