Thermal decomposition of oxalates. Part XI. Dehydration of calcium oxalate monohydrate

A kinetic study has been made of the isothermal dehydration of calcium oxalate monohydrate to the anhydrous oxalate at various pressures of water vapour by use of a thermogravimetric balance. The rate of dehydration was found to be dependent upon the water vapour pressure. The reaction rate for dehydration decreased sharply with increasing water vapour pressure up to 0·0665 kN m–2(0·5 mmHg). With increasing water vapour pressure beyond this value the rate of dehydration increased from this point of minimum rate to a slightly higher one, followed by a steady fall.In order to complement and clarify this work, specific surface area (B.E.T.) and X-ray studies were also completed. The results showed that when the material was dehydrated in a vacuum the product was amorphous, but when dehydrated in the presence of water vapour, the anhydrous salt was crystalline.