Calcium solutions, supersaturated

A disadvantage of the conventional precipitation method in which the supersaturation was allowed to decrease during the reactions, was that different calcium phosphate phases could form and subsequently dissolve during the course of the reactions. In the present work, the constant composition method was used to investigate the influence of sodium chloride, potassium chloride, and potassium nitrate, as background electrolyte upon the rate of crystallization of HAP in solutions supersaturated only with respect to this phase. These experiments were made in solutions containing totaj... 

Fig. IX-21. The histograms showing the distribution of contact strength between two crystals of calcium sulfate dihydrate in supersaturated calcium sulfate solutions upon varying a - the degree of solution supersaturation, and b - duration of contact, t [16,17]...

Surfactants are particularly suitable as additives for the control of crystallization because of their specific molecular stracture. A surfactant molecule consists of an ionic or nonionic hydrophillic headgroup coupled with a hydrophobic tail. In a crystallization system the headgroup binds to the crystal surface while the tail provides steric hindrance for the incorporation of growth units into the crystal lattice. As surfactants are relatively inexpensive and readily available in many different designs, they should be regarded as ideal crystallization modifiers for industrial applications. The ability of anionic surfactants to control nucleation from solutions supersaturated with different calcium oxalate hydrates will be discussed in some detail. 

FIG. 8 Crystallization of calcium oxalates in the presence of anionic surfactants (0.3 M sodium chloride solutions supersaturated to COM and COD, pH 6.5, temperature 37°C). Changes in the mass fraction of COD in the precipitate are shown as a function of the surfactant concentration expressed in multiples of the respective critical micelle concentrations (erne s). The mass percent of COD (vertical axis) was calculated from thermogravi-metric data assuming a formula C2O4 2H2O for COD. (After Ref. 62.)... 

Since the effluent from a softening unit is usually supersaturated with calcium carbonate at the usually high pH values, it is necessary to reduce the pH to a value that allows the solution to be exactiy saturated for the calcium-ion and carbonate-ion concentrations present. The relationship is... 

Appreciable errors may also be introduced by post-precipitation. This is the precipitation which occurs on the surface of the first precipitate after its formation. It occurs with sparingly soluble substances which form supersaturated solutions they usually have an ion in common with the primary precipitate. Thus in the precipitation of calcium as oxalate in the presence of magnesium, magnesium oxalate separates out gradually upon the calcium oxalate the longer the precipitate is allowed to stand in contact with the solution, the greater is the error due to this cause. A similar effect is observed in the precipitation of copper or mercury(II) sulphide in 0.3M hydrochloric acid in the presence of zinc ions zinc sulphide is slowly post-precipitated. 

Sodium carboxymethyl chitin and phosphoryl chitin had most evident influences on the crystallization of calcium phosphate from supersaturated solutions. They potently inhibited the growth of hydroxyapatite and retarded the rate of spontaneous calcium phosphate precipitation. These chitin derivatives were incorporated into the precipitate and influenced both the phase and morphology of the calcium phosphate formed (flaky precipitate resembling octacalcium phosphate instead of spherical clusters in the absence of polysaccharide) [175]. 

A carboxylate derivative of a fully aromatic, water-soluble, hyperbranched polyphenylene is considered as a unimolecular micelle due to its ability to complex and solubilize non-polar guest molecules [23]. The carboxylic acid derivative of hyperbranched polyphenylene polymer (HBP) (My,=5750-7077, Mn=3810-3910) consists of 40-60 phenyl units that branch outward from a central point forming a roughly spherical molecule with carboxylates on the outer surface. The free acid form of HBP was suspended in distilled water and dissolved by adding a minimum quantity of NaOH. The solution was adjusted to pH 6.2 with aqueous HCl. Calcium carbonate crystals were growth from supersaturated calcium hydrogencarbonate solution at room temperature. HBP gave... 

