Thermoanalytical and calorimetric investigations on the formation and decomposition of some alunites

Abstract Under hydrothermal conditions the formation of oxonium alunite (OxAl) and potassium alunite (KAI) was experimentally determined as a function of temperature. The straight lines In K = f(T) obtained are the result of the experiments described. From the calorimetric determination of the thermal capacity, the standard formation enthalpy and the standard entropy. In K = f( T ) functions could be calculated. Comparison of experimental and calculated results has proved to be satisfactory for KAI, where the non-stoichiometry is the main consideration. The agreement is not very good for OxAl because of foreign phase components. Investigation of the thermal decomposition under quasi-isothermal and quasi-isobaric conditions shows that the alunites have a defined decomposition temperature. Reactions preceding this decomposition are due to the loss of excess water.