Successive addition of electrons to sodium quinizarin-2- and -6-sulphonate in aqueous solution. a pulse and γ-radiolysis study
Absorption characteristics of the semiquinone free radicals formed by one-electron reduction of quinizarin 2- and 6-sulphonates (Q2S and Q6S, respectively) have been studied. Second-order rate constants have been determined for the reactions of e–aq, CO˙–2, H˙ and O˙–2 with the two quinones and also for the reaction of the semiquinones with O2 and with themselves. The one-electron reduction potentials (vs. NHE) are E17=–270 mV for Q2S and –249 mV for Q6S. They vary with pH in accordance with the pKa values of the parent and the semiquinone. The radicals are stable within the pH range 5–11. The stability constant is highest at pH 8.5, viz. ca. 1.45 for Q2S and 0.95 for Q6S. The one-electron reduction potentials of the semiquinones and the two electron reduction potentials of the quinones can be calculated to be E27=–246 mV for Q2S and –213 mV for Q6S, and Em7=–258 mV for Q2S and –231 mV for Q6S. The variation of these quantities with pH has been related to pKa values for the relevant species. Properties of the radiolytically prepared fully reduced Q2S and Q6S are reported. Their acid forms are stable to oxygen. Adjustment to alkaline pH results in a loss of the sulphonate group from fully reduced Q2S but not from fully reduced Q6S. Reasons are given.