Molybdenum(VI) complex formation. Part 6. Reactions with aspartate in 1.0 mol dm-3 sodium chloride medium at 25 °c
Complex formation between molybdate and aspartate -O2CCH2CH(NH2)CO2- (L2-) 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 βpqr where the subscripts p, q and r refer to the stoichiometric coefficients in the general formula [(MoO4)p(HL)qHr] (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.