An assessment of the Thermodynamic Difference Rule for mixed inorganic oxides and comments on the enthalpies of formation of phosphates

The applicability of the Thermodynamic Difference Rule to mixed inorganic oxides is assessed for a range of aluminates, borates, ferrites, phosphates, silicates and vanadates. The deviation of the phosphate data from the linear relation between the enthalpy of formation, ΔfH°, of the solid solvating oxide and ΘHf, the Thermodynamic Difference Rule Constant, is discussed and we conclude that the use of ΔfH°(P4O10, s) to provide the solvating oxide data is inappropriate and, instead, a value for ΔfH°(P2O5, s) of −1618 ± 9 kJ mol−1 is proposed.

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