Stability, Stoichiometry, and Structure of Fe(II) and Fe(III) Complexes with Di-2-pyridyl Ketone Benzoylhydrazone: Environmental Applications

Abstract

The stoichiometry, structure, and thermodynamics of Fe-(II) and Fe(III) complexes of di-2-pyridyl ketone benzoylhydrazone (DPKBH) were studied at pH 5.3 and 25.0 °C in water-ethanol solutions. The spectrophotometric method of corresponding solutions was used to obtain overall formation constants for the 1:1 (β_1 = 4.693 X 10^6 M^(-1)) and 1:2 (β_2 = 2.195 X 10^(10) M^(-2)) Fe(III)(H_20)_6^(3+)DPKBH complexes and 1:1(β_1 =1.217 X 10^5 M^(-1)) and 1:2 (β_2 = 7.814 X 10^9 M^(-2)) Fe(II)(H_2O)_6^(2+)-PKBH complexes. Conductance and pH titrations indicate that DPKBH coordinates preferentially as an anion, while FTIR analysis shows that DPKBH exists in the enol form and exhibits tridentate ligand behavior. DPKBH has been shown to be a useful analytical reagent for the simultaneous spectrophotometric determination of Fe(II) and Fe(III) in natural waters: Fe(H_20)_6^(3+) + DPKBH ⇆ Fe(H_2O_)_3- (DPKBH)^(3+) + 3H_2O and Fe(H_2O)_6^(3+) + 2DPKBH ⇆ Fe-(DPKBH)_2^(3+) + 6H_2O.