Calcium oxalate, decomposition

The amount of oxygen desorbed during TPD as CO and CO2 can be related to the amount of active sites. In order to quantify the evolved CO and CO2, a previous calibration with calcium oxalate was performed [7]. Fig. 2 shows the relationships between the theoretical amount of CO and CO2 produced from calcium oxalate decomposition and the experimental peak area values from the CO and CO2 evolution profiles during the calibration tests. 

Table 5 Rate constants at various temperatures for calcium oxalate decomposition...

In the UPO3 example, it seemed to be possible to make this assumption. The present example illustrates the more common case that Eq. (6) with the first two factors alone, does not represent the kinetic data for different heating rates and masses. The calcium oxalate decomposition is a reaction with a substantial g(T,p). 

Fig. 2.39 TG, DTG, DSC and MS curves of the calcium oxalate decomposition. TG and FTIR curves of the calcium oxalate decomposition...

The sodium formate process is comprised of six steps (/) the manufacture of sodium formate from carbon monoxide and sodium hydroxide, (2) manufacture of sodium oxalate by thermal dehydrogenation of sodium formate at 360°C, (J) manufacture of calcium oxalate (slurry), (4) recovery of sodium hydroxide, (5) decomposition of calcium oxalate where gypsum is produced as a by-product, and (6) purification of cmde oxahc acid. This process is no longer economical in the leading industrial countries. UBE Industries (Japan), for instance, once employed this process, but has been operating the newest diaLkyl oxalate process since 1978. The sodium formate process is, however, still used in China. 

A. The thermal decomposition of calcium oxalate monohydrate. This determination may be carried out on any standard thermobalance. In all cases the manufacturer s handbook should be consulted for full detailed instructions for operating the instrument. 

As additional experiments, investigate the decomposition of calcium oxalate in a static air atmosphere and in a nitrogen atmosphere at a flow rate of 10 mL min -. Compare the final stage of the decomposition, i.e. the conversion of calcium carbonate to calcium oxide, using different furnace atmospheres. 

The thermogram for calcium oxalate monohydrate (CaC204.H20) is presented in Figure 11.2. The successive plateaus correspond to the anhydrous oxalate (100-250°C), calcium carbonate (400-500°C), and finally calcium oxide (700-850°C). In other words, these plateaus on the decomposition curve designate two vital aspects, namely ... 

In TA the mass loss can be due to such events as the volatilization of liquids and the decomposition and evolution of gases from solids. The onset of volatilization is proportional to the boiling point of the liquid. The residue remaining at high temperature represents the percent ash content of the sample. Figure 2 shows the TA spectrum of calcium oxalate as an example. 

A popular and useful device is a combined DTA/TG (simultaneous thermal analysis STA) system in which both thermal and mass change effects are measured concurrently on the same sample. An example STA study comprising DTA, TG, and DTG for the decomposition of calcium oxalate is shown in Figure 5.7. 

Figure 5.7 Decomposition of calcium oxalate hydrate (CaC204-H20) in a Setaram TG-DTA [4], A heating rate of 10°C/min and an argon atmosphere were used. Mass spectroscopy (see subsequent discussion) was also used. Three successive steps in the decomposition are shown (1) CaC204-H20 = CaC204 + H20, (2) CaC204 = CaC03 + CO, and (3) CaC03 = CaO + C02. Note that there is a low concentration of C02 measured with mass spectroscopy (MS) associated with the release of CO. The exotherm associated with the oxidation of CaC204 is not present because of the inert atmosphere.

Cano, G. Simultaneous Thermogravimetry and Gas Analysis. Application to the Study of the Decomposition of Calcium Oxalate. Bull. Soc. Chim. France 2540/2542 (1963). 

H2) Freeman, S., and B. Carroll The application of Thermoanalytical techniques to reaction kinetics. The Thermogravimetric Evaluation of the Kinetics of the Decomposition of Calcium Oxalate Monohydrate. J. Phys. Chem. 62, 394/ 397 (1958). 

Fig. 2. Calibration plots representing the number of moles of CO and CO2 against the area under the peaks from the decomposition of calcium oxalate.

