Simple spectrophotometric method for the determination of sulfur dioxide by its decolorizing effect on the peroxovanadate complex

A simple, rapid, and economical spectrophotometric method is developed for the determination of sulfur dioxide in sugar and air samples. The developed method is based on a red-brown peroxovanadate complex (lambda(max) = 470 nm) produced in 2 M sulfuric acid when ammonium metavanadate is treated with hydrogen peroxide. Under fixed concentrations of hydrogen peroxide and ammonium metavanadate, when sodium metabisulfite (Na2S2O5 = 2SO(2)) is added, it preferentially reacts with hydrogen peroxide producing sulfuric acid, and the unreacted hydrogen peroxide then reacts with ammonium metavanadate; therefore, the concentration of sulfur dioxide is directly proportional to a decrease in the concentration of the peroxovanadate complex. The stoichiometric ratio between hydrogen peroxide and ammonium metavanadate as well as the stability constant of the complex are determined by the modified Job's method and the respective values are found to be 1 : 1 and 2.5 x 10(4) mol(-1) L, respectively. The system obeys Lambert-Beer's law in the concentration range 3.57-64.26 ppm of sulfur dioxide. The molar absorptivity, correlation coefficient, and Sandell's sensitivity values are found to be 0.649 x 10(3) L mol(-1) cm(-1), 0.9908, and 0.1972 mu g cm(-2), respectively. The method is applied to the determination of sulfur dioxide present in commercial sugars and air samples. The results obtained are reproducible with a standard deviation of 0.02-0.05. For method validation, sulfur dioxide is also determined separately following the AOAC method for an air sample and the ICUMSA method for commercial sugars. The results obtained by the developed and official methods are in good agreement.