Synthesis and thermal decomposition of potassium trioxalatoaluminate hydrate

Abstract

The preparation of pure K3Al(C2O4)3·mH2O (2<m<3) is described. Dependent on the mode of preparation, the following were found to be contaminants of the desired product: K2C2O4·1H2O; KHC2O4; KHC2O4·H2C2O4·2H2O; H2C2O4·2H2O; different forms of aluminium oxide hydrate; K4Al2(OH)2(C2O4)4· (2+x)H2O (0.7<x<1.7) and K2Al2(H2O)2(C2O4)4· 4H2O.

The thermal behaviour of K3Al(C2O4)3·mH2O was investigated by means of thermogravimetry, differential thermal analysis, differential scanning calorimetry, infrared spectral analysis, evolved gas analysis and high temperature X-ray diffraction. The dehydration proceeds via the intermediate 2- and 0.75-hydrates (room temperature to 120°C). The existence of the 3-hydrate could not be confirmed. On increasing the temperature, anhydrous potassium trioxalatoaluminate decomposes into potassium oxalate, potassium carbonate and alumina (320–435°C). The potassium oxalate decomposition (435-475°C) is overlapped by the formation of KAlO2 from potassium carbonate and alumina; the procedural final temperature of the latter process is dependent on the CO2 partial pressure. After the thermal decomposition of the excess K2CO3 present, KAlO2 is the final solid product.