The kinetics of the decomposition of a series of alkali-metal oxalate perhydrates and their related hydrates have been studied by a combination of thermogravimetric analysis, differential scanning calorimetry, and optical microscopy. The behaviour of the perhydrates is strikingly similar to that of the related hydrate (where this exists). The kinetic formulations which best fit the decompositions have been ascribed and activation energies obtained for experiments carried out in a static atmosphere (€, = 64-208 kJ mol-lfor perhydrates, 98-250 kJ mo1-l for hydrates).PERHYDRATES are of considerable technical importance as carriers of hydrogen peroxide. However, all known perhydrates are more or less unstable. Decomposition is continuous under typical laboratory conditions, where the time for complete loss of peroxide ranges from a few hours to several months. Typically, the rate of decomposition at room temperature is a function of relative humidity.lThe structures of perhydrates are akin to those of hydrates: hydrogen bonds hold the H202 molecules in the lattice. The thermal decompositions of the two classes of compound also show similar features : heating perhydrates releases H202 into the atmosphere,l just as the dehydration of hydrates releases water. However, while the kinetics of dehydration reactions have been reported for a number of hydratesJ2 no thorough investigation has been made of the deperhydration of perhydrates.We report in this paper our attempt to establish the kinetics of decomposition of a series of oxalate perhydrates, M2[C20,]*H202 (M = Li, Na, K, Rb, or NH,), using thermogravimetric analysis (t.g.a.) , differential scanning calorimetry (d.s.c.) , and hot-stage optical microscopy.2 These techniques were also applied to the dehydration of the hydrates M2[C20,]*H20 (M = K, Rb , or NH,) which are isostructural with the corresponding +e~hydrates.~-~ Thus, we are able to present a direct comparison of the decomposition kinetics of hydrates and perhydrates. The decomposition products of the deperhydration reaction are H20, vapour and anhydrous oxalate (determined by X-ray diffraction).
EXPERIMENTAL
