Calcium in solution

The hardness test sometimes is performed on the mud as well as the mud filtrate. The mud hardness indicates the amount of calcium suspended in the mud as well as the calcium in solution. This test usually is made on gypsum-treated muds to indicate the amount of excess CaSO present in suspension. To perform the hardness test on mud, a small sample of mud is first diluted to 50 times its original volume with distilled water so that any undissolved calcium or magnesium compounds can go into solution. The mixture then is filtered through hardened filter paper to obtain a clear filtrate. The total hardness of this filtrate then is obtained using the same procedure used for the filtrate from the low-temperature low-pressure API filter press apparatus. 

In arid and semi-arid soils with a pH range of 6-9, free calcium is the major Ca speciation form in soil solution. When pH > 9.2-9.5, CaP04 becomes a major calcium species in soil solution of neutral and calcareous soils, especially when the activity of H2P04 is > 10 5 M (Lindsay, 1979), such as after phosphate fertilizers are used. Lindsay (1979) further pointed out that CaS04° contributes significantly to the total calcium in solution when S042- is > 10 M. 

The virial methods differ conceptually from other techniques in that they take little or no explicit account of the distribution of species in solution. In their simplest form, the equations recognize only free ions, as though each salt has fully dissociated in solution. The molality m/ of the Na+ ion, then, is taken to be the analytical concentration of sodium. All of the calcium in solution is represented by Ca++, the chlorine by Cl-, the sulfate by SO4-, and so on. In many chemical systems, however, it is desirable to include some complex species in the virial formulation. Species that protonate and deprotonate with pH, such as those in the series COg -HCOJ-C02(aq) and A1+++-A10H++-A1(0H), typically need to be included, and incorporating strong ion pairs such as CaSO aq) may improve the model s accuracy at high temperatures. Weare (1987, pp. 148-153) discusses the criteria for selecting complex species to include in a virial formulation. 

NRS [New Regeneration System] A process for regenerating the ion-exchange resin used for removing calcium from sugar solution. If sodium chloride were used, the waste calcium chloride solution would have to be disposed of if sodium hydroxide were used, calcium hydroxide would be precipitated in the resin. The NRS process uses sodium hydroxide in the presence of sucrose, which retains the calcium in solution as calcium saccharate. Developed by the IMACTI Division of Duolite International, The Netherlands. 

Selected entries from Methods in Enzymology [vol, page(s)] Chelation, 238, 74, 76, 297 buffers [for analysis of exocytosis, 221, 132 preparation, 219, 186 modulation of cytosolic buffering capacity with quin2, 221, 159] fluorescence assay, 240, 724-725, 740-742 fluorescence imaging, 225, 531 238, 303-304, 322-325, 334-335 free intracellular levels after bacterial invasion, 236, 482-489 free calcium in solutions for membrane fusion analysis, calculation and control, 221, 149 homeostasis mechanisms, 238, 80 hormonal elevation, 238, 79 inositol phosphate effect on release, 238, 207 determination of cytosolic levels [computer methods, 238, 73-75 with fura-2, 238, 73, 146 with indo-1, 238, 298, 316-317 with quin-2, 238, 297] hormone effects, 238, 79 ionomycin effects, 238, 79 membrane depolarization effects,... 

The calcium taken up by living systems will be found mainly as extracellular deposits or structures, with a small amount present in intracellular stores, together with some in various extracellular fluids. The calcium in fluids will be bound to protein or will be free in solution, with a few percent bound to small molecules or anions. The calcium in solution will exchange with the calcium in structures such as bone and teeth, due to the activity of hormones such as l,25-(OH)2D3, parathyroid hormone and calcitonin,455 which are responsible for the remodelling and repair of bone. 

The level of calcium in solution will depend upon the presence of precipitating anions, notably phosphate and carbonate. Calcium will precipitate as the phosphate to give hydroxyapatite, Caio(P04)6(OH)2, in bones and teeth, and as the phosphate or carbonate to give other structures, including small crystals, or non-crystalline deposits in cells. Small crystals of calcium carbonate, found in the inner ear of some animals, are responsible for the control of balance. Various calcified tissues result from the precipitation of calcium salts, such as hydroxyapatite in the calcification of the aortic wall, and the oxalate in various stones. 

Figure 7. Comparison of observed calcium in solution (15) during free drift experiment at 0.954 atm COj and 25°C loith simulated reaction. The dashed line is calculated using the reported surface area and the solid line is calculated assuming the area is half the reported value.

The normal urinary excretion range of citric acid is 200-1000 mg/day, as has been established by 0stberg (012). The urinary citrate plays a fundamental role in keeping calcium in solution and so preventing the precipitation of calcium salts in the urinary tract (C9, H8, S20). This role is illustrated by the frequency of calcium lithiasis after administration of substances which, lie acetazoleamide (Diamox), reduce the urinary citrate without afifecting the urinary calcium (G16). 

XsCa + = equivalents of calcium in solution/total equivalents in solution... 

The solubility of sparingly soluble salts may also be affected by the presence of impurities. Kemmer [1988] gives as an example the solubility of CaCO in the presence of magnesium. If magnesium is precipitated along with CaCO the residual concentration of calcium in solution may be increased. The inclusion of other impurities, such as strontium, has also been shown to increase the solubility of CaCOj. The use of empirical data from process plant therefore, has to be treated with caution since the presence of impurities may have a significant effect on the fouling propensity under certain circumstances, and will depend on location. 

Acid medium. Absorption of calcium is favored in an acid medium (a lower pH) because it keeps the calcium in solution hence, most of the absorption occurs in the duodenum. 

The power of a silicate to sequester calcium can be evaluated by a test called the Calcium Sequestration Kinetics Test. This test consists in introducing 1 g/L of builder into a solution containing 3 X 10 M of calcium chloride and then measuring the free calcium content with a previously calibrated specific electrode. After a given time, often 10, 15, or 20 min, the experiment is halted. This test can be used to compare the action of different builders in eliminating free calcium in solution. 

Calcium may be determined by atomic absorption spectrophotometry. This technique has been used for the estimation of calcium in biological fluids and agricultural materials. It is a speedy method, superior to the more tedious chemical determination by oxalate precipitation and more specific than the EDTA titration method. The accuracy is of the order of 2 per cent and sensitivity limits have been reported at 0 08 to 1 p.p.m. of calcium in solution. The interference problems are very similar to those experienced with the emission method although not quite so formidable. 

Recently a sensitive specific colour reagent for microgram quantities of calcium in solution has been described. This is di-(o-hydroxyphenyl-imino) ethane, also known as glyoxal-bis-(2-hydroxyanil) and this has been adapted to the quantitative determination of small quantities of calcium in magnesium carbonate by Leonard. 

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