Determination of thiosulphate with iodine

Dilute solutions of sodium thiosulphate (e.g. 0.001 M) may be titrated with dilute iodine solutions (e.g. 0.005M) at zero applied voltage. For satisfactory results, the thiosulphate solution should be present in a supporting electrolyte which is 0.1 M in potassium chloride and 0.004 M in potassium iodide. Under these conditions no diffusion current is detected until after the equivalence point when excess of iodine is reduced at the electrode a reversed L-type of titration graph is obtained. 

Dilute solutions of iodine, e.g. 0.0001 M, may be titrated similarly with standard thiosulphate. The supporting electrolyte consists of 1.0 M hydrochloric acid and 0.004M potassium iodide. No external e.m.f. is required when an S.C.E. is employed as reference electrode. 

Reagents, ca 0.001 M Sodium thiosulphate solution, 0.1 M with respect to potassium chloride and 0.004M with respect to potassium iodide. 

Standard 0.005M iodine solution in 0.004 M potassium iodide. 

Procedure. Place 25.0 mL of the thiosulphate solution in the titration cell. Set the applied voltage to zero with respect to the S.C.E. after connecting the rotating platinum micro-electrode to the polarising unit. Adjust the range of the micro-ammeter. Titrate with the standard 0.005 M iodine solution in the usual manner. 

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