Calcium oxalate, anhydrous

Calcium oxalate normally is precipitated as the monohydrate from hot solution. The thermolysis curve shows several plateaus, corresponding to the monohydrate from room temperature to 100°C, anhydrous calcium oxalate from 226 to 398°C, calcium carbonate from 420 to 660°C, and calcium oxide above 840 to 850°C. Sandell and Kolthoff concluded that the monohydrate is not a reliable weighing form because of its tendency to retain excess moisture. Coprecipitated ammonium oxalate also remains undecomposed, so the results are usually 0.5 to 1.0% too high when the precipitate is dried at 105 to 110°C. Anhydrous calcium oxalate also is unsuitable as a weighing form because of its hygroscopicity. 

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... 

Anhydrous calcium oxalate decomposes at 745 K to yield calcite [88]. Freeberg et al. [89] reported E, = 284 kJ mol for the first-order reaction ... 

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. 

Figure 16.2 TGA thermal curve of hydrated calcium oxalate, Ca(COO)2 XH2O, with both adsorbed water from the precipitation process and water of crystallization. There is a loss of adsorbed water starting at about 90°C, and loss of bound water at about 150°C. The stable compound above 225°C is anhydrous calcium oxalate, Ca(COO)2. This loses CO at about 450°C to form CaCOs. The calcium carbonate is stable until approximately 600°C, when it loses CO2 to form CaO. (Compare this step with Fig. 16.1.)...

Figure 3.25 TGA-DTG-MS curves of the thermal decomposition of calcium oxalate monohydrate measured at 30 K/min in a 70-p.L alumina pan. Purge gas argon, 50 mL/min. The diagram shows that calcium oxalate monohydrate decomposes in three distinct steps. The MS fragment ion curves for water (rrVz 18), CO (rrVz 28) and CO2 Mz44) display peaks that correspond closely to the individual steps in the TGA curve. The first mass loss step relates to the elimination and vaporisation of water of crystallisation (1) the second step to the decomposition of anhydrous calcium oxalate with formation of CO (2) and the third step to the decomposition of calcium carbonate to calcium oxide and CO2 (3). The m/z44 ion curve shows that CO2 is also formed in the second step at 550 C (besides CO). This is a result of the disproportion reaction of CO to CO2 and carbon.

A) Preparation of 4-Acetyl-7-Chloro-1,2,3,4-Tetrahydro-1-Methyl-5H-1,4-Bemodiazepin-5-one A mixture of 68.5 g (0.37 mol) of 5-chloro-N-methylanthranilic acid, 51 g (0.51 mol) of calcium carbonate, 76 g (0.37 mol) of bromoethylamine hydrobromide and 2.5 liters of water was stirred and heated under reflux for 3 hours. A solution of 23.4 g (0.26 mol) of anhydrous oxalic acid in 250 ml of water was slowly added to the refluxing mixture. The precipitated calcium oxalate was filtered off, and the filtrate adjusted to pH 7 with concentrated ammonium hydroxide. The filtrate was then concentrated to dryness in vacuo and the residue heated on the steam bath with 400 ml of 6 N ethanolic hydrogen chloride until the residue was crystalline. Filtration gave 122 g of N-(aminoethyl)-5-chloro-N-methylanthranilic acid hydrochloride as a solid. 

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 ... 

Calcium Oxalate. CaCtOj, white precipitate, insoluble in weak acids, but soluble in strong acids, formed by reaction of soluble calcium salt solution and ammonium oxalate solution. Solubility at I8°C 0.0056 g anhydrous salt per liter of saturated solution. 

From the mass-loss curve, then, the following data are obtained D, mass of dry precipitate at 100°C , mass of water of hydration F, mass of carbon monoxide formed by the decomposition of the anhydrous metal oxalates G, mass of carbon dioxide formed by the decomposition of calcium carbonate and L, the mass of carbon dioxide formed by the decomposition of strontium carbonate. From these data, the amounts of calcium, C, strontium, S, and barium, By can be calculated from... 

CHEMICAL PROPERTIES combustible solid gives off water of crystallization at 215°F and begins to sublime anhydrous form sublimes best at 157°C (315°F) decomposes into carbon dioxide, carbon monoxide, formic acid, and water reacts with strong alkalies, strong oxidizing materials, chlorites, and hypochlorites forms highly insoluble calcium oxalate FP (NA) LFL/UFL (NA) AT (NA) HF (-821.7 kJ/mol crystal at 25°C). 

A sample of mixed calcium oxalate monohydrate and anhydrous silica weighed 7.020 mg. After heating to 600°C, the weight of the mixture was 6.560 mg. What was the weight of calcium oxalate in the original sample What information can be obtained by DTA that cannot be obtained by TGA Describe the components of a DTA instmment and discuss the differences between a DTA and a TGA instmment. 

Fig. 1. Solubilities S of stone-fonning substances in urine-like liquors at 37°C. (Cys) L-Cystine in standard-reference artificial urine (dots), real urine (circles) and O.SOmolkg NaCl (triangles) [39], (COM) Calcium oxalate monohydrate (as [Ca +lj, ) in standard-reference artificial urine (dots) [45], (UAA) Anhydrous uric acid in standard-reference artificial urine (diamonds) and 0.30molkg NaCl -l- 0.30molkg urea (squares) [37,38], (Xan) Xanthine in 0.30 mol kg NaCl (squares) [40]. The experimental data are compared to model calculations (lines) described in the respective publications.

A. Triethyl oxalylsuccinate. In a 2-1. three-necked flask equipped with a sealed stirrer and a reflux condenser bearing a calcium chloride drying tube is placed 356 ml. (276 g., 6.00 moles) of anhydrous ethanol (Note 1). Sodium (23 g., 1.0 g. atom) is added in small portions at a rate sufficient to keep the ethanol boiling. External heating is required to dissolve the last portions of the metal. After all the sodium has dissolved, the excess ethanol is removed by distillation at atmospheric pressure as the mixture becomes pasty, dry toluene is added in sufficient amounts to permit stirring and to prevent splattering of the salt. Distillation and addition of toluene is continued until all the ethanol is removed and the contents of the flask reach a temperature of 105° (Note 2). The sodium ethoxide slurry is cooled to room temperature and 650 ml. of anhydrous ether is added, followed by 146 g. (1.00 mole of diethyl oxalate. To the yellow solution there is added 174 g. (1.00 mole) of diethyl succinate, and the mixture is allowed to stand at room temperature for at least 12 hours. 

Yttrium oxalate is then ignited to its oxide, Y2O3. The oxide is heated at 750°C in a stream of anhydrous hydrogen fluoride to yield yttrium fluoride, YF3. Alternatively, the oxide is mixed with ammonium hydrogen fluoride NH4HF2 and heated at 400°C in a stream of dry air or helium. Yttrium metal may be produced from its fluoride either by metallothermic reduction or electrolysis. The more common metallothermic reduction involves reducing the fluoride with redistilled calcium in 10% excess over the stoichiometric amounts at elevated temperatures ... 

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