Example crystallization slow cooling

As with pH, proteins may vary in solubility as a function of temperature, and some are quite sensitive. One can take advantage of this property with both bulk and microtechniques (Jacoby, 1968 McPherson, 1999). Many of the earliest examples of protein crystallization were based on the formation of concentrated solutions at elevated temperatures followed by slow cooling. Osborne in 1892 successfully crystallized over 20 plant seed globulins by cooling relatively crude extracts from 60°C to room temperature in the presence of varying concentrations of sodium chloride. 

This allotrope is usually obtained by heating of powdered a-Sa to about 369 K. In single crystals of a-Ss the transformation is kinetically hindered, for example due to the absence of impurities introduced by grain boundaries. Another way to obtain pSs is by slow cooling of molten sulfur, or by crystallization from organic solvents. 

Microscopic Examination. Seven crystal types have been described from optical microscope examination. All of these phases were seen in this work. Figures 3 and 4 show some examples. A new form, air spherulite, crystallized on slow cooling of the melt (1°C. per minute or less). 

Occasionally, a pure component (a) phase may exhibit properties markedly different from those of the intermetallic phase which is vicinal to it on the constitutional diagram. Thus, the a-phase may dissolve more readily in a solvent or it may be attacked more readily by a reagent. In such cases it may be possible to use an excess of the pure component during the high-temperature synthesis and then liberate the intermetallic product by leaching out the matrix phase. Occasionally, also, slow cooling of the alloy melt may yield well-formed crystals of the intermetallic phase embedded in the pure component matrix, which may then be removed by some solvent. Dependii on circumstances, the matrix phase may be removed by electrolytic oxidation, by aqueous acids, by bases, or by liquid NHa. For example ... 

Hydrates can sometimes be obtained by simply suspending the anhydrous material in water, whereupon a form of Ostwald ripening occurs. For instance, aqueous suspensions of anhydrous metronidazole benzoate are metastable, and storage at temperatures lower then 38°C leads to monohydrate formation accompanied by crystal growth [65]. Sorbitol provides another example of this behavior, where slow cooling of a saturated aqueous solution yields long thin needles of sorbitol hydrate [66]. When suspended in water, anhydrous carbamazepine is transformed to carbamazepine dihydrate [67]. In other instances, hydrates can be obtained from mixed solvent systems. 

It may be instructive to consider some examples of solvate formation. The compound 5-methoxysulphadiazine forms 1 1 host-guest solvates with dioxane, chloroform, and tetrahydrofuran [78]. These were prepared by heating to boiling a solution of the sulfonamide in the appropriate solvent, followed by slow cooling to obtain large crystals. Spironolactone forms 1 1 solvates with methanol, ethanol, ethyl acetate, and benzene. It also forms a 2 1 spironolactone-acetonitrile solvate [79,80]. The spironolactone solvates were prepared by crystallization in a refrigerator from solutions that were nearly saturated at room temperature. 

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