SBF is a solution that has inorganic ion concentrations similar to those of human blood plasma but does not contain any cells or protein. A brief summary of SBF, introduced by Cho et al. [17], follows. The ion concentrations of SBF are given in Table 11.1 [17]. The pH of SBF is typically adjusted to 7.25 or 7.40 at 36.5 °C. This fluid is a metastable solution containing calcium and phosphate ions supersaturated with respect to hydroxyapatite. SBF is prepared by successively dissolving the reagent-grade chemicals in ultra-pure water in the order given in Table 11.2 [17]. Each new chemical is added after the previous one has completely dissolved. The temperature... 

Figure 8-2 shows the depth profiles of the saturation index omegadel), the solution rate, and the respiration rate. At the shallowest depths, the saturation index changes rapidly from its supersaturated value at the sediment-water interface, corresponding to seawater values of total dissolved carbon and alkalinity, to undersaturation in the top layer of sediment. Corresponding to this change in the saturation index is a rapid and unresolved variation in the dissolution rate. Calcium carbonate is precipitating... 

As the precursor, e.g., an amorphous phase, precipitates and brings down the supersaturation of the solution, the more stable phase to be precipitated is using the precursor phase as a substrate for its own precipitation (Steefel and Van Cappellen, 1990). A classical example that documents this principle is the precipitation of calcium phosphates, where a metastable calcium phosphate precursor phase is nucleated initially and is then replaced, in some instances via an intermediate phase, by apatite. (Nancollas, 1990 Steefel and Van Cappellen, 1990). 

Body fluids have a very high supersaturation with respect to hydroxyapatite, which cannot be explained by the small particle size of bone mineral. In fact, they behave as agueous solutions which are in metastable eguilibrium with DOHA. However, the minerals in bone, dentin, dental enamel and dental calculus contain considerable amounts of Na, Mg and CO3, in addition to calcium and phosphate which are the major components. Therefore, the phases mentioned above which all show a solubility comparable to that of DOHA, all come into consideration as components of these minerals. 

Calcium phosphate precipitation may also be involved in the fixation of phosphate fertilizer in soils. Studies of the uptake of phosphate on calcium carbonate surfaces at low phosphate concentrations typical of those in soils, reveal that the threshold concentration for the precipitation of the calcium phosphate phases from solution is considerably increased in the pH range 8.5 -9.0 (3). It was concluded that the presence of carbonate ion from the calcite inhibits the nucleation of calcium phosphate phases under these conditions. A recent study of the seeded crystal growth of calcite from metastable supersaturated solutions of calcium carbonate, has shown that the presence of orthophosphate ion at a concentration as low as 10-6 mol L" and a pH of 8.5 has a remarkable inhibiting influence on the rate of crystallization (4). A seeded growth study of the influence of carbonate on hydroxyapatite crystallization has also shown an appreciable inhibiting influence of carbonate ion.(5). 

Despite the importance of the precipitation of calcium phosphates, there is still considerable uncertainty as to the nature of the phases formed in the early stages of the precipitation reactions under differing conditions of supersaturation, pH, and temperature. Although thermodynamic considerations yield the driving force for the precipitation, the course of the reaction is frequently mediated by kinetic factors. Whether dicalcium phosphate dihydrate (CaHPO HoO, DCPD), octacalcium phosphate (Ca HfPO, 2.5 H20, OCP), hydroxyapatite (Cag (PO fOH), HAP), amorphous calcium phosphate (ACP), or a defect apatite form from aqueous solution depends both upon the driving force for the precipitation and upon the initiating surface phase. Thermodynamically, the relative supersaturation, o, is given by... 

Further measurements were made by Khan (16) and Mukhopadhyay (17). A large excess of either gypsum or hemlhydrate crystals was added to an acid solution of measured composition. This was agitated at constant temperature until equilibrium was reached. Duplicate runs were carried out both from undersaturated and supersaturated solutions. After settling, the solution was analysed for calcium content by atomic absorption. 

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