Thermodynamics of zirconium and hafnium orthophosphates (arsenates)

The enthalpies of solution of M IV [HP(As)O 4 ] 2 (where M IV is zirconium or hafnium) compounds and their monohydrates have been measured calorimetrically and the standard enthalpies of formation derived. The results obtained have been used for the calculation of dehydration enthalpies of M IV [HP(As)O 4 ] 2 .H 2 O. The standard enthalpies of formation of Zr(MAsO 4 ) 2 .nH 2 O (where M is lithium, sodium or potassium) compounds have also been determined and enthalpies of M + -H + exchange on crystalline Zr(HAsO 4 ) 2 .H 2 O have been obtained. Qualitative explanations for the observed ion exchange results are presented.