Enthalpies of Formation of Alunite and Natroalunite (According to Calorimetric Data)

A sample of natural alunite of the composition K0.6Na0.4Al3(SO4)(2)(OH)(6) was studied by powder X-ray diffraction, infrared spectroscopy, electron microprobe, and thermal analysis. The enthalpy of formation of the alunite from elements increment Delta H-f(0)(298.15 K) = -5149 +/- 13 kJ/mol was determined by the method of melt solution calorimetry, using a Setaram (France) Calvet microcalorimeter. The calculated enthalpies and Gibbs energies of formation of the end members of the series alunite KAl3(SO4)(2)(OH)(6) to natroalunite NaAl3(SO4)(2)(OH)(6) are -5164 +/- 13 and -4651 +/- 13 kJ/mol, respectively, for alunite and - 5127 +/- 13 and -4615 +/- 13 kJ/mol, respectively, for natroalunite.

Automated summary

A natural alunite sample was studied using various analytical techniques, and its thermodynamic properties were determined. The results were compared with different compositions of alunite.