Synthesis, characterisation and thermal decomposition of double sulphates of Al and Cr(III) with diisopropylammonium ion

On evaporation at room temperature of an aqueous mixture of M(III) sulphate and diisopropylammonium sulphate in a molar ratio 1:3, double sulphates of Al and Cr as crystalline products were obtained. The stoichiometry of the obtained compounds was determined by means of elemental and TG analysis. The above compounds were characterized by their IR spectra and X-ray powder diffraction patterns, as well. It was found that they have a general empirical formula M(CH₃CHCH₃)₂NH₂ (SO₄)₂·6H₂O and that they are isostructural. TG, DTG and DTA curves were done in static air atmosphere from room temperature up to 1000°C. The dehydration of the compounds takes place in one step and it points out that the six water molecules are bound in the same way. This assumption is confirmed by their IR spectra, where single sharp HOH deformation band at 1694 cm^-1 appears in both compounds. The thermal decomposition of the anhydrous compounds starts at about 260°C and is very complex. This process takes place in many steps which are not well resolved. The diisopropylammonium sulphate is prepared from the diisopropylamine by neutralization up to pH 3-4 with diluted sulphuric acid and permanent cooling in an ice-water bath.