Molybdenum(VI) complex formation. Part 6. Reactions with aspartate in 1.0 mol dm–3 sodium chloride medium at 25 °C

Abstract

Complex formation between molybdate and aspartate ^–O₂CCH₂CH(NH₂)CO₂^–(L^2–) has been investigated in the pH range 1.2–7.5 by potentiometric, spectrophotometric and enthalpimetric titrations at 25 °C in 1.0 mol dm^–3 NaCl. The ‘best’ reaction model, based on computer treatment of the data and supported by analysis of spectrophotometric measurements, comprises nine complexes representing various 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 [(MoO₄)_p(HL)_qH_r]^{(2p+q–r)–}, have the values log β₁₁₁= 6.54, log β₁₁₂= 9.84, log β₁₂₁= 6.57, log β₁₂₂= 11.47, log β₂₁₄= 21.20, log β₂₁₅= 23.50, log β₂₄₈= 37.74, log β₄₄₉= 50.86 and log β₄₄₁₀= 53.48. Enthalpy and entropy changes for the major complexes were calculated from the calorimetric data using these formation constants; ΔH^⊖₁₁₁=–26.8, ΔH^⊖₁₂₂=–38, ΔH^⊖₂₁₄=–97 and ΔH^⊖₂₁₅=–83 kJ mol^–1. Thermodynamic parameters for the protonation of aspartate in 1.0 mol dm^–3 NaCl have also been determined: log β₀₁₁= 3.64 and log β₀₁₂= 5.65; ΔH^⊖₀₁₁=–5.8 and ΔH^⊖₀₁₂=–10.2 kJ mol^–1.