TUNGSTEN(VI) COMPLEX-FORMATION - EQUILIBRIA AND THERMODYNAMIC QUANTITIES FOR THE REACTIONS WITH MALATE

Complex formation between tungstate and malate has been investigated i n the range pH 1.5-8.5 by potentiometric and enthalpimetric titrations at 25 degrees C in 1.0 mol dm(-3) (Na)Cl. The 'best' reaction model c omprises mono-, di- and tetra-nuclear complexes. The formation constan ts of the complexes, denoted by beta(pqv), where the subscripts p, q a nd r refer to the stoichiometric coefficients in the general formula [ (WO4)(p)(mal)(q)H-r], have the values: log beta(111) = 8.85, log beta( 111) = 14.78, log beta(113) = 17.26, log beta(122) = 17.20, log beta(1 23) = 21.70, log beta(124) = 25.61, log beta(213) = 23.07, log beta(22 2) = 18.87, log beta(223) = 25.72, log beta(224) = 31.88 and log beta( 4411) = 73.60. Enthalpy and entropy changes for complexes occurring in sufficiently high concentrations were calculated from the calorimetri c data using these formation constants: Delta H degrees(111) = -64, De lta H degrees(112) = -65, Delta H degrees(113) = -67, Delta H degrees( 122) = -86.0, Delta H degrees(123) = -92, Delta H degrees(124) = -91, Delta H degrees(m) = -115, Delta H degrees(223) = -121, Delta H degree s(224) = -120 and Delta H degrees(4411) = -240 kJ mol(-1).