Tungsten(VI) equilibria: A potentiometric and calorimetric investigation
Tungsten(VI) equilibria has been investigated in the range pH 5-7.8 (corresponding to a degree of protonation, Z ≲ 1.2) by potentiometric and enthalpimetric titrations at 25 °C in 1.0 mol dm-3 NaCl. The potentiometric data were treated with the computer program SUPERQUAD to examine a large number of reaction models. The model that gave the best fit to the data comprises [WO4]2- and the four polyions [W6O20(OH)2]6-, [W7O24]6-, [HW7O24]5- [H2W12O42]10- with formation constants log β6,6 = 49.01, log β7,8 = 65.19, log β7,9 = 69.96, and log β12,14 = 115.38. The enthalpy and entropy changes for the formation of the polyions were calculated from the enthalpimetric data using these constants. The enthalpy values are ΔH⊖6,6 = -231, ΔH⊖7,8 = -333, ΔH⊖7,9 = -328, and ΔH⊖12,14 = -542 kJ mol-1. The energetics of condensation is discussed in terms of the thermodynamid quantities for heptatungstate and heptamolybdate.