Thermal decomposition of some alkali-metal oxalate monohydrates and monoperhydrates

Abstract

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

Solubility and Phase Diagram for the Ternary Sodium Oxalate + Hydrogen Peroxide + Water System at (283.15 and 293.15) K

“…The structure is built up of endless chains of alternating hydrogen peroxide molecules and oxalates ions linked together by hydrogen bonds. In 1980, Adams et al studied the thermal decomposition of Na 2 C 2 O 4 ·H 2 O 2 under isothermal conditions . Nevertheless, to the best of our present knowledge, further investigations on the compound Na 2 C 2 O 4 ·H 2 O 2 have not been reported in the literature.…”

“…It is well-known that solid−liquid phase equilibrium data are important in crystallization processes. As described by Pederson and Pederson 2 and Adams et al, sodium oxalate reacted with hydrogen peroxide solution, and Na 2 C 2 O 4 ·H 2 O 2 crystallized from the system. Obviously, the preparing process of Na 2 C 2 O 4 ·H 2 O 2 was based on the phase diagram of the ternary Na 2 C 2 O 4 + H 2 O 2 + H 2 O system.…”

Phase Diagram for the System Ammonium Oxalate + Hydrogen Peroxide + Water at 283.15 K and 293.15 K

“…The interatomic dimensions of the oxalate ion are normal, but the ion is nonplanar. In 1980, Adams and co-workers 4 studied the thermal decomposition of (NH 4 ) 2 C 2 O 4 ‚H 2 O 2 under isothermal conditions. Nevertheless, to the best of the present authors' knowledge, further investigation on the adduct, (NH 4 ) 2 C 2 O 4 ‚ H 2 O 2 , has not been described elsewhere in the literature.…”

“…It is well-known that solid-liquid phase equilibrium data are important in crystallization processes. Pederson and Pederson 3 and Adams et al 4 showed that ammonium oxalate can react with a hydrogen peroxide solution of a given concentration and that (NH 11 and demonstrate the temperature dependence of the ternary phase diagram.…”

Studies of layered and channel complexes using combined thermogravimetry and mass spectrometry

“…Perhydrates are compounds which contain hydrogen peroxide of crystallisation and are important in bleach formulations for powder detergents. Much work has focused on sodium carbonate perhydrate [11] and the oxalate perhydrates [ 12]. However, little has been published on the properties of the isostructural inclusion compounds 4Na2SO4.MC1-2H202 (M=Na [13], K [14] and NH4 [15]).…”