Ion Chromatographic Determination of L-Ascorbic Acid, Sulfite, Sulfide and Thiosulfate Using a Cation-Exchanger of Low Crosslinking

Abstract

l-Ascorbic acid, sulfite, sulfide and thiosulfate have each been completely separated from their mixtures by a single run of ion chromatography using columns of a sulfonated styrenedivinylbenzene copolymeric cation-exchange resin of low crosslinking (1%) and an eluent of 2.3 × 10⁻³ M (M=mol dm⁻³) sulfuric acid containing 4% (v/v) acetonitrile. l-Ascorbic acid and sulfite as sulfurous acid were eluted early by an ion-exclusion effect of the cation-exchanger in the hydrogen form, but both the sulfide as hydrogen sulfide and the thiosulfate ion were eluted late, owing to their adsorption onto the resin bed. The sample species in the effluent were monitored by photometric measurement of the excess of iodine (as triiodide) for its postcolumn reaction with each species separated. The chromatograms obtained gave negative peaks, based on the decrease in the absorbance from background. Calibration graphs for the four species, plotted as peak-heights vs. concentrations, were linear up to 3.00 × 10⁻⁵ M for l-ascorbic acid, 1.25 × 10⁻⁴ M for sulfite, 5.00 × 10⁻⁴ M for sulfide and 1.20 × 10⁻⁴ M for thiosulfate, respectively. Detection limits as S/N=3 were 1.00 × 10⁻⁷ M for l-ascorbic acid, 1.15 × 10⁻⁶ M for sulfite, 1.80 × 10⁻⁶ M for sulfide and 6.80 × 10⁻⁶ M for thiosulfate. The proposed method was successfully applied to the determination of l-ascorbic acid in soft drinks, sulfide in hot-spring waters and sulfite in wines.