Spectrophotometric Determination of Sulphite

Similar to hydrogen sulphide and sulphides, sulphur dioxide and sulphites can be determined by titration with iodine or by oxidation to sulphate followed by gravimetric determination as barium sulphate. Additionally a spectrophotometric method was developed based on the colour of p-nitroaniline which is discharged on acidification but is restored by adding formaldehyde and sulphur dioxide or sulphite. 

The reaction is rather slow but is best carried out in dilute solution. To avoid the aerial oxidation of the sulphite, it is added to a known excess of standardised iodine solution, back titrating the excess with thiosulphate. 

Prepare freshly 0.05 M iodine solution. Pipette 25.0 cm of this solution into a 400 cm conical flask, add 5 cm 2 M HCl and dilute with water to 180 cm. Prepare freshly a solution which is 0.05M in sulphite or SO2. Add from a burette 15-20 cm of this solution to the iodine solution. This should be done slowly with stirring and keeping the tip of the burette close to the iodine solution. Then titrate the excess of iodine against standardised 0.01 M thiosulphate solution prepared by 5-fold dilution of 0.05 M solution. Follow the procedure in Sec.7.3.1. Alternatively weigh out accurately 0.09-0.13 g of the purest sulphite and dissolve it in the acidified diluted iodine solution and continue as above. 

Sulphites are oxidised when boiled with an excess of ammoniacal hydrogen peroxide. The sulphate is then precipitated as barium sulphate. 

Dissolve 0.138 g of purest p-nitroaniline in 25 cm ethanol and make up to 1 dm with water. Dilute formaldehyde to make a 0.1 M stock solution. 

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