Determination of trace sulfite by direct and indirect methods using differential pulse polarography

Sulfite determination plays an important role since it is used in some food stuff as protecting agent. The objective of this study was to develop a new and simple method for the determination of sulfite so that it can be used in routine food analysis. A differential pulse polarographic (DPP) method has been used for this purpose since the results obtained with this method are very reproducible and the instrument used is not expensive. Various acids and pH values have been studied such as 4 M, 1.0 M, 0.1 M HCl and HClO4, and HAc-NaAc electrolyte. The best medium was pH 3-5 acetate medium, where the reduction peak of sulfite at about -0.6 V was used. At lower pH values sulfite was escaping as SO2 while nitrogen gas was passing. At higher pH values, on the other hand, the peak was too small for the accurate determination. Interference of most common elements such as Fe, Cu, Pb and Cd was investigated and it was found that only Cd had an overlapping peak. This interaction could be eliminated using EDTA. The detection limit found was 2x10(-6) M in acetate buffer. An indirect method was developed also where the reaction between selenite and sulfite has been used. In this method a known amount of selenite was added and the selenite after reaction has been determined. The detection limit (7x10(-7) M) was lower than direct method. The quantity of sulfite present in dried apricots was found between 1.5 and 1.7 g kg(-1) of dried fruit.