Sodiumthiosulphates (Na2S2O3.5H2O)
It is prepared by boiling sulphur with an aqueous solution of sodium sulphate Na2SO3 + 1/8 S8 ------> Na2S2O3 Properties 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