Solubility calcium

The total hardness of the treated water may be as low as 35 ppm or so (as CaC03). The pH level normally should be approximately 9.4 to 9.6 to obtain minimum calcium solubility. If it is much above this level, it indicates an excess of lime the treated water will not be stable, and some secondary precipitation can be expected. 

The in vitro estimate of potential availability was defined, somewhat arbitrarily, as calcium solubility (18,000 x g supernatant) after complete digestion. Potentially available calcium was expressed as a percentage of the total food calcium (Figure 1). With the exception of a low in vitro calcium solubility value for whole milk, our earlier data compared reasonably well with calcium bioavailability information in the literature (7.) ... 

The in vitro procedure was tested in "critical" experiments designed to make direct comparisons of in vivo and in vitro estimates of exchangeability and potential bioavailability and to test the use of in vitro exchangeability values in in vivo experiments. (8). Three foods which were expected to show different levels of calcium solubility and exchangeability, collards, soybeans and spinach, were intrinsically labeled with 45Ca in nutrient solution culture. They were used together with 47 Ca as an extrinsic label in both in vitro and in vivo experiments. 

Experiments were conducted to determine if varying the conditions in the in vitro digestion procedure would affect post-digestion calcium solubility and in some cases, exchangeability. This was done with two purposes to test the use of the in vitro digestion procedure for studying factors which might influence calcium bioavailability and to use the results to modify the standard procedure. 

Figure 2. Relationship between calcium solubility and pH after complete digestion for four soy products. Key solid line, full-fat soy flour long-dashed line, soy protein isolate short-dashed line, soy protein concentrate and dotted line, defatted soy flour...

Table III. Effect of In Vitro Pancreatic Digestion pH on Carbohydrate and Protein Digestion and Calcium Solubility...

Figure 3. Calcium solubility dependence on bile salt preparation and concentration used in in vitro digestion (pH 6.8-6.9). Key ...

Choice of Potential Bioavailability Criterion. It is usually assumed that calcium must be soluble and probably ionized in order to be available for absorption ( ). For the in vitro procedure, as a first approximation we chose calcium solubility after centrifugation at 18,000 x g as the measure of potential bioavailability (Figure 1). We assumed that this would probably overestimate the available calcium and later work based on fractionation might define the bioavailable calcium more precisely. The data in Table IV illustrate how the choice of criterion for "solubility" could affect the in vitro estimate of potential availability, even if in vitro conditions closely resembled in vivo conditions. Since our in vitro criterion unexpectedly underestimated calcium bioavailability for two of the three foods in the direct in vivo - in vitro comparison (8), it was necessary to determine the in vitro digestion conditions which might be limiting solubility before addressing the choice of appropriate criterion. 

Digestion Conditions. Peptic conditions were not emphasized since calcium solubility is high at the peptic stage (Table I) and chyme release to the duodenum is more dependent on particle size than completeness of digestion (23>24). 

The term "standard" is not meant to denote a digestion procedure which should be routinely used to determine potentially available calcium. Since the number of foods tested so far is limited, it will be more useful to think of the "standard" procedure as a set of conditions to be used to see how well we understand food chemistry and calcium solubility in the gastrointestinal environment. It should be used for measuring the relative solubility of calcium from foods and meals, but mostly in the context of comparisons with in vivo results to define factors which require further study. 

Table I. Effects of 17o Orthophosphate, Tripolyphosphate, and Hexametaphosphate on Calcium Solubility from Ground Beef or Soy Protein Concentrate Subjected to In Vitro Gastric and Gastrointestinal Digestions...

Although all three phosphates tested caused substantial decreases in calcium solubility following the gastric digestion, this trend was reversed upon completion of the second phase of the digestion. Only orthophosphate caused a decrease in calcium solubility from the soy following the complete digestion, while all three phosphates enhanced calcium solubility from the meat. Thus, phosphates are unlikely to interfere with calcium absorption due to the formation of insoluble salts in the gut. 

A,A, A" Calcium concentrations, operating, satd. with gypsum, satd. with hemlhydrate (Figure 4). nucleatlon rate, /min. 1 o slurry, calcium concentration g Ca /I soln. under saturation, g Ca /I soln. equilibrium calcium solubility, g Ca Vg soln. equlHbrium calcium concentration at transition temperature g Ca /g soln. 

Titratable acidity Total soluble calcium Soluble unionized calcium Ionized calcium Soluble magnesium Soluble citrate... 

Citrate. Addition of citrate reduces the concentrations of calcium ions and colloidal calcium phosphate and increases the soluble calcium, soluble phosphate and pH. 

Calcium solubility curves for TPN solutions containing 1.5% (w/v) amino acid and 10% (w/v) dextrose at pH 5.5 are shown in Fig. 10.1. The broken straight lines show the calcium and phosphate concentrations at 3 1 and 2 1 ratios. The dotted curve for Aminosyn solutions shows the concentrations at which precipitation occurs after 18 h at 25°C followed by 30 min in a water bath at 37°C. The full curve is for TrophAmine solutions, and represents calcium or phosphate concentrations at which visual or microscopic precipitation or crystallisation occurs. Compositions to the left of the curves represent physically compatible solutions. 

Calcium solubility as a function of calcium concentration and pH was investigated and the concentrations of calcium species in solution as well as calcite precipitate shown. 

In the last section, a rough estimate for the solution speciation in the presence of a dissolved organic matter (DOM) was attempted. DOM is the MinteqAl equivalent of DOC. This involved the speciation of DOM as a function ofpH for two calcium concentrations. Subsequently the impact of DOM on calcium solubility and calcite precipitation was examined. Again, the calcium speciation was determined as a function of pH. 

Figure A5.4 Calcium solubility (at pH 8). Calcite concentrations are 0.17 mM at 3 mM calcium and 2.17 mM at 5 mM calcium.

The effect of DOM on calcium solubility is less apparent than expected. Calcite precipitates, as in the absence of DOM (Figure AS.4), from a calcium concentration of 3 mM, as shown in Figure A5.8. However, the calcite concentration is now varied with a concentration of 0,12 at 3 mM and 2,12 at 5 mM. The calcite concentration is reduced by the concentration of the Ca-DOM complex (see Figure A5.9). 

Figure A5.8 Calcium solubility (at pH 8) in the presence of 100 jaeq Li organic matter (12.5 mg Li as DOC).

This example provides a further illustration of the way in which com-plexation increases the solubility of sparingly soluble solids. The curves in Fig. 6-15 show that the consideration of calcium complexes becomes significant in determining calcium solubility at pH values above 9. 

Verduch analyzed the W spacing of the primary diffraction line of cristobalite to determine the extent of calcium solubility. Since the procedure was not the usual lattice parameter determination, his results may be questionable. It is noted that Verduch confirmed the work of Jander and Hoffman with respect to the order in appearance of products. 

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