Precipitate selectivity

Group II consists of six different cations, all of which form very insoluble sulfides (Figure A). These compounds are precipitated selectively by adding hydrogen sulfide, a toxic, foul-smelling gas. at a pH of 0.5. At this rather high H+ ion concentration. 0.3 M, the equilibrium... 

A popular source of sulfide ions is hydrogen sulfide gas. This is a versatile reagent capable of precipitating a number of metal sulfides, some of which can be precipitated selectively from solution by suitably adjusting the solution pH. In the aqueous medium the following reactions take place by which hydrogen sulfide releases, in two stages, sulfide ions to the medium ... 

Cerium is obtained from its ores by chemical processing and separation. The process involves separation of cerium from other rare-eartb metals present in the ore. Tbe ore is crushed, ground, and treated with acid. Tbe extract solution is buffered to pH 3 and tbe element is precipitated selectively as Ce4+ salt. Cerium also may be separated from other metals by an ion-exchange process. 

Various processes separate rare earths from other metal salts. These processes also separate rare earths into specific subgroups. The methods are based on fractional precipitation, selective extraction by nonaqueous solvents, or selective ion exchange. Separation of individual rare earths is the most important step in recovery. Separation may be achieved by ion exchange and solvent extraction techniques. Also, ytterbium may be separated from a mixture of heavy rare earths by reduction with sodium amalgam. In this method, a buffered acidic solution of trivalent heavy rare earths is treated with molten sodium mercury alloy. Ybs+ is reduced and dissolved in the molten alloy. The alloy is treated with hydrochloric acid, after which ytterbium is extracted into the solution. The metal is precipitated as oxalate from solution. 

Selective adsorption on glass Ammonium sulfate fractionation Ammonium sulfate fractionation Ethanol precipitation Sonic disintegration Selective adsorption on cellulose phosphate CM-cellulose chromatography Acetone precipitation Ammonium sulfate fractionation Ethanol precipitation Selective adsorption on cellulose phosphate Ammonium sulfate fractionation Lysozyme and trypsin digestion Ammonium sulfate fractionation DEAE-cellulose chromatography Ammonium sulfate fractionation... 

It should be noted that the majority of the methods of synthesis of block and graft copolymers yield, in addition to the copolymer, fractions of the associated homopolymers to a greater or lesser extent. Therefore in order to determine the composition and the structure of the copolymer, it is first of all necessary to separate it from any homopolymer which may be present. This separation is usually based upon solubility differences between the different components (203). Although the turbidimetric titration may be very useful for analytical purposes (157), nevertheless it gives samples too small for further examination preparative methods, which are based on fractional precipitation, selective extraction or selective precipitation should be preferred. It is beyond the scope of this review to discuss these separation methods the reader is referred to existing literature concerning this subject (51). 

The most typical route for metal ion removal/ recovery from polluted aqueous media is precipitation (see Section 10.2). However, in some cases the target metal ion cannot be precipitated in a pure form because other metal ions may co-precipitate. Selective complexation can then be used to promote the solubilization of either the target ions or the accompanying ions. Here, the constant for metal complexation must be greater than the solubility product. (Note that in order for this comparison to be valid, one has to be aware of the differences in the units of the constants). For example, one can recover Ag... 

Linear polystyrene can be functionalized by various methods . The functional group capacity in these polymers diould not be too high otherwise, steric complications may arise. Poly(ethylene ycol) has been found to be most suitable for liquid-phase synthesis. This linear polyether and the block copolymers with functional groups at defined distances are chemically stable and soluble in a large number of solvents including water and can be precipitated selectively. Partially hydrolyzed poly(vinylpyrrolidone) and its copolymers with vinyl acetate were successfully applied in peptide synthesis. Poly(acrylic acid), poly(vinyl alcdiol), and poly-(ethylenimine) are less suitable for the sequential type synthesis because of the... 

The bisulfite adducts of electrophilic pyrimidines are relatively stable, and several have been isolated. Thus, iV-substituted 5-chloro-2(l/7)-pyrimidinones readily form bisulfite adducts in aqueous solution. Both the 3,4- (329) and the 3,6-bisulfite (330) adducts are formed, with the former as the predominant isomer. In water, the 3,4-isomer was significantly less soluble and was precipitated selectively from concentrated solutions. Solubilization in more dilute aqueous solution, and addition of TFA, precipitates the parent heterocycle <87ACS(B)448,91ACS177). 

Table 5.7 omits references in which transesterification in PEST/PEST blends is brought about under static conditions either by annealing or heating in a DSC chamber. Blends in such examples were prepared by solution precipitation. Selected recent references in this class include PC + polyester urethane [Ahn et al., 1997] PBT + PET [Backson et al., 1995] Polyarylate + PBT [Denchev et al., 1996] PCL + PBT-CL copo-... 

Cyclodextrins are prepared on a commercial scale by the enzymatic degradation of starch. The corresponding cylodextrin transferases are nonspecific with respect to ring size. Pure a-5 cyclodextrins are precipitated selectively, and the transferases then establish a new equilibrium of all four cyclodextrins. a-Cy-clodextrin can be obtained in 40% yield in one step by precipitation with 1-de-canol, P-cyclodextrin in 60% yield by precipitation with toluene, and y-cyclodextrin in 50% yield with cyclohexadec-8-en-one. 

Some polysaccharides, like pullulan, can be precipitated selectively in plates with ethanol (8). Polyanionic polysaccharides like pectin or carboxy methyl cellulose (CMC) can be precipitated with cetylammonium salts (9, 10). Degradation zones in these plates will not turn opaque upon addition of the precipitant thus revealing the activity of polysaccharases. 

The properties of, say, natural rubber grafted with poly(methyl methacrylate) cannot be evaluated unless the copolymer is isolated from either homopolymer species. The methods used are based on fractional precipitation, selective solution, or a combination of these basic techniques. For details, refer to Chapter 3. In many cases, though, technologists are concerned with the materials as manufactured, so we consider in this context also the properties of the block and graft copolymers without homopolymer removed. 

Alternatively, borinane is precipitated selectively from the reaction mixture as its bis adduct with either N,N,Ar,Ar-tetramethylethylenediamine (TMEDA) or 1,4-diazabicyclo [2.2.2] octane (DABCO) in nonpolar solvents like hexane. However, the bis adducts of both 9-BBN and borinane precipitate, quantitatively, from the hexane solution of the boranes at 0 °C. Fortunately, there is a large kinetic difference between the rates of adduct formation 9-BBN takes a couple of hours to react with difunctional amines, whereas borinane reacts almost instantaneously. Thus, this selective reaction affords almost quantitative precipitation of the bis adduct of borinane with TMEDA and DABCO (Scheme 17.3) [6]. 

In each step of the reaction sequence, the polymer-bound products were precipitated selectively and excess reagents and by-products were removed by filtration methods before the next step of reaction sequence. 

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