ON THE PH-DEPENDENT FORMATION-CONSTANTS OF IRON(III) SULFUR(IV) TRANSIENT COMPLEXES

An experimental study is described of Fe(III)-S(IV) formation constants measured as a function of pH (1-3), ionic strength (0.2-0.5 M) and [Fe(III)]T (2.5-5.0 x 10(-4) M) using a continuous-flow spectrophotometric technique to make observations 160 ms after mixing. Preliminary experiments using pulse-accelerated-flow (PAF) spectrophotometry to measure rate constants on a microsecond timescale are also described. The conditional formation constant at 25-degrees-C can be modeled with the following equation: K(app) = alpha1alpha4K7 + alpha0alpha4K8, where alpha0 = [Fe3+]/[Fe(III)]T, alpha1 = [FeOH2+]/[Fe(III)]T, alpha4 = [HSO3-]/[S(IV)T], K7 = 8.5 x 10(2) M-1, and K8 = 5.5 x 10(1) M-1. K7 and K8 can be interpreted as intrinsic constants for the coordination of HSO -3 by FeOH2+ and Fe3+, respectively, but until further evidence is obtained they should be regarded as fitting constants. PAF spectrophotometry showed that the initial reaction of Fe(III) with S(IV) (pH 2.0) is characterized by a second-order rate constant of almost-equal-to 4 x 10(6) M-1 s-1 which is comparable to rate of reaction of FeOH2+ with SO42-. However, the PAF results should be regarded as preliminary since unexpected features in the initial data indicate that the reaction may be more complex than expected.