The calibration and application of a heat flux DSC in the study of heterogeneous reactions has been discussed in the literature (248). The possibilities and limitations of this technique were demonstrated for methanation and methanol synthesis on Cu/ZnO catalysts. More recently, Rejai and Gonzalez (222, 223) used a DSC to investigate the reduction of Pt02, PtCl2, and H2PtCl6, the decomposition of calcium oxalate, and the formation of supported Pt-Ru bimetallic catalysts. The results were consistent with values based on standard enthalpies of formation reported in the literature. This work illustrates the power of calorimetry for studying the important processes involved in catalyst preparation and treatment. 

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... 

If you perform the next step without waiting for the solution to cool fully, your purple product will form an inseparable precipitate with calcium oxalate. Waiting too long to add the calcium chloride solution, however, will result in the formation of a red powdery precipitate. This has Xmax of 375 and 525 nm and is highly insoluble in H2O. Attempts to revive the solution by addition of more calcium chloride and by the application of heat result in decomposition to a similar red precipitate. 

Calcium salts. The thermal decompositions of calcium oxalate, malonate, maleate and fumarate were studied in significantly higher temperature ranges (above 720, 612 to 653, 733 to 763 and 733 to 803 K, respectively) than those of the same salts of the transition metals. This is evidence of a stabilizing influence on these anions of the strong bond formed with this strongly electropositive cation. 

It is common practice to check the performance of a TGA system by running a sample of calcium oxalate monohydrate. This salt is known to thermally decompose in three stages over well-defined temperature ranges. The first step involves the loss of the single water of hydration molecule followed sequentially by the conversion of anhydrous calcium oxalate to calcium carbonate with the loss of carbon monoxide, and thence the decomposition of calcium carbonate to calcium oxide with the evolution of carbon dioxide. 

Thermogravimetric analysis has been widely used to study the thermal decomposition of oxy-salts, such as metal oxalates and metal sulfates. Dollimore, Griffiths and Nicholson have reported TGA data for a wide range of metal oxalates. In an atmosphere of air, these all decompose in three stages, similar to the thermal decomposition of calcium oxalate 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. 

Few data are available on the concentration of dicarboxylic acid anions in subsurface waters. C2 through C q saturated acid anions have been reported in addition to maleic acid (cz5-butenedioic acid) (5. 15-16L Oxalic acid (ethanedioic) and malonic acid (propanedioic) appear to be the most abundant. Reported concentrations range widely from 0 to 2540 mg/1 but mostly are less than a few 100 mg/1. Concentrations of these species in formation waters are probably limited by several factors, including the very low solubility of calcium oxalate and calcium malonate (5), and the susceptibility of these dicarboxylic acid anions to thermal decomposition (16). This paper will focus on the monocarboxylic acids because they are much more abundant and widespread, and stability constants for their complexes with metals are better known. We do recognize that dicarboxylic acid anions may be locally important, especially for complexing metals. 

Calcium Oxalate—Oxalate of lime—Ca.O,Oi—1S8—exists in the-sap of many plants, and is formed as a white, crystalline precipitate, by double decomposition, between a Ca salt and an all.-aline oxalate. It is insoluble in HaO, acetic acid, or NH,HO soluble in the mineral acids and in solution of HaNaP04. 

Calcium oxalate minerals were found oidy in the rhizosphere soil. In soil, oxalates come from the acid dissociation of oxalic acid produced during the decomposition of organic matter or by the metabolism of microorganisms and fungi (Cromack et al., 1979 Malajczuk and Cromack, 1982 Fox, 1995 Jones, 1998 Caviglia and Modenesi, 1999 Tail et al., 1999). Plant roots can also excrete oxalic acid to increase the availability of Fe and P (Graustein et al., 1977 Jurinak et al., 1986 Bar-Yosef, 1991 Staunton and Leprince, 1996) or to inactivate toxic elements (Kochian, 1995). Once released, oxalic acid participates in mineral alteration (Shotyk and Nesbitt, 1992 Jones, 1998). The observation that at Mount... 

We would lose 73 mg for every 100 mg of pure magnesium oxalate dihydrate we had in the mixture on heating the mixture to 500°C. Note that the decomposition of magnesium oxalate does not follow the same process as the decomposition of calcium oxalate. Calcium oxalate first forms calcium carbonate, which only loses carbon dioxide to form CaO at temperatures above 600°C. Magnesium oxalate appears to decompose directly to MgO in one step at about 500°C. The weight remaining from the magnesium oxalate is equal to ... 

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). 

---