Dehydration curves oxalate

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... 

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. 

Recently, it has been shown [1071] that CoC204 2 H20 exists in two crystalline modifications, a and 3. Taskinen et al. [1072] prepared anhydrous cobalt oxalate of different particle sizes by dehydration of the (3 (coarser grained) phase and the a/(3 mixture (finer grained). The coarser grained preparation decomposed at 590—700 K with a sigmoid a—time curve fitted by the Avrami—Erofe ev equation [eqn. (6), n = 2] and below and above 625 K, E values were 150 and 57 kJ mole-1, respectively. Reaction of the fine preparation obeyed eqn. (6) (n = 3) and below and above 665 K, values of E were 120 and 59 kJ mole-1, respectively. Catalytic properties of the products of decomposition of cobalt oxalate have been investigated [1073]. 

The r-time curves for the decomposition of anhydrous cobalt oxalate (570 to 590 K) were [59] sigmoid, following an initial deceleratory process to a about 0.02. The kinetic behaviour was, however, influenced by the temperature of dehydration. For salt pretreated at 420 K, the exponential acceleratory process extended to flr= 0.5 and was followed by an approximately constant reaction rate to a = 0.92, the slope of which was almost independent of temperature. In contrast, the decomposition of salt previously dehydrated at 470 K was best described by the Prout-Tompkins equation (0.24 < a< 0.97) with 7 = 165 kJ mol . This difference in behaviour was attributed to differences in reactant texture. Decomposition of the highly porous material obtained from low temperature dehydration was believed to proceed outwards from internal pores, and inwards from external surfaces in a region of highly strained lattice. This geometry results in zero-order kinetic behaviour. Dehydration at 470 K, however, yielded non-porous material in which the strain had been relieved and the decomposition behaviour was broadly comparable with that of the nickel salt. Kadlec and Danes [55] also obtained sigmoid ar-time curves which fitted the Avrami-Erofeev equation with n = 2.4 and = 184 kJ mol" . The kinetic behaviour of cobalt oxalate [60] may be influenced by the disposition of the sample in the reaction vessel. 

Broadbent et al. [69] showed that ar-time curves for the decomposition of copper(II) oxalate (503 to 533 K) were sigmoidal and that data for the vacuum reaction fitted the Avrami-Erofeev equation with values of = 2.9 initially and later n = 3.5 ( , = 136 kJ mol ). Electron transfer was identified as the step controlling the reaction. There was no evidence from X-ray diffraction studies for the intervention of the Cu salt the orthorhombic structure was present until disappearance of the reactant and product copper metal was detected. However, many metal carboxylates, chilled after dehydration, yield anhydrous salts that are amorphous to X-rays or poorly crystalline, see, for example [70]. 

The thermal decomposition of barium titanyl oxalate tetrahydrate, BaTi0(C204)2.4H20, occurs in three stages [105] (i) dehydration, (ii) decomposition of the anhydrous oxalate to the carbonate, and (iii) decomposition of the carbonate forming barium titanate. Isothermal ar-time curves for stage (ii), 509 to 599 K in vacuum, derived from separate measurements of pressures of evolved CO and COj, were deceleratory and superimposable up to ar= 0.3. CO evolution was slower beyond ar= 0.3 and a diffusion mechanism was proposed, , = 189 kJ mol . 

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. 

Figure 7.24. A representative DSC curve for the dehydration and decomposition reactions detected on heating (at 20 K min" ) a 5.21-mg sample of manganese (I ) oxalate dinydrate in nitrogen using a gold sample pan. Water loss and anion breakdown are distinct and separate processes. The latter reaction is complex the three areas marked are in the ration 63 8 29 for 1 2 3. respectively the dotted line indicates the baseline trace during sample rerun 51).

The DTA curve for oxalic acid dihydraie. the only acid studied containing water of hydration, had dehydration peaks with A7 n values ofl 10.120, and 125 C, respectively. All other curve peaks for the organic acids were caused by fusion and decomposition reactions. For example, the second endothermic peak in the succinic acid curve was probably caused by dehydration reaction. 

Thermogravimetric curves for solid K2[Pd(C204)2],3H20 and other transition-metal oxalates indicate that the thermal stability of the anhydrous complexes decreases with increase in electron affinity of the central metal ion. AH values were obtained for both dehydration and decomposition. Subsequent studies showed carbon dioxide as the only gaseous product, the decomposition occurring via electron transfer from a 304 ligand to the central palladium. ... 

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.)... 

The TG plots show two-step decomposition whereas DTA curves show four peaks. The two steps observed in TG correspond to the formation of titanyl/zirconyl oxalate hydrazine and Ti02/Zr02 respectively. The DTA peaks are assigned to dehydration, dehydrazination, and decomposition reactions that can be written as follows ... 

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