Crystallisation by cooling

When boric acid is made from colemanite, the ore is ground to a fine powder and stirred vigorously with diluted mother Hquor and sulfuric acid at about 90°C. The by-product calcium sulfate [7778-18-9] is removed by settling and filtration, and the boric acid is crystallised by cooling the filtrate. 

Phosphorus pentabromide [7789-69-7] M 430.6, m <100 , b 106 (dec). Dissolved in pure nitrobenzene at 60°, filtering off any insoluble residue on to sintered glass, then crystallised by cooling. Washed with dry Et20 and removed the ether in a current of dry N2. (All manipulations should be performed in a dry-box.) [Harris and Payne J Chem Soc 3732 1958]. Fumes in moist air because of hydrolysis. HARMFUL VAPOURS. 

Crystallisation by Cooling.—The ideal solvent is one in which the compound to be obtained in pure crystalline form is insoluble in the cold, but readily soluble in the hot. Further, the impurities should either be insoluble or else very soluble. In practice such a solvent is seldom obtained, but the nearest approach to it should be selected. 

Crystallisation by Evaporation.—This method is employed when the substance is so easily soluble in all solvents (hot and cold) that it will only separate after partial evaporation. The solvent is allowed to evaporate spontaneously in the air or in a desiccator if in the latter the evaporation is greatly hastened by using a suitable absorbent as well as evacuating the desiccator. The type of vessel employed depends on the volatility of the solvent obviously the conical flask already recommended for crystallisation by cooling is not suitable for spontaneous evaporation, while a beaker or shallow crystallising dish is. When the latter type of vessel is used, crusts often form on the sides above the surface of the liquid. Such crusts seldom consist of pure substance, and they should be carefully removed with a spatula before attempting to filter off the crystals. 

Methods of Crystallisation. — The crude solid product obtained directly as the result of a reaction is generally amorphous or not well crystallised. In order to obtain the compound in uniform, well-defined crystals, as well as to separate it from impurities like filter-fibres, inorganic substances, by-products, etc., it is dissolved, usually with the aid of heat, in a proper solvent, filtered from the impurities remaining undissolved, and allowed to cool gradually. The dissolved compound then separates out in a crystallised form, while the dissolved impurities are retained by the mother-liquor. (Crystallisation by Cooling.) Many compounds are so easily soluble in all solvents, even at the ordinary temperature, that... 

It also crystallises from toluene/pet ether in the cold, from pet ether (b 40-60°) or benzene. Its solution in diethyl ether has been washed with aqueous 10% NaOH until colourless, then dried (Na2S04) and crystallised by cooling in powdered Dry-ice. It has also been purified by dissolving in benzene, washing with aqueous 5% NaOH, then dried with K2CO3 or MgS04, and distilled under reduced pressure and can be sublimed at high vacuum [Ebel Chem Ber 60 20867927]. [Beilstein 11 IV 375.]... 

Potassium chloride [7447-40-7] M 74.6, m 771°, d 4 1.98. Dissolve it in conductivity water, filter it, and saturate it with chlorine (generated from cone HCl and KMn04). Excess chlorine is boiled off, and the KCl is precipitated by HCl (generated by dropping cone HCl into cone H2SO4). The precipitate is washed with water, dissolved in conductivity water at 90-95 , and crystallised by cooling to about -5 . The crystals are drained at the centrifuge, dried in a vacuum desiccator at room temperature, then fused in a platinum dish under N2, cooled... 

Figure 4.80. Apparent heat capaeity measured by DSC and MTDSC for PE035000 crystallised by cooling from the melt at 10 K min T The solid curve was obtained by DSC at 10 K min , and the open circles represent the apparent reversing heat capacity, obtained by a series of quasi-isothermal experiments [52].

Figure 6.9 Degree of crystallinity versus molecular weight for pellets and ultradrawn fibres. The pellets were crystallised by cooling from 147 °C at 20 °C/min [6]...

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