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 (2.5-5.0×10^-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 °C can be modeled with the following equation: {ie307-1} where {ie307-2}K₇ and K₈ can be interpreted as intrinsic constants for the coordination of HSO₃^- 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 ≈4×10⁶ M^-1 s^-1 which is comparable to rate of reaction of FeOH²⁺ with SO₄^2-. 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.