MOLYBDENUM(VI) AND TUNGSTEN(VI) COMPLEX-FORMATION .9. EQUILIBRIA AND THERMODYNAMIC QUANTITIES FOR THE REACTIONS OF MOLYBDENUM(VI) AND TUNGSTEN(VI) WITH MANDELATE (ALPHA-HYDROXYBENZENEACETATE)

The complex formation of molybdate and tungstate with mandelate [PhCH(OH)CO2-] have been investigated in the range pH(c) 7.5-1.5 by potentiometric, spectrophotometric and enthalpimetric titrations at 25 degrees C in 1.0 mol dm(-3) NaCl. The potentiometric data were treated with the computer program SUPERQUAD taking into account the side-reactions of molybdate and tungstare with hydrogen ions, For both systems the 'best' reaction model comprises only one major complex and a number of minor complexes. Thermodynamic quantities have been determined for each of the major complexes. For the molybdenum (VI) complex [MoO2(C8H6O3)(2)](2-), log beta(122) = 15.93, Delta H degrees = -78.2 and T Delta S degrees = 12.7 kJ mol(-1) respectively and for the tungsten (VI) complex. [WO2(C8H6O3)(2)](2-), log beta(122) = 17.59, Delta H degrees = -86.3 acid T Delta S degrees = 14.1 kJ mol(-1). The equilibrium constant and the enthalpy change for the protonation of mandelate have also been determined, log K = 3.15 and Delta H = -0.05 kJ mol(-1). The energetics of complexation is discussed in terms of the thermodynamic quantities.