Extraction–spectrophotometric determination of sulphur dioxide

Abstract

A sensitive spectrophotometric method for the determination of trace amounts of sulphur dioxide after fixing in a modified buffered formaldehyde solution is described. The reaction of iodate with sulphur dioxide in the presence of acid and excess of chloride leads to the formation of ICI which is stabilized as the ICI^–₂ ion. The species formed reacts with 2′,7′-dichlorofluorescein to form 2′,7′-dichloro-4′,5′-diiodofluorescein and is extracted into a solvent mixture of 15% isopentyl acetate in isopentyl alcohol. The colour system obeys Beer's law in the range 0–40 µg of sulphur dioxide. The relative standard deviation is 3.5% for ten determinations of 15 µg of sulphur dioxide. The effect of interfering gases on the determination is discussed. The method has been applied to the determination of sulphur dioxide at low concentrations and the results obtained were compared with the widely used pararosaniline method. The method can be used to determine as low as 2 µg of sulphur dioxide.