Solid solution solubility behavior

Fvaluation of some systems of solid solutions according tvaxud fco their solubility behavior... 

The logarithm of the solubility product for hydroxyapatite is -58.6 and that of fluorapatite (CajtPO jF) is -60.6 (57), and thus, D = 0.01 in favour of fluoride incorporation into the solid apatite precipitate. Accordingly, it should be difficult to prepare solid solutions of these compounds by precipitation from aqueous solution and if prepared batchwise, they are expected to contain logarithmic gradients in their internal composition. Yet, Moreno et al.(M3) report linear changes in the lattice parameters of such solid solutions. They also determined their solubility behavior. 

A thermodynamically acceptable explanation for the solubility behavior of solid solutions at x = 0.868 is needed. First, we shall assume that OHA-FA solid solutions are ideal. If the composition of the surface layer of the solid particles is given by Equation (49), then the following equations can be derived (2) ... 

The data of Table III show that the surface layer of the solid particles is indistinguishable from pure fluorapatite in all equilibrations at x = 0.110, 0.190 and 0.435 and 0.595. However, some equilibrations at x = 0.763 and all at x = 0.868 do deviate significantly from the behavior of pure fluorapatite. A peculiar aspect is that the activity of fluorapatite becomes significantly larger than 1. Simutaneously, the activity of hydroxyapatite approaches unity. This would mean that at all values of x both activities would become smaller than 1, and thus an ideal behavior of the solid solutions would not explain the observed solubility behavior. 

Thus, the assumption of a regular behavior of the solid solutions of OHA and FA does not explain the observed solubility behavior either. 

Calculation of the extreme values of the activities at the spinodal compositions xgp for variable values of W/2.303 RT results in the data presented in Figure 10. It appears that values as high as log ap/ = 2 are reached in the range xgp >0.63. Thus, the assumption of a subregular behavior of the solid solutions of OHA and FA explains the observed solubility behavior qualitatively. It follows further from the calculations that W/2.303 RT. 8 so that W, > 4.6 104 J mol"1. 

Potassium hexacyanochromate(III) is a yellow solid, highly soluble in water, that crystallizes as large square platelets. It crystallizes in the orthorhombic system, with the space group Pcan. The unit parameters are a = 8.53, b = 10.60, c = 13.68 A. Because of its disorder behavior, it presents a complex crystallographic problem. The compound shows a j/cn band at 2131 cm. The molar extinction coefficients in aqueous solution of the two observable d d bands at 376 nm ( A2g —> 72) and 309 nm (4 2 T l ) are 93 and 62 L mol ... 

The crystallography of the f.c.c.— b.c.t. martensitic transformation in the Fe-Ni-C system (with 22 wt. %Ni and 0.8 wt. %C) has been described in Section 24.2. In this system, the high-temperature f.c.c. solid-solution parent phase transforms upon cooling to a b.c.t. martensite rather than a b.c.c. martensite as in the Fe-Ni system. Furthermore, this transformation is achieved only if the f.c.c. parent phase is rapidly quenched. The difference in behavior is due to the presence of the carbon in the Fe-Ni-C alloy. In the Fe-Ni alloy, the b.c.c. martensite that forms as the temperature is lowered is the equilibrium state of the system. However, in the Fe-Ni-C alloy, the equilibrium state of the system in the low-temperature range is a two-phase mixture of a b.c.c. Fe-Ni-C solid solution and a C-rich carbide phase.5 This difference in behavior is due to a much lower solubility of C in the low-temperature b.c.c. Fe-Ni-C phase than in the high-temperature f.c.c. Fe-Ni-C phase. If the high-temperature... 

Half a century later, the work of Carson and Katz (1942) provided a second reason for considering the dissociation condition of the hydrate equilibrium point (see Chapter 3, Figure 3.1b for more details). Their work clearly showed the solid solution behavior of hydrates formed by gas mixtures. This result meant that hydrate preferentially encapsulated propane from a methane + propane gas mixture, so that a closed gas volume was denuded of propane (or enriched in methane) as more hydrates formed. On the other hand, upon hydrate dissociation, when the last crystal melted the initial gas composition was regained, minus a very small amount to account for solubility in the liquid phase. 

It is unusual to find systems that follow the ideal solution prediction as well as does (benzene+ 1,4-dimethylbenzene). Significant deviations from ideal solution behavior are common. Solid-phase transitions, solid compound formation, and (liquid 4- liquid) equilibria often complicate the phase diagram. Solid solutions are also present in some systems, although limited solid phase solubility is not uncommon. Our intent is to look at more complicated examples. As we do so, we will see, once again, how useful the phase diagram is in summarizing a large amount of information. 

Zircon, complete solid-solution behavior is observed, and a plot of the unit cell volume against x shows that Vdgard s Law is followed. When the end members are not is structural, a systematic change in the solubility range in both structures is found as A is varied, and the data have been systematized in terms of a simple, potentially predictive, structure-field map. The pervasive polymorphism of these ABO4 compounds, involving both reconstructive and displacive transformations and metastable structures produced by different sample preparation methods, indicates that the crystal structural stability of substituted compounds needs to be carefully evaluated as a function of temperature to assess the structural integrity of waste-form materials. 

Mackenzie F.T., Bischoff W.B., Bishop F.C., Loijens M., Schoonmaker J. and Wollast R. (1983) Magnesian calcites Low-temperature occurrence, solubility and solid-solution behavior, In Carbonates Mineralogy and Chemistry (ed. R.J. Reeder), pp. 97-144. Mineral. Soc. Amer., Chelsea, Mich. 

A second type of solubility behavior is exhibited by mixtures that form solid solutions. Consider, for example, a hypothetical system containing R and S whose... 

FIGURE 7.1 Solubility phase diagrams of diastereoisomeric salts, (a) Ideal behavior (b) end solid-solution behavior (c) full solid-solution behavior and (d) double salt formation. 

---