Co-determination of sodium metabisulfite and starch in corn syrup by flow injection coulometry

During the processing of corn syrup for commercial use, starch, in the form of alpha -amylose, must be completely broken down to its D -glucopyranose units. Sodium metabisulfite is added to the corn syrup as a preservative. Flow Injection Coulometry was used to perform an automated assay of these analytes, both individually and jointly. The sodium metabisulfite concentration, over a range of 3.5 x 10(-4)-2.9 x 10(-2)M, is determined by coulometric flow injection titration with generated iodine, using spectrophotometric endpoint detection at 530 nm. Analysis over this range produced a relative standard deviation of < 1.5% and was found to correlate very well with manual titrations. The determination was performed in the presence of varying amounts of starch, and was found to be independent of the starch concentration. Starch was determined, when no sodium metabisulfite was present, from the absorbance level after the reaction of the sample with a specific amount of iodine. In the presence of sodium metabisulfite, the rate of the accumulation of the starch/iodine interaction product after the metabisulfite titration endpoint, at a constant reagent generation rate, was used. A relative standard deviation of < 1.4% was obtained for all starch analyses, with a very good correlation (correlation coefficients 0.997) with the known relative concentration. The use of the FIC technique to perform analyses by specific amount and excess reagent generation is demonstrated, along with dual analyte determination.