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

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 mete-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.