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

6-Hydroxydopamine [5-(2-aminoethyl)benzene-1,2,4-triol, protonated form H3L H+] reacts anaerobically with aqueous iron(III) to produce four types of qu inone, namely the p-(pQ), o-(oQ), triketo- (tQ) and the deprotonated quinon e (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 equi librium between their respective semiquinones. Above a pH of about 2, tQ di sappears 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 discusse d. The speciation over the pH range 0.5-5 has been determined as a necessar y prelude to a full kinetic study of the system.