Dithionite, Sulphite and Thiosulphate

Although dithionites can be determined by reaction with iodine in a similar way to that described above for sulphites, and thiosulphates, the problem is the disproportionation of the dithionite. 

A method was therefore devised to determine the three anions simultaneously. This is based on their oxidation by hexacyanoferrate(III) under different alkali concentration and using osmium tetroxide as a catalyst. The corresponding reactions are  

The tetrathionate formed from the reaction of thiosulphate with iodine is also oxidised by the iron(III) complex  

Na2S406+14K3[Fe(CN)6]+14K0H=Na2S04+14K4[Fe(CN)ej]+4H20+3H2S04 (7.10) 

When the alkali concentration is 0.5-1.0M, dithionite can be titrated with standard hexacyanoferrate(IIl) solution. Because of the possibility of oxidation in air, the titration vessel should be provided with an inlet and an outlet for purging and maintaining an oxygen-free nitrogen atmosphere. Under these conditions, the dithionite is oxidised to sulphite. When the alkali concentration is raised to 4-4.5 M and the temperature is raised to 50°-60°C, the sulphite thus produced or already present together with the thiosulphate, the anions are oxidised by the iron(IIl) complex to sulphate. Titration is carried out in presence of osmium tetroxide as a catalyst. A second titration with iodine in acid solution oxidises the thiosulphate to tetrathionate. This can be titrated with the iron(III) complex at 50°C when the alkali concentration is 5 M. The end-point could be detected potentiometrically or by the dead-stop set up. 

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A recent study has shown that the same reaction may take place smoothly at room temperature in 7—IOM-H2SO4. Redox potentials of aqueous solutions containing dithionite, sulphite, and thiosulphate and for the irreversible system sulphite-dithionite have been measured. 

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