Evaluation of antioxidants using oxidation reaction rate constants |
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Authors: | Yan Shi Xiancheng Zhan Lie Ma Linli Li Chengrong Li |
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Institution: | (1) Key Laboratory of Drug Targeting, West China School of Pharmacy, Sichuan University, Chengdu, 610041, China |
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Abstract: | An evaluation method for the capacity of antioxidants to protect drugs against oxidation is presented. As a new viewpoint,
to determine the priority of the competitive oxidations between the antioxidant and the protected drug, and to compare the
drug-protection capacity of antioxidants, it is important to determine their oxidation rate constants using chemical kinetics
instead of standard oxidation (or reduction) potentials. Sodium sulfite, sodium bisulfite and sodium pyrosulfite were used
as models for the determination of oxidation reaction rate constants in aqueous solutions. In the experiments, sufficient
air was continually infused into the solution to keep the concentration of dissolved oxygen constant. The residual concentrations
of the antioxidants were 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 pseudo zero-order kinetics in the buffer solutions. Because of the ionization
equilibrium, these three antioxidants have the same ion form in solutions at a definite pH value and therefore their apparent
rate constants were essentially the same. The average apparent rate constants of the three antioxidants at 25°C are (1.34
± 0.03) × 10−3 at pH 6.8, (1.20 ± 0.02) × 10−3 at pH 4.0 and (6.58 ± 0.02) × 10−3 mol·L−1·h−1 at pH 9.2, respectively.
Translated from Acta Chimica Sinica, 2006, 64(6): 496–500 (in Chinese) |
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Keywords: | antioxidant drug stability oxygen oxidation rate constant sodium sulfite sodium bisulfite sodium pyrosulfite |
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