Solubility retrograde

Calcium carbonate solubility is also temperature and pressure dependent. Pressure is a 6r more important fector than temperature in influencing solubility. As illustrated in Table 15.1, a 20°C drop in temperature boosts solubility 4%, whereas the pressure increase associated with a 4-km increase in water depth increases solubility 200-fold. The large pressure effect arises from the susceptibility of the fully hydrated divalent Ca and CO ions to electrostriction. Calcite and aragonite are examples of minerals whose solubility increases with decreasing temperature. This unusual behavior is referred to as retrograde solubility. Because of the pressure and temperature effects, calcium carbonate is fer more soluble in the deep sea than in the surfece waters (See the online appendix on the companion website). 

Pb, where retrograde solubility for the solid in equilibrium with the liquid can also occur. As a critical value of n " is approached the liquid forms its own miscibility gap and the diagram then exhibits two forms of liquid invariant reaction, the lower temperature reaction being either eutectic or peritectic, while the higher temperature reaction becomes monotectic. Examples of such systems are Cu-Pb and Cu-Tl. When n becomes even larger, the top of the liquid miscibility gap rises above scale of the graph and there is little solubility of either element in the liquid. Such a diagram is typical of Mg systems such as Mg-Fe or Mg-Mn. 

An understanding of the phase behavior of a particular system of interest is important because complex results can sometimes occur. A dramatic example, which occurs frequently for solubilities in supercritical systems, is the retrograde behavior. Figure 3 clearly shows the presence of a retrograde region. For an isobaric system at some pressure, such as 12.7 MPa (1841.5 psi), an increase in temperature of a solution of ethylene and naphthalene from 300 to 320 K results in an increase in the equilibrium solubility of naphthalene. This behavior is typical of liquid solvent systems. For the same increase in temperature (300 to 320 K) but at a pressure of 8.1 MPa (1174.5 psi), the solubility of naphthalene decreases by nearly an order of magnitude. Because this latter behavior is the opposite of typical liquid solvents, it is termed retrograde solubility. 

In the supercritical phase, both temperature and pressure play a significant role in determining the extraction efficiency. After the short-lived retrograde solubility effect subsides at about 55-60°C, a transition of the system back to the mass transfer controlled situation will take place where increasing temperature will, once again, bring about a surge in the extraction efficiency. In fact, for the supercritical phase,... 

Some systems exhibit the phenomena known as retrograde solubility. In such cases the solubility decreases in the normal way as the temperature is decreased, passes through a minimum, and then increases as the temperature is continually decreased. The curve of In x2 plotted as a function of 1/T then passes through a minimum and, at the minimum, — s)) must be... 

The precursor has retrograde solubility and its concentration is about the same as the saturation. 

For lower consumed C02 the quantity of extracted lupanine decreases with temperature but this situation inverts for the lower temperatures with the increase of consumed C02. Thus, a retrograde solubility effect between the curves at 298 K and 313 K seems to exist. 

Reactions involving carbonates present a confusing situation. Precipitation and dissolution reactions involving carbonates can both be written to evolve CO2. The retrograde solubility of calcite and the role of Pco in controlling carbonate solubility are further complications. Across the... 

Employing cryoscopic measurements, Pelton and Flengas (1) and Bell and Flengas (2) determined the melting point of PbS to be 1113.4 C and 1111.9 C, respectively. Pelton and Flengas (1 ) estimated an accuracy of 1 C. Miller and Komarek (3), in studying the retrograde solubility in the Pb-S system, reported = 1113.3 0.5 C. The latter authors have summarized many early... 

Precipitation from supercritical fluids is of interest not only in relation to the production of uniform particles. The thermodynamics of dilute mixtures in the vicinity of the solvent s critical point (more specifically, the phenomenon known as retrograde solubility, whereby solubility decreases with temperature near the solvent s critical point) has been cleverly exploited by Chlmowitz and coworkers (12-13) and later by Johnston et al. (14). These researchers implemented an elegant process based on retrograde solubility for the separation of physical solid mixtures which gives rise to high purity materials. 

