Effect of reagent (dye) addition in wet chemical method of sulfur dioxide determination in ambient air.

For measurement of sulfur dioxide (SO2) in ambient air, the United States Environment Protection Agency (US EPA) had recommended the PRA dye-based colorimetric method as a reference technique. The method has been developed and applied in many countries for a longtime; however information regarding the sensitivity of the method with respect to sampling and analysis conditions is not available. Collaborative studies conducted in some of the South East Asian countries indicated substantial variations (of the order of 50%) in the measured concentrations against the true values. It was observed that dye used for color development plays an important role and even a slight variation in the way the dye solution is prepared can cause substantial variation in the measured SO2 concentration. Because a major objective of air quality measurements is to relate air pollution concentration to the effects of air pollution, it is important that the method used should yield accurate levels of the pollutant, so that appropriate management plans can be devised and implemented effectively. In the present investigation, therefore, the role of dye used in SO2 monitoring method has been highlighted. Different makes of dyes prepared in different modes were used to study the variation in the measured SO2 concentration levels. Specifications of all the dyes were also tested. Need for use of certified reference material (CRM) for SO2 has also been emphasized.