Study of photostability of three synthetic dyes commonly used in mouthwashes

Photostability of three synthetic dyes (Carmoisine, CI 14720; Allura Red AC, CI 16035; and Patent Blue V, CI 42051) commonly used in mouthwashes was investigated for the first time. Aqueous solutions of dyes and commercial mouthwashes were submitted to simulated outdoor and indoor conditions of irradiation. All dyes demonstrated high stability in aqueous solutions but discolouration was observed in commercial products after irradiation, highlighting a strong effect of matrix composition on the dyes stability. The photochemical behaviour of dyes corresponded to a first-order reaction. Degradation half-life times of Carmoisine, Allura Red AC, and Patent Blue V under solar standard conditions of irradiation were 18.4 ± 3.3, 11.9 ± 1.6, and 6.6 ± 1.2 h respectively, for 5.0 μg mL⁻¹ of dye spiked in a blank matrix (a mouthwash without the presence of dyes among the ingredients). Factors potentially affecting the photostability, such as temperature and the presence of oxygen, also were studied.