Equilibria and thermodynamic quantities for the reactions of molybdenum(VI) and tungsten(VI) with mandelate (α-hydroxybenzeneacetate)

Abstract

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 °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.