Molybdenum(VI) complex formation. Part 7. Equilibria and thermodynamic quantities for the reactions with nitrilotriacetate

Abstract

Complex formation between molybdate and nitrilotriacetate (nta) has been investigated in the range pH 1.0–9.0 by potentiometric, spectrophotometric and enthalpimetric titrations at 25 °C in 1.0 mol dm^–3 NaCl. The ‘best’ reaction model comprises seven complexes, of which three are mononuclear and four dinuclear. The formation constants of the complexes, denoted by β_pqr′ where the subscriptsp, q and r refer to the stoichiometric coefficients in the general formula [(MoO₄)_p(nta)_qH_r]^{(2p + 3q – r)–}, have the values log β₁₁₂= 17.78, log (β₁₁₃= 21.02, log β₁₁₄= 22.57, log β₂₂₇= 45.16, log β₂₂₈= 47.95. log β₂₁₅, = 30.74 and log β₂₁₆= 33.09, Enthalpy and entropy changes for complexes occurring in sufficiently high concentrations were calculated from the calorimetric data using these formation constants: ΔH⁰₁₁₂=–69. ΔH⁰₁₁₃=–71, ΔH⁰₂₂₇=–123 and ΔH⁰₂₂₈=–132 kJ mol^–1.Thermodynamic parameters for the protonation of nta in 1.0 mol dm^–3 NaCl have also been determined: log β₀₁₂= 8.78, log β₀₁₂= 11.04, logβ₀₁₃= 12.73 and log β₀₁₄= 1 3.69; ΔH⁰₀₁₁=–24.2. ΔH⁰₀₁₂=–24.4 and ΔH⁰₀₁₃=–26.6 kJ mol^–1.