Solvent-Floatation-Photometric Determination of SO_3^2- in Foodstuff

In a phosphate buffer medium of pH 7.0, free iodine liberated through the oxidation of I- by IO₃ ^-, reacts with excess I- to form a binary complex of I₃ ^-, which reacts further with CTMAB to form a ternary ion association complex of CTMA⁺ldrI₃ ^-. This ternary complex is easily soluble and kept stable in organic solvents, in which it has absorption maximum at 365 nm. In the presence trace amounts of SO₃ ^2- ion, the I₃ ^- in the complex is reduced, leading to a decrease in absorbance of the ternary complex (reductive color-fading) in organic solvent. It was found that the magnitude of decrease in absorbance (DeltaA) keeps a linear relationship with the concentration of SO₃ ^2- in the range of 0~30 mug per 50 mL of solution. Based on this phenomenon and applying the solvent floatation of technique with 5.0 mL of benzene, the sensitivity of this method is remarkably enhanced, epsiv₃₆₅=3.0times10⁵ Lldrmol^-1ldrcm^-1. In the application of this method to the determination of SO₃ ^2- in vegetables and wine samples, RSD <4.3% (n=5), and recoveries found by standard addition method were in the range from 97% to 100%.