Studies on the solid state decomposition of oxalic acid and ammonium oxalate Ph.D. Thesis
Abstract
This thesis embodies studies on the photolysis of oxalic acid and ammonium oxalate and the low-temperature thermal decomposition of ammonium oxalate.
From a detailed review of the literature, it was evident that very few decomposition studies have been made on nonpolar and non-metallic ionic compounds. Hence, the present investigation was primarily carried out to shed more light on the mechanism of photochemical and thermal decomposition of these systems.
A complete understanding of the photolytic decomposition of metallic salts (e.g., alkali metal azides) is complicated by the presence of metal atoms in the crystal lattice. The role of metal atoms in such cases has been poorly understood. The oxalate systems chosen for this investigation have the advantage that these complexities are absent, enabling a clearer understanding of decomposition mechanisms.
Another objective of the present work was to seek a correlation, if any, between the thermal and photodecomposition mechanisms of oxalate systems.
Scope of Investigation
Photolysis of:
Oxalic acid (dihydrate, anhydrous, and pretreated)
Ammonium oxalate (monohydrate, anhydrous, and pretreated)
Temperature range: 0°C – 30°C
Thermal decomposition of ammonium oxalate monohydrate
Temperature range: 105°C – 130°C
Effects studied:
Particle size
Doping with aliovalent ions
Precompression on photo and thermal decomposition behavior
Techniques employed:
Constant volume pressure-rise method
Mass spectrometry
UV and IR spectroscopy
Results and Discussion
Photolysis of Oxalic Acid:
When fresh oxalic acid is irradiated with UV light at low intensities:
Rate decreases initially until it reaches a minimum, then increases to a constant value.
The initial deceleratory region becomes less prominent as intensity increases and finally vanishes.
This region depends on dislocation density of crystals.
Activation energy of photolysis: 2 kcal·mol?¹
A phenomenological mechanism involving radical formation and radiation damage is suggested.
Photolysis of Ammonium Oxalate:
Rate decreases from an initial maximum to a constant value.
Initial region affected by pretreatments altering defect density.
Intensity dependence follows:
R=AI+BI2R = A I + B I^2R=AI+BI2
where RRR = rate, III = intensity, A,BA, BA,B = constants.
Activation energy: 2 kcal·mol?¹
Mechanism interpreted as decomposition at selective sites and formation of excited species.
Thermal Decomposition of Ammonium Oxalate:
Proceeds incompletely in the range 105°C – 130°C.
Activation energy: 14.8 kcal·mol?¹
Desensitization caused by:
Doping with Pb²? ions
Annealing
Sensitization caused by:
Mechanical pretreatment
Increased surface area
A proton transfer mechanism is suggested.
General Observations
Photolysis of molecular compounds like oxalic acid depends strongly on dislocation density and radiation intensity.
Intensity dependence of photolysis is complex compared to ionic azides.
Role of free radicals in photolysis emphasized.
Activation energy is low for both oxalic acid and ammonium oxalate.
No correlation found between photo and thermal decomposition of ammonium oxalate:
Photolysis proceeds via electron transfer mechanism
Thermal decomposition involves proton transfer as the rate-determining step.