Photolysis of Aqueous Solutions Of 9,10-Anthraquinone-2-Sulfonate. Part II. Dependence of Rates of Hydroxylation on Reaction Conditions

The rates of formation of hydroxy-9,10-anthraquinone-2-sulfonates, produced by irradiation of neutral and alkaline aqueous solutions of 9,10-anthraquinone-2-sulfonate (A) at 365 nm, have been measured for solutions with initial [A] ranging from 5 × 10−5 to 8 × 10−2 mol dm−3, saturated with either O2, air, or N2. The rate of monohydroxylation was independent of the saturating gas and, for neutral solutions absorbing all the incident light, increased with increasing initial [A] reaching a constant maximum quantum yield of 0.08 above 2 × 10−2 mol dm−3. In 0.1 mol dm−3 aqueous NaOH solution, the rate was about three times larger but had not attained a limiting value at the maximum [A] imposed by its solubility. Dihydroxy-9,10-anthraquinone sulfonates are proposed to arise from reaction of O2 with an intermediate, rather than by further hydroxylation of monohydroxy products. The rate of dihydroxylation decreased on increasing [A] above 5 × 10−4 mol dm−3, becoming essentially zero at high concentrations.