The oxidation of 6-hydroxydopamine in aqueous solution. Part 2. Speciation and product distribution with iron(III) as oxidant

Abstract

6-Hydroxydopamine [5-(2-aminoethyl)benzene-1,2,4-triol, protonated form H₃LH⁺] reacts anaerobically with aqueous iron(III) to produce four types of quinone, namely the p- (pQ), o- (oQ), triketo- (tQ) and the deprotonated quinone (Q⁻). The relative concentrations depend strongly on the pH. At low pH pQ, oQ and tQ predominate and are in meta-stable equilibrium. However, the pH dependency of the concentrations of pQ and tQ at low pH suggests an equilibrium between their respective semiquinones. Above a pH of about 2, tQ disappears and above about 2.5 pQ and oQ equilibrate via Q⁻, which, above a pH of about 6, is the only species present. The equilibrium constants and molar absorbances have been measured for all these species and are discussed. The speciation over the pH range 0.5–5 has been determined as a necessary prelude to a full kinetic study of the system.