Molybdenum(VI) complex formation. Part 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°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 βpqr, where the subscriptsp, q and r refer to the stoichiometric coefficients in the general formula [(MoO4)p(nta)qHr](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, ΔH 0227 = -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.