Complexation of tungsten(VI) with citrate

Abstract

Complex formation between tungstate and citrate (citric acid, H₄cit = 3-carboxy-3-hydroxypentane-1,5-dioic acid) has been investigated in the pH_c range 1.5–9.5 by potentiometric and enthalpimetric titrations at 25 °C in 1.0 mol dm^–3 NaCl. The potentiometric data were treated with the computer program SUPERQUAD taking into account side reactions of tungstate and citrate with hydrogen ions. The ‘best’ reaction model comprises eight complexes, representing four different tungstate–citrate stoichiometries. The formation constants of the complexes, denoted by β_pqr, where the subscripts p, q and r refer to the stoichiometric coefficients in the general formula [(WO₄)_p(Hcit)_qH_r]^(2p+3q-r)–, have the values log β₁₁₁= 10.21, log β₁₁₂= 17.03, log β₁₁₃= 21.67, log β₁₁₄= 22.82, log β₂₂₄= 34.89, log β₂₂₅= 39.3, log β₁₂₆= 34.51 and log β₂₁₄= 31.7. Enthalpy and entropy changes for the complexation reactions were calculated from the calorimetric data using the values of these formation constants. The enthalpy values for the major complexes are ΔH°₁₁₁=–66, ΔH°₁₁₂=–67 and ΔH°₁₁₃=–78 kJ mol^–1.