MOLYBDENUM(VI) COMPLEX-FORMATION .6. REACTIONS WITH ASPARTATE IN 1.0 MOL DM(-3) SODIUM-CHLORIDE MEDIUM AT 25-DEGREES-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-degrees-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 beta(pqr) where the subscripts p, q and r refer to the stoichiometric coefficients in the general formula [(MoO4)p(HL)(q)H(r)](2p+q-r)-, have the values log beta111 = 6.54, log beta112 = 9.84, log beta121 = 6.57, log beta122 = 11.47, log beta214 = 21.20, log beta215 = 23.50, log beta248 = 37.74, log beta449 = 50.86 and log beta4410 = 53.48. Enthalpy and entropy changes for the major complexes were calculated from the calorimetric data using these formation constants: DELTAH(formal)111 = -26.8, DELTAH(formal)122 = -38, DELTAH(formal)214 = -97 and DELTAH(formal)215 = -83 kJ mol-1. Thermodynamic parameters for the protonation of aspartate in 1.0 mol dm-3 NaCl have also been determined: log beta011 = 3.64 and log beta012 = 5.65; DELTAH(formal)011 = -5.8 and DELTAH(formal)012 = -10.2 kJ mol-1.