Equilibria and thermodynamic quantities for the reactions of molybdenum(VI) and tungsten(VI) with mandelate (α-hydroxybenzeneacetate)
The complex formation of molybdate and tungstate with mandelate [PhCH(OH)CO2-] have been investigated in the range pHc 7.5-1.5 by potentiometric, spectrophotometric and enthalpimetric titrations at 25°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 tungstate 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 β122 = 15.93, ΔH°= -78.2 and TΔS°= 12.7 kJ mol-1 respectively and for the tungsten(VI) complex, [WO2(C8H6O3)2] 2-, log β122 = 17.59, ΔH°= -86.3 and TΔS°= 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 ΔH = -0.05 kJ mol-1. The energetics of complexation is discussed in terms of the thermodynamic quantities.