General Crystallization Procedure

Step 2. Dissolve the material to be purified in the rriirdmum amount of warm solvent. Remember that most organic solvents are extremely flammable and that many produce very toxic vapor. 

Step 3. Once the solid mixture has fully dissolved, filter the heated solution, and then bring it to the point of saturation by evaporating a portion of the solvent. 

Step 4. Cool the warm saturated solution to reduce the solubility of the solute this usually causes the solid material to precipitate. If the material has a low melting point or is very impure it may come out of solution sometimes as an oil. If so, reheat the solution and allow it to recool slowly. 

Step 5. Isolate the solid by filtration, and then remove the last traces of solvent. 

The crystallization is successful if the solid is recovered in good yield and is purer than it was before the crystallization. This cycle, from solid state to solution and back to solid state is called recrystallization when both the initial and final materials are crystalline. 

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The general reaction procedure and apparatus used are exactly as described in Procedure 2. Ammonia (465 ml) is distilled into a 2-liter reaction flask and to this is added 165mlofisopropylalcoholandasolutionof30g(0.195 mole) of 17/ -estradiol 3-methyl ether (mp 118.5-120°) in 180 ml of tetrahydrofuran. The steroid is only partially soluble in the mixture. A 5 g portion of sodium (26 g, 1.13 g-atoms total) is added to the stirred mixture and the solid dissolves in the light blue solution within several min. As additional metal is added, the mixture becomes dark blue and a solid (matted needles) separates. Stirring is inefficient for a few minutes until the mass of crystals breaks down. All of the sodium is consumed after 1 hr and 120 ml of methanol is then added to the mixture with care. The product is isolated as in Procedure 4h 2. After being air-dried, the solid weighs 32.5 g (ca. 100% for a monohydrate). A sample of the material is dried for analysis and analyzed as described in Procedure 2 enol ether, 91% unreduced aromatics, 0.3%. The crude product may be crystallized from acetone-water or preferably from hexane. 

Zeolite membranes are generally synthesized as a thin, continuous film about 2-20 xm thick on either metallic or ceramic porous supports (e.g., alumina, zirco-nia, quartz, siHcon, stainless steel) to enhance their mechanical strength. Typical supported membrane synthesis follows one of two common growth methods (i) in situ crystallization or (ii) secondary growth. Figure 10.2 shows the general experimental procedure for both approaches. 

There are two general crystal preparations for x-ray analysis. The hrst is for x-ray powder analysis and the second is for single-crystal analysis. These procedures complement one another. For x-ray... 

Crystallization Method. Such methods as mechanical separation, preferential crystallization, and substitution crystallization procedures are included in this category. The preferential crystaUization method is the most popular. The general procedure is to inoculate a saturated solution of the racemic mixture with a seed of the desired enantiomer. Resolutions by this method have been reported for histidine (43), glutamic acid (44), DOPA (45), threonine (46), A/-acetyl phenjialanine (47), and others. In the case of glutamic acid, the method had been used for industrial manufacture (48). 

There are also a limited number of mechanical separation procedures basically similar to that originally used by Pasteur to separate the different tartaric acids. Such methods usually require the use of special crystallization techniques, or the use of chiral additives such as cyclodextrin. These types of crystallization procedures, however, can be somewhat tedious to operate and not usually applicable. Nevertheless, a limited number of processes have been successfully established, where a concentrated racemic solution is carefully maintained at a critical temperature and then seeded with one isomer only. Under these conditions the crystals produced are almost entirely of that of a single isomer only. However, these are rather special cases and the field of application for mechanical separation methods, in general is rather limited. 

The most general preparative procedures used in the synthesis of side-chain polymer liquid crystals are (a) homopolymerization or copolymerization of the mesogenic (or non-mesogenic) groups themselves or (b) attachment of the mesogenic units via a reactive site to an existing polymer backbone. As shown schematically in Fig. 2, for either route, the nett effect is a semi-flexible backbone with pendant side-groups. A, B, C, etc. 

This experiment assumes a familiarity with the general semimicroscale crystallization procedure (Technique 11, Section 11.3). In this experiment. Step 2 in Technique 11, Figure 11.3 (removal of insoluble impurities) will not be required. Although the impure sample may have a slight color, it will also not be necessary to use a decolorizing agent (Technique 11, Section 11.7). Leaving out these steps makes the crystallization easier to perform. Furthermore, very few experiments in... 

First, consider the extraction of a solid carboxylic acid (the desired product) with sodium bicarbonate, followed by precipitation of the acid and isolation of the solid acid by filtration. It is also necessary to air-dry the filtered solid. See the flowchart for all of these steps in Figure 12.11. Although this procedure sometimes gives the acid in a pure form, it may also be necessary to perform a crystallization procedure. In this example, the carboxylic acid is assumed to be a solid at room temperature. If the carboxylic acid were a liquid at room temperature, the separation scheme would involve different techniques including a distillation. The neutral impurities are not identified here since this example represents a general case rather than a specific experiment. In many experiments it will be possible to determine the identity of most of the impurities. For example, in most synthesis experiments, some of the impurities will be leftover reactants. 

Column chromatography is a powerful technique for the purification of organic materials. In general, it is significantly more efficient than crystallization procedures. Recrystallization is often best avoided until the last stages of purification, where it will be most efficient. Rely instead on chromatography to do most of the separation. 

Once the carotenoids have been isolated as described in Sect. 2.2, they can generally be crystallized as an initial step to purification. Actoally, what is most likely to happen is a co-crystallization. When working with a nonpolar fraction, a- and p-carotene may co-crystallize. In the same way, a polar fraction may yield lutein — zeaxanthin crystals. A pure carotenoid product may be obtained by crystallization of a fraction derived from a preparatory chromatographic procedure, which can be done using TEC, HPLC (Sect. 4), or in some cases column chromatography [84]. The crystallization procedure is shown here. 

Melt crystallization procedures generally produce crystalline modifications of the a and P forms close to the limit ordered a" and limit disordered P models, respectively [4]. 

Low Soda Hydroxide. Tlie Na20 content of nomial Bayer hydroxide is around 0.2—0.4%, 0.1% of wliich can be removed by thorough wasliing. Tlie remaining soda is trapped within the hydroxide crystal. Experience shows that the occluded soda content is reduced when cry staUization is carried out under low aluniina-supersaturation conditions and at relatively higher temperatures (80—95°C). Soda contents as low as 0.05% Na20 can be obtained by tliis procedure. However, these conditions also reduce hydroxide ield and thus increase the production cost. Low soda aluminum hydroxide is generally employed in the production of aluminas for the ceramics industries. 

An average crystal size can be used to characterize a CSD. However, the average can be determined on any of several bases, and the basis selected must be specified for the average to be usehil. More than 20 different averaging procedures have been proposed, yet none is generally satisfactory or preferred (5). 

General References AIChE Testing Procedures Crystallizers, American... 

This procedure can be checked against experimental values which are obtained from the energy to cleave single crystals along specific directions. The agreement is good (see Table 7.2), and since it is of a general nature, the method could even be extended to the elemental semiconductors. 

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