COMPLEXATION OF THE AQUA-IRON(III) DIMER BY TIRON - KINETICS OF COMPLEX-FORMATION AND DISSOCIATION

The reaction of Tiron (1,2-dihydroxy-3,5-benzenesulfonate) with aqueous iron(III) has been studied at moderate acidities (0.1-0.02 M H+) by stopped-flow spectrophotometry at 25-degrees-C in 1.0 M NaClO4/HClO4. The concentration of the bis(mu-hydroxo) dimer of iron(III) has been systematically varied by controlling the H+ and iron(III) concentrations in the iron(III) solutions before mixing with Tiron solutions at various acidities to give the desired final [H+]. The absorbance change at 660 nm is biphasic, and the absorbance-time profiles for the slower absorbance decrease have been modeled by numerical integration. It is found that the faster absorbance increase is due to reaction of (H2O)8Fe2(mu-OH)2(4+) with Tiron to give a blue complex with k = 5.1 X 10(3) M-1 s-1, but no protons are released during the complexation, and the formation equilibrium constant is 6.3 X 10(3) M-1. The dimer complex decomposes to the less strongly colored monomeric complex and Fe(OH2)6(3+) with k = 0.2 s-1. The possible nature of the dimeric complex is considered, and the relevance of these observations to previous studies on catechols, squaric acid, and ascorbic acid is discussed.