Thermal Decomposition of Potassium, Di-µ-Hydroxo-Bis(Dioxalatoaluminate) Hydrate

Abstract

The preparation of pure K4Al2(OH)2(C2O4)4 · (2 + x)H2O (0.7<×<1.7) is described. The thermal behaviour of this compound was investigated by means of thermogravimetry, differential thermal analysis and differential scanning calorimetry. At increasing temperature, the hydrate water is split off via the intermediate 2- and 1-hydrates leading to the anhydrous hydroxosalt. Afterwards, the bridging hydroxo groups in the dimeric oxalatoaluminate are broken down with the evolution of 1 mole of water per mole of anhydrous salt. At higher temperatures, the thermal decomposition is analogous to the normal potassium trioxalatoaluminate decomposition, i.e., (1) oxalatoaluminate decomposition with formation of oxalate, carbonate and alumina; overlapped by (2) the decomposition of oxalate to carbonate; this in turn overlapped by (3) the solid-solid reaction of potassium carbonate and alumina with formation of potassium aluminate; and (4) the final step is ascribed to the decomposition of potassium carbonate.The tentative conclusions from thermal analysis measurements about the formula of the relevant compound are fully confirmed by its infrared spectrum.