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 degrees C in 1.0 mol dm(-3) (Na)Cl. The ''best'' reaction model comprises mononuclear, dinculear and tetranuclear complexes. 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)(mal)(q)H-r], have the values: log beta(111) = 7.47, log beta(112) = 13.23, log beta(113) = 15.87, log beta(122) = 15.48, log beta(123) = 20.13, log beta(124) = 24.14, log beta(213) = 20.10, log beta(214) = 24.35, log beta(224) = 28.06, log beta(225) = 32.01, log beta(428) = 52.92, log beta(429) = 54.35, log beta(4210) = 55.27, and log beta(4411) = 69.92. Enthalpy and entropy changes for complexes occurring in sufficiently high concentrations were calculated from the calorimetric data using these formation constants. Delta H degrees(111) = -47.9, Delta H degrees(112) = -55.5, Delta H degrees(113) = -57, Delta H degrees(122) = -77.0, Delta H degrees(123) = -83, Delta H degrees(124) = -85.6, Delta H degrees(213) = -75, Delta H degrees(224) = -85, Delta H degrees(224) = -85, Delta H degrees(225) = -85, Delta H degrees(428) = -232, Delta H degrees(429) = -214, Delta H degrees(4210) = -197, and Delta H degrees(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 beta(011) = 4.44 and log beta(012) = 7.57; Delta H degrees(011) = -2.0, and Delta H degrees(012) = -6.5 kJ mol(-1). Copyright (C) 1996 Elsevier Science Ltd