Dehydration, calcium oxalate

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

In conclusion, any metal oxalate can be considered as a compound in a metastable equilibrium. The only sub-spontaneous diagenetic evolution of calcium oxalate is a possible transformation of weddellite into whewellite by dehydration (Frey-Wyssling, 1981 Verrecchia et al., 1993). Consequently, activation energy has to be provided for a complete oxidation of COM or COD, and life is the best and the most obvious candidate. Therefore, biogenic activity could be the key explanation for the absence of oxalate in paleosols as well as in the geological sedimentary record. 

Thermogravimetry can be coupled with DSC. Most companies offer the TG-IR or the TG-MS coupling.f Synergic chemical analysis by coupling TG-FT-IR, TG-MS or TG-GC-MS has been recently discussed. Fig. 5 demonstrates the ability of TG-MS for the study of dehydration and decomposition of calcium oxalate dihydrate. The steps correspond to the dehydration into anhydrous calcium oxalate, followed by the transformation into calcium carbonate then by the formation of cacium oxide. ° The sample... 

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... 

Gardner, G.L. (1976) Kinetics of the dehydration of calcium oxalate trihydrate crystals in aqueous solution. Journal of Colloid and Interface Science, 54, 298-310. 

An example is given in Fig. 4, which shows partial thermogravimetric curves (dehydration only) obtained from calcium oxalate precipitates prepared at different initial reactant concentrations. Curve 1 represents dehydration curves typically obtained from samples of COM of different morphologies formed by heterogeneous nucleation (including compact crystals and dendrites) curve 2 is... 

FIG. 4 Partial TG curves (dehydration only) showing the loss of water from (1) compact and dendritic crystals of COM and (2) microcrystaUine aggregates with the structure of COD, dmi and dm2 are the total mass loss (i.e., loss of hydration water) corresponding to 1 mol of HjO (rfm, for COM) and 2.5 mol of HjO dm2 Irom microcrystalline aggregates) per mole of calcium oxalate. (Adapted from Ref. 44.)... 

By determining the mass loss due to dehydration (rfwi and dm2 in Fig. 4), it is possible to quantitatively determine the phase composition of mixtures of crystal hydrates provided it was qualitatively ascertained by some other method (for instance, by X-ray powder diffraction). This method has been used to determine the influence of various experimental parameters on the phase composition of calcium oxalate precipitates. Several examples are given in Figs. 5-9. 

At low and medium supersaturations, hydrophilic cations form different crystal hydrates by heterogeneous nucleation and subsequent crystal growth and phase transformation. Dehydration curves give information on the modes of water incorporation resulting from different modes of crystallization. A useful application of thermal analysis is the analytical approach by determining the mass loss due to dehydration, it was possible to quantitatively determine the proportion of different calcium oxalate hydrates in mixtures, which have been qualitatively analyzed by other techniques (X-ray powder diffraction, IR spectroscopy, etc.). The method yielded excellent results in studies of the kinetics of phase transformation and has been successfully used to demonstrate the potential of surfactant micelles to control the nature of the crystallizing phase. 

Decomposition of model substances method The third method of calibration is by carrying out an experimental run using certain well studied model substances such as copper sulfate pentahydrate, calcium carbonate, calcium oxalate mono hydrate, potassium carbonate, sodium hydroxide, zinc oxalate dihydrate, and benzoic acid. These model substances show well resolved dehydration and decomposition temperatures over a wide temperature range. 

A classic example of a solid—fluid ceramic powder synthesis reaction is that of calcination and dehydration of natural or synthetic raw materials. Calcination reactions are common for the production of many oxides from carbonates, hydrates, sulfates, nitrates, acetates, oxalates, citrates, and so forth. In general, the reactions produce an oxide and a volatile gaseous reaction product, such as CO2, SOg, or HgO. The most extensively studied reactions of this type are the decompositions of magnesium hydroxide, magnesium carbonate, and calcium carbonate. Depending on the particular conditions of time, temperature, ambient pressure of CO2, relative humidity, particle size, and so on, the process may be controlled by a surface reaction, gas diffusion to the reacting... 

Oxalic acid and its soluble salts are poisonous to humans and animals, whereas insoluble salts of calcium and magnesium oxalate are not. Oxalates ingested by humans may be precipitated by calcium as an insoluble complex, which then is excreted in feces [21, 22, 23]. In both cases reported by Chien et al [21], patients ingested sour carambola juice on an empty stomach so that the protective effect of calcium and magnesium in food was not present. The dehydration state may have contributed to the development of carambola- associated acute nephropathy. The authors do not report any concomitant neurological signs or symptoms [21]. 

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