Solubility fractional precipitation

Separation of Ions Using Differences in Solubility Fractional Precipitation Qualitative Analysis of Metal Ions in Solution... 

The theory of the process is as follows. This is a case of fractional precipitation (Section 2.8), the two sparingly soluble salts being silver chloride (Xsol 1.2 x 10 10) and silver chromate (Kso] 1.7 x 10 12). It is best studied by considering an actual example encountered in practice, viz. the titration of, say, 0.1M sodium chloride with 0.1M silver nitrate in the presence of a few millilitres of dilute potassium chromate solution. Silver chloride is the less soluble salt and the initial chloride concentration is high hence silver chloride will be precipitated. At the first point where red silver chromate is just precipitated both salts will be in equilibrium with the solution. Hence ... 

The precipitated metallic hydroxides or hydrated oxides are gelatinous in character, and they tend to be contaminated with anions by adsorption and occlusion, and sometimes with basic salts. The values presented in Table 11.2 suggest that many separations should be possible by fractional precipitation of the hydroxides, but such separations are not always practical owing to high local concentrations of base when the solution is treated with alkali. Such unequal concentrations of base result in regions of high local pH and lead to the precipitation of more soluble hydroxides, which may be occluded in the desired precipitate. Slow, or preferably homogeneous, precipitation overcomes this difficulty, and much sharper separations may be achieved. 

The reported molar masses of polyesters obtained by enzymatic catalysis are relatively low, generally below 8000, except for polymers recovered by precipitation.336 This procedure results in the elimination of a soluble fraction consisting of low-molar-mass linear and cyclic oligomers.336 An Mw as high as 46,400 has thus been reported for a poly(tetramethylene decanedioate) obtained... 

Water soluble protein with a relative molecular mass of ca. 32600, which particularly contains copper and zinc bound like chelate (ca. 4 gram atoms) and has superoxide-dismutase-activity. It is isolated from bovine liver or from hemolyzed, plasma free erythrocytes obtained from bovine blood. Purification by manyfold fractionated precipitation and solvolyse methods and definitive separation of the residual foreign proteins by denaturizing heating of the orgotein concentrate in buffer solution to ca. 65-70 C and gel filtration and/or dialysis. 

These opposing tendencies may defeat the purpose of the fractional precipitation process. The fractional precipitation of crystalline polymers such as nitrocellulose, cellulose acetate, high-melting polyamides, and polyvinylidene chloride consequently is notoriously inefficient, unless conditions are so chosen as to avoid the separation of the polymer in semicrystalline form. Intermediate fractions removed in the course of fractional precipitation may even exceed in molecular weight those removed earlier. Separation by fractional extraction should be more appropriate for crystalline polymers inasmuch as both equilibrium solubility and rate of solution favor dissolution of the components of lowest molecular weight remaining in the sample. 

Water-soluble polymers and polyelectrolytes (e.g., polyethylene glycol, polyethylene imine polyacrylic acid) have been used successfully in protein precipitations, and there has been some success in affinity precipitations wherein appropriate ligands attached to polymers can couple with the target proteins to enhance their aggregation. Protein precipitation can also be achieved by using pH at ustment, since proteins generally exhibit their lowest solubility at their isoelectric point. Temperature variations at constant salt concentration allow for fractional precipitation of proteins. 

The unsoluble fluorophosphates can be prepared from the sodium salt by reaction with heavy metal nitrates, whereas the watersoluble salts are appropriately prepared by the reaction of the sligthly soluble AgjPOjF with heavy metal chlorides (24). Acid salts M H[P03F] with = Na, K, NH4 result from fractional precipitation by addition of alcohol and ether to the aqueous solution of the free acid (17). Further methods for the synthesis of fluorophosphates are given by Eqs. (12) and (13), which lead to dimethylin fluorophosphate (32) and to tin (II) difluorophosphate (55) ... 

Principles and Characteristics Fractional solution procedures usually consist of consecutive extractions with solvents of increasing solvent power. These labour intensive methods benefit from a larger surface area to mass ratio. Other methods for fractionation by solubility rely on fractional precipitation through addition of a nonsolvent, lowering the temperature or solvent volatilisation (Section 3.7). 

In modern terms, asphaltene is conceptually defined as the normal-pentane-insoluble and benzene-soluble fraction whether it is derived from coal or from petroleum. The generalized concept has been extended to fractions derived from other carbonaceous sources, such as coal and oil shale (8,9). With this extension there has been much effort to define asphaltenes in terms of chemical structure and elemental analysis as well as by the carbonaceous source. It was demonstrated that the elemental compositions of asphaltene fractions precipitated by different solvents from various sources of petroleum vary considerably (see Table I). Figure 1 presents hypothetical structures for asphaltenes derived from oils produced in different regions of the world. Other investigators (10,11) based on a number of analytical methods, such as NMR, GPC, etc., have suggested the hypothetical structure shown in Figure 2. 

Evidence for the block-type nature of the copolymers is provided by an analysis based on the ability of to form a complex with methylene chloride that is insoluble in methylene chloride. (11) When each of the coupled copolymers described above is dissolved in methylene chloride, a precipitate forms and can be isolated by filtration. Both 1 and the second polymer are present in the soluble fraction and both are present in the precipitate. Since 1 normally precipitates quantitatively from methylene chloride solution and the other polymers remain soluble, the coupled products must be block copolymers. 

For example, consider a solution that contains three halide ions Cl , Br , and l. Since these halides all come from the same group on the periodic table, they share many properties. When they are the anions in slightly soluble ionic compounds, however, they have different solubilities. (See Table 9.4.) Therefore, chemists can use fractional precipitation to separate them from solution. 

Fractional precipitation is dependent on the slight change in the solubility with molecular weight. When a small amount of miscible nonsolvent is added to a polymer solution, the product with the highest molecular weight precipitates first. The procedure is repeated after the precipitate is removed. Molecular weights are run for each fraction and a curve developed that is similar to Figure 3.6. 

For m -> oo, the critical value is identical with that in a d- solvent, i.e., A2 = 0 and X = 0-5- Since the solubility of macromolecules decreases with increasing molecular weight, it is possible to separate these materials with respect to their molecular weights by changing the composition of the solvent and/or the temperature. In general, one roughly distinguishes between two methods, namely fractional precipitation and fractional extraction. 

This parallel reaction is unavoidable in the present reaction mode. However, it is possible to eliminate this reaction in such a way to enhance reactions (4) and (5) while decreasing (6). This can be achieved by increasing the [I /[RSH] ratio. We can also apply the fractional precipitation method utilizing the difference of solubility to a mixed solvent. 

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