Photostability of Crystal Violet (CI 42555)

Commercially available Crystal Violet, when subjected to adsorption chromatography, was shown to contain Crystal Violet, Methyl Violet 2B and Methyl Violet 6B. All three compounds were stable on ii-radiation with simulated sunlight. In the presence of equimolar amounts of hydrogen peroxide these dyes were rapidly decomposed by light, the reaction following second-order kinetics. Addition of acetaldehyde inhibited the photochemical degradation of the dyes, suggesting some hydroperoxy radical involvement in this reaction. Incorporation of mannitol reduced the rate constants of the reactions when hydrogen peroxide was present: this suggests some hydroxyl radical mediation. These findings may have considerable implications when Crystal Violet (pure or impure) is used in bacteriological assays which involve differentiation of microbial colonies by colour detection alone.