Solubility methods, distinguishing precipitation

Solubility and kinetics methods for distinguishing adsorption from surface precipitation have the common features of being essentially macroscopic in nature and of not utilizing a direct examination of sorbed material. The essential difference between an adsorbate and a surface precipitate lies with molecular structure, however, and it is inevitable that methodologies not equipped to explore that structure directly will produce ambiguous results requiring ad hoc assumptions in order to interpret them. The principal technique for... 

Solubility and kinetics methods for distinguishing adsorption from surface precipitation suffer from the fundamental weakness of being macroscopic approaches that do not involve a direct examination of the solid phase. Information about the composition of an aqueous solution phase is not sufficient to permit a clear inference of a sorption mechanism because the aqueous solution phase does not determine uniquely the nature of its contiguous solid phases, even at equilibrium (49). Perhaps more important is the fact that adsorption and surface precipitation are essentially molecular concepts on which strictly macroscopic approaches can provide no unambiguous data (12, 21). Molecular concepts can be studied only by molecular methods. 

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. 

The homopolymer of DMP dissolves readily in methylene chloride but precipitates on standing as a crystalline polymer-CH2Cl2 complex, providing a method for distinguishing between block copolymers and mixtures of homopolymers. Random copolymers prepared by methods a and b form stable solutions in methylene chloride. Copolymers with a 1 1 ratio of DMP and DPP prepared by methods c and d also yield stable methylene chloride solutions. Since the NMR spectrum shows that the DMP portion of these materials is present as a block and the solubility in methylene chloride shows that DMP homopolymer is absent, these copolymers have the block structure. They can be separated by crystallization from m-xylene into an insoluble DPP-rich fraction and a soluble DMP-rich fraction, both fractions having the NMR spectra characteristic of block copolymers. A typical 1 1 copolymer prepared by adding DMP to growing DPP polymer yielded 35% of insoluble material... 

Solubility in Methylene Chloride. The methods described above can show the presence of blocks of DMP and blocks of DPP units, but they do not distinguish between block copolymers and blends of homopolymers. Gel permeation chromatograms of the copolymers are sharp and symmetrical, indicating that they are indeed copolymers rather than blends, but this alone is not conclusive as blends of the homopolymers do not produce binodal or badly skewed curves under the conditions used unless the two polymers differ considerably in molecular weight. A partial answer to this question is provided by the solubility behavior in methylene chloride. Dimethylphenol homopolymer dissolves readily in methylene chloride but precipitates quantitatively on standing for a short... 

PURPOSE AND RATIONALE Solubility assays are gaining growing attention in drug discovery, because many pharmaceutically active compounds can be adjusted to in vivo testing merely with co-solvents. Furthermore, in vitro assays may also lead to false results, simply for precipitation of a compound in the assay media. Solubility assays vary in one main point they are either performed from solids or stock solutions. A nomenclature has been established in the literature which tries to distinguish between these methods. Determinations from stock solutions are often called kinetic solubility whereas thermodynamic solubility stands for solubility of solids (Kerns). Thermodynamic solubility takes the crystal lattice forces into account. Batch to batch variations, polymorphism... 

A number of methods of analysis are important in wood-cellulose chemistry. One of the most important is the determination of solubility in sodium hydroxide solution at room temperature. This distinguishes between alpha-cellulose (not dissolved by 17.5% sodium hydroxide), beta-cellulose (dissolved by 17.5% sodium hydroxide, but precipitated when acidified with acetic acid), and gamma-cellulose (dissolved by 17.5% sodium hydroxide, and not precipitated on acidification). 

The selectivity of the method is given first by the ability to stay dissolved in a solution containing tartrate, second by the color of the sulfide precipitate, and finally by the fact that the sulfide salt dissolves in sodium hydroxide. The first property distinguishes it from bismuth(III) and the other cations forming insoluble oxides in neutral or alkaline solutions. But since the test does not show that a precipitate is formed in pure water, which dissolves when tartrate is added, all water-soluble cations are not excluded. So it should be viewed as a trick to facilitate dissolution only and not a part of the identification. The color of the sulfide precipitate is unique, and it is the most important criterion for a positive identification if there is any doubt when judging the result, preparing a positive control would be constructive. The solubility of the sulfide salt in sodium hydroxide is a characteristic shared with, for example, the sulfide salt of arsenate, and in classic inorganic separation the sulfide precipitate solubility in hydrochloric acid or polysulfide is used instead. " ... 

Fractionation of antibody by precipitation has been used for 150 years and is well established in commercial processes for manufacture of polyclonal antibodies. A variety of difierent methods can be used, exploiting different physicochemical properties of antibodies which distinguish them from typical contaminants Table 2). Control of pH is important because proteins are least soluble at their isoelectric point. Compared with other serum proteins, most antibodies are comparatively basic (high pi), but of course there is considerable variation between different monoclonals. 

Usually, one distinguishes among five polymerization methods, which may show different kinetic behavior bulk, solution, precipitation, suspension, and emulsion polymerization. In general, water is used as the continuous phase in suspension and emulsion polymerization. Due to the solubility of AN and the insolubility of PAN in water, the differences among solution, suspension, and emulsion polymerization of AN are small, and the kinetics differ strongly from those of normal suspension and emulsion polymerization. 

A classical method of distinguishing between the types of amines is based on the difference in the reactivity of amines with benzenesulfonyl chloride followed by reaction vyith sodium hydroxide. The procedure is called the Hinsberg test. First, benzenesulfonyl chloride is shaken with a mixture of an amine and aqueous base. Then, the reaction mixture is examined to determine which of three possible events occurred. If the amine was tertiary, no precipitate appears, and there is no evidence of any reaction. If the amine was secondary, a water-insoluble sulfonamide forms and appears as a precipitate. Primary amines also give no evidence of a reaction because the sulfonamide is soluble in the base, but if the solution is neutralized with an acid, the neutral water-insoluble sulfonamide precipitates. Thus, the experimental results are unique for each class of amine. Infrared spectroscopy and NMR allow us to identify classes of amines (Section 23.14). 

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