Retrograde solubility describes the change in impurity concentration in a solid above Teu, i.e., a maximum solubility is observed at a temperature Tmax lower than melting temperature of silicon Tm, but above Teu. In this frequently encountered case, impurities tend to precipitate upon cooling. 

Thermodynamically, retrograde solubility requires a large positive value for A if 1, the solid solution enthalpy of mixing. 

Retrograde solubility can be usefully applied in solidification refining of impurities in SoG-Si materials. Yoshikawa and Morita proposed the methods for the removal of boron [3,4] and phosphorus [5] by addition of Al and Ti. Shimpo et al. [6] and Inoue et al. [7] reported the Ca addition method to remove boron and phosphorus in silicon. 

Dolomite precipitation probably took place at relatively low temperatures (<100°C), on the basis of planar crystal fabrics (Sibley Gregg, 1987) and the dominance of monophase aqueous fluid inclusions (Goldstein Reynolds, 1994). Under such conditions, the precipitation of well-ordered dolomite is favoured by high (Mg " " + Fe " )/Ca ratios, low salinities and high carbonate alkalinity in pore fluids (Folk Land, 1975 Machel Mountjoy, 1986). Low salinity reduces ion pairing, and high COf activities facilitate dehydration of Mg " " ions. Dolomite exhibits retrograde solubility. 

The retrograde solubility of the 7 phase as shown in the radial vertical sections in Figs. 11-13 and in the isopleths at constant Co content (Figs. 14 - 16) in the ternary system is somewhat higher than in both the Cu-Fe and Co-Cu boundaries it is close to 20 at.% Cu at 1300°C. 

Quartz has prograde solubility so it precipitates as the solution cools. Many minerals (e.g. calcite, barite, gypsum) have retrograde solubility so they precipitate as the temperature increases (Rimstidt, 1997a). 

The appropriate reaction enthalpy is the solution enthalpy. In case of an endother-mal solution enthalpy, the equilibrium shifts with higher temperature to higher solubilities, and thus results in a positive slope of the solubility curve. On the other hand, an exothermal dissolution process leads to a retrograde solubility behavior. In most cases, the heat of solution is positive, that is, most substances dissolve with absorption of heat. This means that the (positive) lattice enthalpy exceeds the (negative) solvation enthalpy or more energy has to be spent to break the lattice than is evolved by solvation. 

Here, the most frequent cases where solubility increases with temperature are shown. It becomes clear that when applying the melting enthalpy in Equation 3.8, a negative slope of the line always results, implying a positive slope of the solubility curve. Thus, retrograde solubility can only be explained with the solution enthalpy that can possess negative values. 

Retrograde solubility also occurs for organic substances. Examples are sulfaguani-dine and sparteine, a complicated natural product, both in water. Discontinuities in the solubility curve originating from polymorphic phase transitions are observed for enantiotropically related polymorphs. The transformation of one polymorph to the... 

KCI in a NaCI-saturated solution that still increases with temperature, NaCI shows retrograde solubility in KCI-saturated solutions (lines 3 and 4). This particular solubility behavior is used in the so-called hot leaching process to separate KCI and NaCI from sylvinite crude salts. It is obvious from the solubility curves that cooling a solution saturated with both salts (point of intersection of lines 3 and 4 at about 100 "C) leads to selective crystallization of KCI as target compound. (Reproduced with permission from Ref [8].)... 

Two hydration peaks are observed (similar to C3S hydration). An initial peak is observed on mixing peak height is directly related to the temperature of hydration. An induction period (0.5 hr) was observed at 0.5 hourat25°C. The minimum rates of heat evolution (attemperatures>25°C) increase with temperature. A slower rate of heat evolution, but a larger total amount of heat released was observed at 25 °C. This behavior was attributed to the retrograde solubility of hydroxyapatite along with incongment dissolution of the acidic constituent. 

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