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

Abstract

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 [FeIII(DPKBH)(H2O)]2+ ⇌ [FeIII(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 [FeIII(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.