Influence du pH et de la lumiere sur la stabilité de quelques antioxydants

Synopsis L'influence du pH et de la lumière sur la stabilité intrinsèque de quelques antioxydants naturels et synthétiques a étéétudiée en milieu hydroalcoolique. Des différences importantes de stabilité ont été observées selon le pH du milieu et la présence ou l'absence de lumière. Des mesures ont été effectuées également dans des émulsions H/E contenant un antioxydant. II découle de cette étude que lors du choix d'un antioxydant pour protéger un milieu donné, on doit vérifier au préalable que le pH du milieu et la présence éventuelle de lumière n'affectent pas sa stabilité intrinsèque. Les antioxydants ont été mesurés par voltamétrie anodique. Influence of pH and light on the stability of some antioxidantsSummary Many organic molecules can be oxidized in the presence of oxygen. Light and traces of heavy metal ions catalyse the process of oxidation. The addition of a very small quantity of antioxidant to alcoholic perfumes and cosmetic bases is often made to retard auto‐oxidations. Among the parameters which could influence the efficiency of an antioxidant, its intrinsic stability should be considered in the medium to be protected. This stability might conceivably be influenced by the pH, the presence of light, heavy metal ions and microorganisms. In this study we have concentrated on the role played by the first two factors. To eliminate a possible interference by the last two, analytical grades reagents together with chelators and high proof (80% v/v) hydroalcoholic solutions have been used. The antioxidants tested were: BHT, BHA, ethyl gallate, 2, carboxy‐6, hydroxy, 2, 5, 7, 8, tetramethyl chroman (Trolox C R ) and D–L‐alpha‐tocopherol. Solutions of 0.5 mmol/kg of each antioxidant were prepared in 80% v/v hydroalcoholic solutions and the pH adjusted with citric acid and potassium hydroxyde or hydrochloric acid. The pH extended from 2.5 to 10 and thus largely covers the pH range of cosmetic products. Of each solution, 100ml were kept in hermetically closed 125ml white glass bottles stored at room temperature (22 ± 2° C) and kept in the dark or exposed to the diffuse daylight of the laboratory. The antioxidants concentrations were determined by linear sweep voltametry on gold or glassy carbon electrodes. Significant differences in behaviour were observed. BHA and BHT are stable regardless of light and pH except at high pH (≤9). Ethyl gallate vanishes rapidly in alkaline, neutral and even weakly acid pH independently of the presence of light. Trolox C and D–L‐alpha‐tocopherol demonstrated a marked influence of the presence of daylight and of the pH.