Sulfur speciation and tetrathionate sulfitolysis monitoring by capillary electrophoresis

Capillary electrophoresis (CE) with indirect detection was used to analyse mixtures of sulfur-containing compounds. An optimised electrolyte, composed of 2 mM sulfosalicylic acid–0.5 mM Waters osmotic flow modifier OFM-OH, pH adjusted to 7.00 with Bis–Tris, allowed the analysis of sulfide (H2S/HS−), thiosulfate (S2O32−), tetrathionate (S4O62−), trithionate (S3O62−), sulfite (HSO3−/SO32−), sulfate (SO42−), and peroxodisulfate (S2O82−). Only sulfate showed a single-peak electropherogram that gives evidence of its stability in aqueous solutions. Thiosulfate, tetrathionate and peroxodisulfate showed an additional minor peak of sulfate that did not seem to be time-dependent and was supposed to be due to salt impurities. Sulfide and sulfite showed rapid conversion into their oxidation products in solutions exposed to air. Linear calibration curves were obtained for all these species, taking the oxidation process into account for sulfide and sulfite. Limits of detection were in the low- to mid-μM range, except for sulfide, for which the limit of detection was 10−5 M. The identity of trithionate was not determined via standard solutions (not commercially available) but after the rearrangement of tetrathionate with sulfite that yields thiosulfate and trithionate. The time-dependence of the concentrations of both the reagents and products could be followed simultaneously by the proposed CE method and the reaction kinetics were found to be second order in the pH range 7 to 11. A calibration curve for trithionate could be deduced from this study.