Simultaneous determination of microamounts of thiosulphate and tetrathionate by means of the induced iodine-azide reaction

The iodine-azide reaction is applied for the determination of thiosulphate a nd tetrathionate in the presence of each other by simple titrimetry. In one part of the sample only thiosulphate is determined. In the second part tetrathionate is stoichiometrically converted to thiosulphate by treating with sulphite: S4O62- + SO32- --> S2O32- + S3O62- and the sum of thiosulphate present in the sample and that formed from tetrathionate is determined. Excess of sulphite is masked with formaldehyde. From these two determinations the amount of both compounds is estimated. The method was applied in determination of thiosulphate and tetrathionate in their mixtures. The proposed mettled allows the determination of both compounds in the concentration range from 2.5x10(-4) to 5x10(-3) mol l(-)1 with accuracy +/- 2.5% for 2-ml samples.