Conversion of Thiosulphate to Tetrathionate

When iodine solution in alcohol reacts with a saturated solution of thiosulphate, tetrathionate forms as a crystalline product. 

Prepare a saturated solution of sodium thiosulphate in water and a solution of iodine in ethanol. Add the thiosulphate solution dropwise to the iodine solution cooled in an ice bath, shaking the mixture after each addition until the colour of the iodine fades to a very pale yellow. Allow the tetrathionate formed to crystallise. Collect the crystals by filtration under suction. Wash on the filter with small volumes of ethanol. Dissolve the product in the minimum amount of water and add ethanol to reprecipitate die tetrathionate. This is filtered under suction and washed with small volumes of ethanol and dried by continued suction and then kept in a desiccator with concentrated sulphuric acid as a desiccant. It can be analysed as in Sec.7.3.S. 

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