Journal of Thermal Analysis and Calorimetry, Vol. 68 (2002) 207–213 STUDY OF THE DEHYDRATION/REHYDRATION OF AMMONIUM TRIS-OXALATO ALUMINATE(III) (NH 4 ) 3 Al(C 2 O 4 ) 3 ·3H 2 O AT CONTROLLED ATMOSPHERE M. Ezahri 1 , M. El Hadek 1 , G. Coffy 2 and B. F. Mentzen 2 1 Laboratoire de Chimie Minérale Appliquée et Génie des Procédés, Université Ibn Zohr, Faculté des Sciences, B.P. 28/S Agadir, Maroc 2 Laboratoire de Physico-Chimie Minérale, URA n° 116 du CNRS, Université Claude-Bernard, Lyon I, 43 Bd, du 11 novembre 1918, F-69622 Villeurbanne Cedex, France (Received May 28, 2001; in revised form November 9, 2001) Abstract The study of the dehydration/rehydration of ammonium tris-oxalato aluminate(III) (NH 4 ) 3 Al(C 2 O 4 ) 3 ·3H 2 O in flowing dinitrogen saturated with water vapor at room temperature, using thermogravimetric analysis and X-ray diffraction techniques, allowed the determination of the tem- perature stability domains of (NH 4 ) 3 Al(C 2 O 4 ) 3 ·3H 2 O, (NH 4 ) 3 Al(C 2 O 4 ) 3 ·2H 2 O and the anhydrous salt. The X-ray powder diffraction profiles are reported for each of the three phases. Keywords: ammonium tris-oxalatoaluminate(III), dehydration, rehydration, thermogravimetric analysis, X-ray powder analysis Introduction The oxalato-metallates have been studied for many years in our laboratory [1–5]. Their pyrolysis produces well crystallized solid oxide phases [6–8]. We have under- taken investigations on multicomponent systems including the ammonium tris-oxa- lato aluminate(III) with a double purpose: 1. to get a better knowledge on the precur- sors, particularly on their stoichiometry, the ammonium oxalato-metallates usually crystallizing at room temperature as hydrates (NH 4 ) 3 M III (C 2 O 4 ) 3 ·xH 2 O(M III =Al, Fe, Cr) with x close to 3; 2. to get, after pyrolysis, a good dispersion of the metallic ele- ment at the atomic scale, which is an important step in the production of doped alu- mina crystallites or micronic powders. In the present paper we report the crystalliza- tion conditions of (NH 4 ) 3 Al(C 2 O 4 ) 3 ·xH 2 O hydrates (for x=0, 2, 3) and their dehydra- tion/rehydration in controlled atmosphere. 1418–2874/2002/ $ 5.00 © 2002 Akadémiai Kiadó, Budapest Akadémiai Kiadó, Budapest Kluwer Academic Publishers, Dordrecht