MOLYBDENUM(VI) COMPLEX-FORMATION .7. EQUILIBRIA AND THERMODYNAMIC QUANTITIES FOR THE REACTIONS WITH NITRILOTRIACETATE

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-degrees-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 beta(pqr), where the subscripts p, q and r refer to the stoichiometric coefficients in the general formula [(MoO4)p(nta)qHr](2p+3q-r)-, have the values log beta112 = 17.78, log beta113 = 21.02, log beta114 = 22.57, log beta227 = 45.16, log beta228 = 47.95, log beta215 = 30.74 and log beta216 = 33.09. Enthalpy and entropy changes for complexes occurring in sufficiently high concentrations were calculated from the calorimetric data using these formation constants: DELTAH-112(0) = -69, DELTAH-113(0) = -71, DELTAH-227(0) = -123 and DELTAH-228(0) = -132 kJ mol-1. Thermodynamic parameters for the protonation of nta in 1.0 mol dm-3 NaCl have also been determined: log beta011 = 8.78, log beta012 = 11.04, log beta013 = 12.73 and log beta014 = 13.69; DELTAH-011(0) = -24.2, DELTAH-012(0) = -24.4 and DELTAH-013(0) = -26.6 kJ mol-1.