Spectrophotometric determination of sulfur dioxide in air, using thymol blue

A simple and sensitive spectrophotometric method was developed for the determination of trace amounts of sulfur dioxide. The method is based on the reaction of SO2 with a known excess of ICI as the oxidant. The unreacted ICI iodinates thymol blue under acidic conditions. The λmax of thymol blue is at 545 nm under acidic conditions, and on iodination λmax shifts to 430 nm. This shift results in a decrease in the absorbance at 545 nm. The amount of uniodinated thymol blue present depends on the concentration of unreacted ICI, which in turn depends on the SO2 concentration. The system obeys Beer's law in the range 0-30 μg SO2 in a final volume of 25 mL, having a molar absorptivity of 3.2 × 104 L/mol·cm with a relative standard deviation (RSD) of 2% at 24 μg SO2 (n = 10). The uniodinated dye can be extracted into 5 mL isoamyl alcohol under acidic conditions for measurement of absorbance. The extraction method obeys Beer's law in the range 0-5 μg SO2, having a molar absorpitivity of 4.16 × 104 L/mol·cm with an RSD of 1.9% at 4 μg SO2 (n = 10). The method has been successfully applied to the determination of atmospheric SO2.