Molybdenum(VI) complex formation. Equilibria and thermodynamic quantities for the reactions with malate
Complex formation between molybdate and malate has been investigated in the pH range 1.5-7.5 by potentiometric, spectrophotometric and enthalpimetric titrations at 25°C in 1.0 mol dm-3 (Na)Cl. The "best" reaction model comprises mononuclear, dinuclear and tetranuclear complexes. 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(mal)qHr], have the values: log β111 = 7.47, log β112 = 13.23, log β113 = 15.87, log β122 = 15.48, log β123 = 20.13, log β124 = 24.14, log β213 = 20.10, log β214 = 28.06, log β225 = 32.01, log β428 = 52.92, log β429 = 54.35, log β4210 = 55.27, and log β4411 = 69.92. Enthalpy and entropy changes for complexes occurring in sufficiently high concentrations were calculated from the calorimetric data using these formation constants. ΔH°111 = -47.9, ΔH°224 = -55.5, ΔH°113 = -57, ΔH°122 = -77.0, ΔH°123 = -83, ΔH°124 = -85.6, ΔH°213 = -75, ΔH°224 = -85, ΔH°225 = -85, ΔH°428 = -232, ΔH429 = -214, ΔH°4210 = -197, and ΔH°4411, = -221 kJ mol-1. Thermodynamic parameters for the protonation of malate in 1.0 mol dm-3 (Na)Cl have also been determined : log β011 = 4.44 and log β012 = 7.57 ; ΔH°011 = -2.0, and ΔH°012 = -6.5 kJ mol-1. Copyright © 1996 Elsevier Science Ltd.