Dehydration, calcium oxalate monohydrate

Fig. 37. Effect of sample packing on the dehydration course of calcium oxalate monohydrate shown by mass spectrometry...

Fig. 5 TG and TG-MS of calcium oxalate monohydrate. The TG steps correspond to the dehydration followed by the formation of calcium carbonate, then calcium oxide is obtained. The evolved gas is detected by MS.

The distributed reactivity models used by Burnham and Braun [92] in the kinetic analysis of complex materials (see Section 5.5.12.) deserve further consideration, particularly in view of the results obtained by Christy et ai, [93] for the kinetics of dehydration of calcium oxalate monohydrate. Water loss proceeds at different rates from different lattice sites in this monohydrate. 

Other decompositions, which had previously been accepted as simple reactions proceeding in the solid state, have subsequently been shown to be more complicated than was discerned from overall kinetic data. The thermal breakdown of potassium permanganate exhibits almost symmetrical sigmoid curves, now regarded (39) as proceeding with the intermediate formation of K3(Mn04)2 by at least two, possibly consecutive, reactions. Dehydration of calcium oxalate monohydrate proceeds (75) with the loss of H20 molecules from two different types of site by two concurrent reactions that proceed at slightly different rates. 

Similar discontinuities in Arrhenius plots are observed in thermal analysis (TA) as well, in particular, in the dehydration of crystalline hydrates performed in humid air. For illustration. Fig. 3.2 reproduces an Arrhenius plot for the dehydration of calcium oxalate monohydrate in an air flow, carried out under non-isothermal conditions by Dollimore et al. [28]. The equilibrium pressure of water vapour Pgqp measured at temperatures of up to 400 K and comparatively moderate decomposition rates turns out to be lower than its partial pressure in air which implies that the decomposition occurs in the isobaric mode. Above 400 K, the equilibrium pressure of H2O becomes higher than p with the process becoming equimolar. The slope of the plot decreases to one half of its former value in full agreement with theory (see Sect. 3.7). 

Dehydration of calcium oxalate monohydrate CaC204.H20 Calcination of titania/PVA expanded hectorite CONCLUSIONS... 

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