Solution super-saturated

If to a saturated binary solution of A (a solid solute) and B (a liquid solvent) a small amount of the third component C (also soluble in B) is added, one of four conditions can result. First, nothing may happen, although this is comparatively rare, in which case the system remains in its original saturated state. Second, component C may react or otherwise combine or react chemically with A by forming a complex or compound, thus altering the whole nature of the system. In the third case, the presence of component C may make the solution super-saturated with respect to solute A, which would then be precipitated. In the fourth case, the solution may become unsaturated with respect to A. The terms salting-out and salting-in are commonly used to describe these last two cases, particularly when electrolytes are involved. 

A batch cooling crystallization is one of the most commonly used crystallization method. In this process super saturation of a liquid is achieved by means of a cooling process. The solubility of the solute (in the solvent) decreases with a decrease in temperature this leads to precipitation of the solute. 

For solution on the other hand no such difficulties occur. It has already been noted that the edges of the planes are already stripped under conditions of equilibrium a small decrease in the concentration of the solution simply extends the edges further towards the centre of the plane until it is stripped entirely. The mechanism of crystal growth thus presupposes a finite degree of super-saturation, this no doubt varies with the surface energies of the crystal faces and will probably be greatest for the surfaces for which <7,1 is least,... 

Supersaturation.—One of the most frequent difficulties in crystallization is due to super saturation. This condition arises when the normal saturation concentration of a salt solution is exceeded without the appearance of any crystals, and as the solution cools further it becomes steadily more supersaturated. When crystallization finally starts, it proceeds with great rapidity, forming a mass of poorly defined crystals unsuited to drying. The tendency toward supersaturation is most marked in the case of very soluble substances which form viscous or syrupy solutions. Lead acetate, sodium thiosulfate, ferric nitrate, and sulfuric acid are good examples. 

FIGURE 11.5 A super- saturated solution of sodium acetate in water. When a tiny seed crystal is added, larger crystals begin to grow and precipitate from the solution until equilibrium is reached. 

The temperature dependence of the nucleation rate allows many critical nuclei to be formed on a time scale that is small relative to the growth time when the difference between the actual solution and equilibrium temperatures is greater than a critical value, ATc. If the temperature variations of liquid density are neglected, the critical super saturation, A Tc, will vary with... 

Later work by Lin et al. overcame this problem by bubbling SC-C02 through a vessel containing TBP upstream of the extraction vessel43 Using this approach, super-saturation of the fluid phase by the extractant was avoided and a supercritical phase containing ca. 11% (on a molar basis)44 of TBP was consistently obtained (at 60°C and 120 atm pressure). This TBP-saturated C02 was then employed to extract uranyl and thorium ions from nitric acid solutions of various concentrations. The extraction of both ions increased with rising aqueous acidity, consistent with the extraction reactions observed in conventional systems (e.g., TBP-dodecane), such as that shown here for uranium ... 

In examining a crystalline structure as revealed by diffraction experiments it is all too easy to view the crystal as a static entity and focus on what may be broadly termed attractive intermolecular interactions (dipole-dipole, hydrogen bonds, van der Waals etc., as detailed in Section 1.8) and neglect the actual mechanism by which a crystal is formed, i.e. the mechanism by which these interactions act to assemble the crystal from a non-equilibrium state in a super-saturated solution. However, it is very often nucleation phenomena that are ultimately responsible for the observed crystal structure and hence we were careful to draw a distinction between solution self-assembly and crystallisation at the beginning of this chapter. For example paracetamol, when crystallised from acetone solution gives the stable monoclinic crystal form I, but crystallisation from a molten sample in the absence of solvent... 

The aim of crystallization is not only to separate the solute by purification but also to produce particles. In general, by cooling the solution or vaporizing the solvent, crystals of high purity are produced and separated from the mother liquor. The method based on super-saturated concentration is classified into five groups ... 

In the coprecipitation of a phase associating two (or several) elements, if one of them is contained in an anion and the second in a cation, the precipitate will have a fixed or at least very inflexible composition. If both are cations (or both anions) the characteristics of the reactions with a common anion (or cation) of the solution, the solubility constants, and the super-saturation values will all be different, and the properties of the precipitate will change with time. Consequently, coprecipitation does not in general give homogeneous precipitates. Methods are available to produce homogeneous precipitates (see item c) below). 

Choosing the appropriate solvent is the most crucial aspect of the recrystallization process. The best solvent for recrystallization is one in which the material is less soluble at room temperature but more soluble when hot. At higher temperatures, solvents that form super saturated solutions with certain solids meet this requirement. 

In some cases recrystallization of super saturated solutions can be initiated with a seed crystal. A seed crystal is simply a small crystal of the product, ft is added to the super saturated solution, and the dissolved product begins to grow on the seed crystal. The seed crystal induces recrystallization by giving the dissolved product a surface from which to grow on. The recrystallization of the product stops when equilibrium of the solution is reached. 

The saturation index SI indicates, if a solution is in equilibrium with a solid phase or if under-saturated and super-saturated in relation to a sohd phase respectively. A value of 1 signifies a ten-fold supersaturation, a value of -2 a hundred-fold undersaturation in relation to a certain mineral phase. In practice, equilibrium can be assumed for a range of -0.2 to 0.2. If the determined SI value is below -0.2 the solution is understood to be undersaturated in relation to the corresponding mineral, if SI exceeds +0.2 the water is assumed to be supersaturated with respect to this mineral. 

Hints for super- or undersaturation of minerals can be found in the last paragraph of the initial solution calculations entitled saturation indices . Graphical representation of saturation proportions is often done by means of bar charts, whereas SI = 0 marks the point of intersection between the x-axis and the y-axis, and the bars of super-saturated phases point upwards and those for undersaturated phases downwards (example Fe-bearing mineral phases Fig. 37). 

A super-saturated solution is one that holds more than it can hold. 

Crystals form in supersaturated solutions in which the solute concentration exceeds its solution solubility. Supersaturation is usually expressed as either of the ratios dc or (c — c )lc, where c is the concentration of solute before crystallization and is the solute equilibrium saturation concentration. Supersaturated solutions are thermodynamically metastable. Equilibrium can be restored by reducing the solute concentration through precipitation or formation of nuclei and subsequent crystal growth. The super saturation requirements for nucleation and... 

When the solution is cold (about 5 C), collect the solid by suction filtration (Box 5.2), using the filtrate, to transfer the solid completely from the collection flask. If crystals do not form, cither add a few crystals of the crude solid to seed the super saturated solution or scratch with a Pyrex" glass rod to induce recrystallization. 

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