Tungsten(VI) equilibria: a potentiometric and calorimetric investigation

Abstract

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 β_6,6= 49.01, log β_7,8= 65.19, log β_7,9= 69.96, and log β_12,14= 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.