Solubility supersaturation

In view of the selective character of many colorimetric reactions, it is important to control the operational procedure so that the colour is specific for the component being determined. This may be achieved by isolating the substance by the ordinary methods of inorganic analysis double precipitation is frequently necessary to avoid errors due to occlusion and co-precipitation. Such methods of chemical separation may be tedious and lengthy and if minute quantities are under consideration, appreciable loss may occur owing to solubility, supersaturation, and peptisation effects. Use may be made of any of the following processes in order to render colour reactions specific and/or to separate the individual substances. 

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... 

Basic crystal properties include solubility, supersaturation, metastable zone width, oil, amorphous solid, polymorphism, occlusion, morphology, and particle size distribution. Clearly. 

Of the eleven chapters in this book, the first two deal with fundamentals such as solubility, supersaturation, basic concepts in crystallography, nucleation, and crystal growth, and are aimed at... 

The relationships between solubility, supersaturation, pressure, and temperature depend upon the details of the microscopic mechanisms operative in the context of the detailed active impurity chemistry and the specific geometric relationship set up between the source carbon, molten metal solution, and nucleation site, and or growing diamond. For an introduction into these extensive subjects the reader is referred to Burns and Davies [16] where the issues such as nitrogen and boron active impurity chemistries, diamond surface reconstruction, morphology and solvent/catalyst are discussed. 

With a large temperature dependency of the solubility, supersaturation is achieved by simple surface cooling of the saturated solution from i to t 2, cooling crystallization... 

There are, however, practical limitations to minimizing RSS. Precipitates that are extremely insoluble, such as Fe(OH)3 and PbS, have such small solubilities that a large RSS cannot be avoided. Such solutes inevitably form small particles. In addition, conditions that yield a small RSS may lead to a relatively stable supersaturated solution that requires a long time to fully precipitate. For example, almost a month is required to form a visible precipitate of BaS04 under conditions in which the initial RSS is 5. ... 

Although they may be completely soluble in the lower temperature bulk water, these compounds (eg, calcium carbonate, calcium phosphate, and magnesium siUcate) supersaturate in the higher temperature water adjacent to the heat-transfer surface and precipitate on the surface. 

Fog particles grow because or excess saturation in the gas. Usually this means that the gas is supersaturated (i.e., it is below its dew point). Sometimes, fog can also grow on soluble foreign nuclei at partial pressures below saturation. Increased saturation can occur through a varietv of routes ... 

Crystal Formation There are obviously two steps involved in the preparation of ciystal matter from a solution. The ciystals must first Form and then grow. The formation of a new sohd phase either on an inert particle in the solution or in the solution itself is called nucle-ation. The increase in size of this nucleus with a layer-by-layer addition of solute is called growth. Both nucleation and ciystal growth have supersaturation as a common driving force. Unless a solution is supersaturated, ciystals can neither form nor grow. Supersaturation refers to the quantity of solute present in solution compared with the quantity which would be present if the solution were kept for a veiy long period of time with solid phase in contac t with the solution. The latter value is the equilibrium solubility at the temperature and pressure under consideration. The supersaturation coefficient can be expressed... 

Folding is defined in Background and Definitions and is a significant problem in most process applications, and somewhat of a problem in most water applications. RO membranes may be fouled by sparingly soluble sealants which supersaturate at the membrane. 

The entire amount of the ammonium chloride-mandelic acid mixture may be boiled with the benzene but this gives a supersaturated solution of the acid in the benzene and much difficulty is met in the filtration. The solubility of mandelic acid in hot benzene is approximately i g. in 50 cc. 

For ionie systems, however, the definition of the appropriate driving foree beeomes more eomplex sinee the ionie eoneentrations are not neeessarily in stoiehiometrie ratio and the solubility produet generally applies. Thus the supersaturation ratio of sparingly soluble systems ean be deseribed by... 

It ean be seen from these definitions of driving foree that sparingly soluble substanees ean easily exhibit high levels of relative supersaturation. Henee, the erystallization proeess ean be very fast and the preeipitation diffieult to eontrol, espeeially on the large seale (see Sohnel and Garside, 1992). 

Thus, if Mp is known, from the initial mass of seeds plus the ehange in solubility over the temperature range, then the growth inerement AL and eonsequent produet CSD eould be evaluated. MeCabe (1929b) verified the method in experimental work. It was observed, however, that nueleation oeeurred despite operation at low supersaturation levels - subsequently referred to as seeond-ary nueleation i.e. nueleation in the presenee of erystals (Chapter 5). 

The solubility of oxygen in water is influenced by several factors but, generally, most waters contain, at equilibrium, about 8mg/kg at 25°C, although supersaturation may increase this considerably. Some indication of the amount of dissolved oxygen relevant in different situations is shown, on the one hand, by the value of 0-1 mg/kg found in central heating systems and, on the other hand, by the value of 0 005 mg/kg specified by most users of large boilers as the upper limit at the economiser inlet (see Section 5.5). 

Potassium sulfate has a solubility of 15 g/100 g water at 40°C. A solution is prepared by adding 39.0 g of potassium sulfate to 225 g of water, carefully heating the solution, and cooling it to 40°C. A homogeneous solution is obtained. Is this solution saturated, unsaturated, or supersaturated The beaker is shaken, and precipitation occurs. How many grams of potassium sulfate would you expect to crystallize out ... 

The great importance of the solubility product concept lies in its bearing upon precipitation from solution, which is, of course, one of the important operations of quantitative analysis. The solubility product is the ultimate value which is attained by the ionic concentration product when equilibrium has been established between the solid phase of a difficultly soluble salt and the solution. If the experimental conditions are such that the ionic concentration product is different from the solubility product, then the system will attempt to adjust itself in such a manner that the ionic and solubility products are equal in value. Thus if, for a given electrolyte, the product of the concentrations of the ions in solution is arbitrarily made to exceed the solubility product, as for example by the addition of a salt with a common ion, the adjustment of the system to equilibrium results in precipitation of the solid salt, provided supersaturation conditions are excluded. If the ionic concentration product is less than the solubility product or can arbitrarily be made so, as (for example) by complex salt formation or by the formation of weak electrolytes, then a further quantity of solute can pass into solution until the solubility product is attained, or, if this is not possible, until all the solute has dissolved. 

---