Spectrophotometric determination of sulphite, sulphate and dithionate in the presence of each other

Abstract

In the photochemical reaction of sulphuric acid esters of leuco vat dyes (vat esters), sulphite, sulphate and dithionate are produced, and a spectrophotometric method for the determination of these species in solution has been devised. An aliquot of solution is analysed for sulphite by the method using bleached Fuchsin N and formaldehyde solutions. Another aliquot is treated with hydrogen peroxide in the cold to oxidise sulphite to sulphate. The sulphate due to oxidation of sulphite plus the original sulphate is measured by the barium chloride and potassium chromate solutions method and the original sulphate is calculated by difference. A third aliquot is treated with hydrogen peroxide in the cold, then concentrated hydrochloric acid is added and the resulting solution is boiled. Dithionate is oxidised quantitatively to sulphate. Total sulphate is measured as before and sulphate due to dithionate is calculated by subtraction. Concentrations as low as 0.5 × 10^–5M sulphite and 1 × 10^–4M sulphate can be determined by this method.