Thermodynamics and kinetics of complexation of iron(III) ion by picolinic and dipicolinic acids

Abstract

The complexation reactions of iron(III) with 2-pyridine carboxylic acia (picolinic acid) and 2,6-pyridine dicarboxylic acid (dipicolinic acid) in aqueous solutions have been studied by spectrophotometric and stopped flow techniques. Equilibrium constants were determined for the 1 : 1 complexes at temperatures between 25 and 80°C. The values obtained are: Picolinic Acid (HL): Fe 3++ H 2L +⇌ FeHL 3++H+( K 1 = 2.8,Δ H = 2 kcal mole −1 at 25°C, μ = 2.67 M) Dipicolinic Acid (H 2D): Fe 3++H 2D⇌ FeD ++2H +( K 1 K 1A= 227 M, Δ H = 3.4 kcal mole −1 at 25°C,μ = 1.0 M). The rate constants for the formation of these complexes are also given. The results are used to evaluate the effects of these two acids upon the rate of dissolution of iron(III) from its oxides.