Thermoanalytische und massenspektrometrische Untersuchung der Zersetzung von Zinkoxalat-Dihydrat. Thermoanalytische Untersuchungen XII

The dehydration and pyrolysis of zinc oxalate dihydrate are re-examined by thermo-analytical methods at linearily increasing temperature in air, nitrogen, water vapour, and high vacuum. Simultaneous analysis of the gaseous decomposition products by means of mass spectrometry confirms previous authors' results obtained with other methods. The activation energy of the thermal decomposition of ZnC₂O₄ above 340° has been found to be 40,4 ± 2,6 kcal/mol. The rehydration of ZnC₂O₄ has been investigated at various water vapour pressures. In an atmosphere virtually saturated with moisture, the rehydration leads to the dihydrate and is easily controlled on the thermobalance. The dihydrate itself takes up an ill-defined fraction of surplus water. The thermal effects resulting from surface water on the dihydrate have been investigated by DTA (freezing and melting peaks). They disappear below 5% w/w of surplus water in the dihydrate. The surface of cristallites of the decomposition products calculated from particle size (from X-ray line broadening) is much larger than the surface determined by the BET-method. However, for ZnO in its state immediately after decomposition of ZnC₂O₄, both methods give similar results. Thus, dihydration of ZnC₂O₄ · 2 H₂O produces a considerable inner surface of ZnC₂O₄ not accessible to gas adsorption. This surface is considered to be covered by adsorbed water, which would explain the remarkable amount of water retained during dehydration. The final product, ZnO, sinters rapidly at temperatures higher than 500–600°.