Kinetic Studies of the Fe(III)/DI-2-Pyridyl Ketone Benzoylhydrazone Complex Reduction By Sulfite

The reactions of HSO^? ₃ with Fe(III) in the presence of di-2-pyridyl ketone benzoylhydrazone (DPKBH) have been investigated at 4.2-7.2 pH. The decomposition reaction of Fe(III)/DPKBH has been studied as a function of total sulfite concentration using spectrophotometric techniques. The kinetics are controlled by the equilibrium Fe^III(DPKBH)(H₂O)]²⁺ ? Fe^III(DPKBH)(OH)]⁺+H⁺, for which the hydrolysis constant was potentiometrically determined as 4.3 x 10^-5M, at (25.0 ± 0.1)°C and 1.5 x 10^-3M ionic strength. Both of those species undergo sulfite substitution and the significantly more labile species is the aqua complex. The formation constants of Fe^III(DPKBH)(SO₃)] are 1.2 x 10⁴ at pH 4.2 and 1.9 x 10²M^-1 at pH 6.2. The limiting rate constants, k, at high HSO^? ₃ concentrations are 3.5 x 10^-2s^-1 and 1.4 x 10^-2s^-1, respectively, at pH 4.2 and 6.2, I = 1.3 x 10^-2M. The results are discussed with reference to the available literature data.