Conversion of Sulphite to Thiosulphate

Sulphur dissolves in boiling sulphite solution to form thiosulphate. 

Grind flowers of sulphur to a fine powder and weigh out 2 g and add it to a hot solution of 6 g sodium sulphite in 30 cm water. Boil the suspension until most of the sulphur has dissolved. Filter while hot under suction collecting the filtrate in a clean receiver. Evaporate the filtrate until crystallisation starts. Cool in an ice bath to allow further crystallisation. Filter under suction and dry in an oven at 40 C. Record the weight of your product and calculate the % yield based on sulphite. The purity of the thiosulphate produced can be assayed by titration as in Sec.7.3.3. 

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