Calcium solubility isotherms

In discussions of the precipitation of calciinn phosphate, the phase which is usually emphasized is the thermodynamically most stable, HAP. However, most calcium phosphate solutions in which precipitation experiments are made, are initially supersaturated with respect to four additional phases. The solubility isotherms are shown in Figure 1 as a function of pH. Thus, at a pH of 7.0, in order of increasing solubilities, it is necessary to take into account tricalcium phosphate (Ca3(PO )2 hereafter TCP), octacalcium phosphate (Ca H(PO ). 2 1/2 H2O, hereafter OCP), anhydrous dicalcium phosphate (CaHPO, hereafter DCPA), and dicalciiam phosphate dihydrate (CaHPO. 2H2O, hereafter DCPD). The corresponding thermodynamic solubility products. 

Figure 4.21 shows the pH dependence of the solubility isotherms of the biologically most important calcium phosphates at 25 °C. From this diagram the following order of solubility results (De Groot et al., 1990). 

Figure 5.1 Solubility isotherms of various calcium orthophosphate minerals as a function of the pH value. (After de Groot et al. (1990).)...

Figure 1. Solubility isotherms of calcium phosphate phases in the system Ca(0H)2-H3P04-H20 at 37°C. DCPD is dicalcium phosphate dihydrate, brashite DCPA is dicalcium phosphate anhydrous, monetite OCP is octacalcium phosphate p-TCP is p-tricalcium phosphate and OHAp is hydroxylapatite. pignre 1.1 from Elliott (1994), reprinted with permission from Elsevier Science.]...

Chow [18] calculated, based on these data, the solubility isotherms at 37 °C over a wide pH range (Figure 8.1). These solubility isotherms were shown as the function of the concentration of calcium and phosphate ions in a saturated solution of each calcium phosphate salt. The relative stability of calcium phosphate salts at various values of pH can be obtained by these solubility isotherms. At a given pH, a salt with its isotherm lying below that of another salt is less soluble and more stable than the other. Therefore, HAp is the most stable and least soluble salt among these salts in the range of pH below approximately 4.2 where DCPA is the least soluble. Similarly, TTCP is the least stable and most soluble salt in the range of pH below 8.5, above which pH DCPD is the most soluble. 

Markarov, S.Z. and Shcharkova, E.F., Solubility isotherm (10°C) of ternary systems of calcium and sodium hypochlorites and chlorides, Russ. J. Inorg. Chem., 14, 1632,1969. 

Figure 10.28 Solubility isotherms of calcium orthophosphates at 25 °C. HAp hydroxyapatite DCPD brushite DCPA monetite P-TCP P-tricalcium phosphate OCP octacalcium phosphate TTCP tetracalcium phosphate (De Groot et ai, 1990).

A thermodynamic model developed by Barba, Brandani and di Giacomo (1982) described the solubility of calcium sulphate in saline water. A system of equations based on Debye Hiickel and other models was used to describe isothermal activity coefficients of partially or completely dissociated electrolytes. Using binary parameters, good agreement was claimed between experimental and predicted values of calcium sulphate solubility in sea water and brackish brines including those with a magnesium content. 

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