Calcium oxalate stability

Two nucleation processes important to many people (including some surface scientists ) occur in the formation of gallstones in human bile and kidney stones in urine. Cholesterol crystallization in bile causes the formation of gallstones. Cryotransmission microscopy (Chapter VIII) studies of human bile reveal vesicles, micelles, and potential early crystallites indicating that the cholesterol crystallization in bile is not cooperative and the true nucleation time may be much shorter than that found by standard clinical analysis by light microscopy [75]. Kidney stones often form from crystals of calcium oxalates in urine. Inhibitors can prevent nucleation and influence the solid phase and intercrystallite interactions [76, 77]. Citrate, for example, is an important physiological inhibitor to the formation of calcium renal stones. Electrokinetic studies (see Section V-6) have shown the effect of various inhibitors on the surface potential and colloidal stability of micrometer-sized dispersions of calcium oxalate crystals formed in synthetic urine [78, 79]. 

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. 

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. 

TG has been applied extensively to the study of analytical precipitates for gravimetric analysis [9]. One example is calcium oxalate, as illustrated in Figure 2. Information such as extent of hydration, appropriate drying conditions, stability ranges for intermediate products, and reaction mechanisms can all be deduced from appropriate TG curves. Figure 2 also includes the first derivative of the TG curve, termed the DTG cmve, which is capable of revealing fine details more clearly. 

The stabilization of specific crystal planes by face-selective polymer adsorption is a powerful tool for modifying the crystal morphology and for deriving low dimensional crystal morphologies like plates or fibers. Recent reports show that the DHBCs can also be used for stabilization of specific planes of some crystals for their oriented growth, e.g., Au [280], ZnO [300-302], calcium oxalate [299], PbCOa [291], and BaS04 [295]. 

Unfavorable combinations and/or choice of chalk (natural calcite, precipitated cal-cite/aragonite) may cause discoloration in the form of yellow spots in chalk-filled PVC [685]. The formation of yellow spots is attributed to oxalic acid (salts) detected by FTIR-spectroscopy. The creation of oxalic acid (salts) depends on the presence of calcium carbonate. The type of filler and stabilization determine the rate of calcium oxalate creation [685]. 

Stability constants for calcium complexes of a selection of hydroxycarboxylate ligands are listed in Table VII (239,246,272-274). For tartrate, malate, and citrate stabilities decrease in the expected order Ca2+> Ba2+> Ra2+ (231,275). The stability constant for the complex of pyruvate (logiOifi 0.8 (273)) is similar to that for acetate calcium complexes of a-ketoglutarate and of oxaloacetate are somewhat more stable (logio-Ki = 1.3, 1.6 respectively (273)). The sequence logio-Ki = 3.0, 1.4, 1.1, 0.6 for the dicarboxylate ligands oxalate, malonate, succinate,... 

All other oxalates are sparingly soluble in water which would dramatically effect the bioavailability of the metal ions involved. Table III illustrates both the solubility and stability constants for a few selected metals (35.). It may be seen that with calcium it forms a practically insoluble salt at neutral or alkaline pH, being soluble to the extent of 0.67 mg per 100 ml of water at pH 7.0 sind 13°C. Zinc also has limited solubility (0.79 mg/100 ml, 18°) while Fe+2 and Fe+3 show solubilities of 22.0 ml/100 ml and "very soluble" respectively. These chemical facts and their effedt on iron absorption have recently been substantiated in a biological sense by Van Campen and Welch (36) who investigated the availability to rats of iron from two varieties of spinach. Also they compared the absorption of iron between FeClj and Fe-oxalate as well as the effects of adding 0.75% oxalate to the diet. They found that absorption of iron from both varieties of spinach was comparable to that from FeClj and that the iron was equally avail-... 

The rates of alkaline hydrolysis of the half-esters, potassium ethyl oxalate, malonate, adipate, and sebacate were studied in the presence of potassium, sodium, lithium. thallium(I), calcium(II), barium(II), and hexamminecobalt(III) ions (106). On the basis of the results obtained, chelate formation between the metal ions and the transition state of the substrate was postulated. In these chelate structures (structures XXXVIII), formally similar to those postulated in the hydrolysis of a-amino esters (26), the metal ion facilitates the attack by the hydroxide ion by positioning it in a suitable manner. The rate of hydrolysis of the oxalate half-ester is greater than that of the malonate, which in turn is greater than that of the adipate. This is in the expected order of the stability of the metal chelates. The order for the rate of hydrolysis of the ethyl oxalate and ethyl malonate is Ca2+ Ba2+ > [Co(NH3)6]3+ > T1+. The hexamminecobalt(III) ion seems to be less effective than expected, since it is too large to satisfy the steric requirements of the chelate structures. The alkali metals were found to have marked negative specific salt effects on the rates of reaction of the adipate and sebacate, but only a small negative salt effect on the hydrolysis of potassium ethyl malonate. 

In the survey article [174], devoted to a generalization of the data on the use of soaps for the stabilization of pol3rvinyl chloride, it is noted that in recent years salts of inorganic acids have been gradually displaced by soaps, the most widespread among which are lead stearate and barium and cadmium laurates. Salts of lead, tin, barium, calcium, cadmium, strontium, sodium, and lithium of such acids as formic, oxalic, maleic, caprylic, imdecylenic, lauric, stearic, ricinoleic, etc., have... 

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