Kinetic study of dehydration of calcium oxalate trihydrate

Abstract

The dehydration of calcium oxalate trihydrate was studied using DSC method and open sample crucibles. The dehydration proceeds in two steps where first two molecules of water are released around 80 °C and consequently the last molecule of water is released at a temperature around 130 °C. The kinetic analysis of overlapping peaks was done and the apparent activation energy, pre-exponential factor and kinetic exponents of kinetic model (reaction order model) were evaluated for both effects.

Thermal decomposition of mixed calcium oxalate hydrates – kinetic deconvolution of complex heterogeneous processes

“…In order to compare the dehydration enthalpy values, the data have to be normalized with respect to the mass of the same material COM (all the above-listed values are normalized on the overall sample mass, where the composition differs). For the second dehydration step (COM→anhydride) this transformation gives values 389 J•g_ COM -1 [55], 360 J•g_ COM -1 [57] and 340 J•g_ COM -1 (present data), which is in reasonable agreement -the differences may be caused by slightly different dehydration enthalpies being associated with the particular COM morphologies (prisms, dendrites, elliptical cylinders). Similar calculation gives 681 J•g_ COM -1 [55], 713 J•g_ COM -1 [57] and 112 J•g_ COM -1 (present data) for the first dehydration step.…”

“…For the second dehydration step (COM→anhydride) this transformation gives values 389 J•g_ COM -1 [55], 360 J•g_ COM -1 [57] and 340 J•g_ COM -1 (present data), which is in reasonable agreement -the differences may be caused by slightly different dehydration enthalpies being associated with the particular COM morphologies (prisms, dendrites, elliptical cylinders). Similar calculation gives 681 J•g_ COM -1 [55], 713 J•g_ COM -1 [57] and 112 J•g_ COM -1 (present data) for the first dehydration step. Considering that a mixture of COT and COM was prepared during the present synthesis, the TG data suggest that approx.…”

“…The DSC curve shows two distinct dehydration steps; similar two steps were reported in the DSC studies of COT [55,57] but with reverse proportional magnitude. In [55] the enthalpies associated with the first and second step were approx. 546 (attributed to the loss of 2 water molecules, i.e.…”

“…In addition to ionic strength the precipitation products are determined by pH and temperature. [55] The following parameters were used for the present synthesis: 0. The DSC measurements were realized by using heat-flow differential scanning calorimeter Q2000 (TA Instruments) equipped with an autosampler, RCS90 cooling accessory, and T-zero technology.…”

“…They obtained E = 40.9 kJ•mol -1 and order of the reaction (with the n-th order reaction model [49]) n = 0.71 for the dehydration, and E = 51.4 kJ•mol -1 and order of the reaction n = 0.38 for the decomposition to carbonate. Honcová et al [55] studied dehydration of COT via differential scanning calorimetry (DSC) -the two overlapping peaks were described with the following kinetic parameters: activation energies equal to 69 and 83 kJ•mol -1 for the first and second endothermal effect, respectively; reaction orders equal to 0.5 for both dehydration steps. Budrugeac et al [56] reported activation energy for thermogravimetric dehydration of COM to be 125 kJ•mol -1 in the range of degree of conversion (0 -15 %) and 94 kJ•mol -1 for the range (20 -70 %).…”