Tungsten(VI) complex formation. Equilibria and thermodynamic quantities for the reactions with malate

Abstract

Complex formation between tungstate and malate has been investigated in the range pH 1.5–8.5 by potentiometric and enthalpimetric titrations at 25 °C in 1.0 mol dm ^-3 (Na)Cl. The ‘best’ reaction model comprises mono-, di- and tetra-nuclear complexes. 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 [(WO ₄ ) _p (mal) _q H _r ], have the values: log β ₁₁₁ = 8.85, log β ₁₁₂ = 14.78, log β ₁₁₃ = 17.26, log β ₁₂₂ = 17.20, log β ₁₂₃ = 21.70, log β ₁₂₄ = 25.61, log β ₂₁₃ = 23.07, log β ₂₂₂ = 18.87, log β ₂₂₃ = 25.72, log β ₂₂₄ = 31.88 and log β ₄₄₁₁ = 73.60. Enthalpy and entropy changes for complexes occurring in sufficiently high concentrations were calculated from the calorimetric data using these formation constants: ΔH° ₁₁₁ = -64, ΔH° ₁₁₂ = -65, ΔH° ₁₁₃ = -67, ΔH° ₁₂₂ = -86.0, ΔH° ₁₂₃ = -92, ΔH° ₁₂₄ = -91, ΔH° ₂₂₂ = -115, ΔH° ₂₂₃ = -121, ΔH° ₂₂₄ = -120 and ΔH° ₄₄₁₁ = -240 kJ mol ^-1 .