Sodiumthiosulphates (Na2S2O3.5H2O)

It is prepared by boiling sulphur with an aqueous solution of sodium sulphate
Na2SO3 + 1/8 S8 ------> Na2S2O3
Sodium thiosulphate is a water soluble crystalline substance.
a) Reaction with iodine
Sodium thiosulphate is oxidised by iodine to form sodium tetra thionate. This reaction is the basis of iodometric titrations.
2Na2S2O3 + I2 ---------> Na2S4O6 + 2NaI
b) Reaction with chlorine
Sodium thiosulphate can remove excess chlorine by forming HCl. Hence it is used as an antichlor.
Na2S2O3 + Cl2 + H2O -----------> Na2SO4 + 2HCl + S
c) Reaction with silver halides
Silver halides dissolve in sodium thiosulphate solution due to formation of a complex sodium argento thiosulphate. This reaction is the basis of its use in photography as fixer.
AgCl + Na2S2O3 -------------> NaAgS2O3 + NaCl

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