To evaluate the antioxygenic property of the antioxidants, the oxidation reaction rate constants of sodium sulfite, sodium bisulfite and sodium pyrosulfite in aqueous solutions were determined. In the experiments, sufficient air was continually inflated into the solution to keep the concentration of dissolved oxygen constant. The residual concentrations of sodium sulfite, sodium bisulfite or sodium pyrosulfite in the solution were respectively determined by iodimetry, and the concentration of dissolved oxygen by oxygen electrode. The data were fitted by linear regressions to obtain the reaction rate constants. It was found that the degradation of sodium sulfite, sodium bisulfite or sodium pyrosulfite obeyed the zero-order kinetics in the buffer solutions. Because of the ionization equilibrium, the same ions were formed when the three antioxidants respectively dissolved in aqueous solutions of definite pH values, and therefore their apparent rate constants were essentially the same. The average apparent rate constants of the three antioxidants at 25 ℃ are (1.34±0.03)×10^－3 mol•L^－1•h^－1 at pH 6.8, (1.20±0.02)×10^－3 mol•L^－1•h^－1 at pH 4.0 and (6.58±0.02)×10^－3 mol•L^－1•h^－1 at pH 9.2, respectively.

Key words: drug stability, oxygen, oxidation rate constant, antioxidant, sodium sulfite, sodium bisulfite, sodium pyrosulfite