Crystallisation by evaporation

The formation of solids in evaporators is not always undesirable and, indeed, this is precisely what is required in the evaporator-crystalliser discussed in Chapter 15. The evaporator-crystalliser is a unit in which crystallisation takes place largely as a result of the removal of solvent by evaporation. Cooling of the liquor may, in some cases, produce further crystallisation thus establishing conditions similar to those in vacuum crystallisation. The true evaporator-crystalliser is distinguished, however, by its use of an external heat source. Crystallisation by evaporation is practised on salt solutions having... 

The cooled solution is freed from tetrammino-platinous chloroplatiuite by filtration, and tlie salt crystallised by evaporation in vacuo.1... 

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. 

Crystallisation by Evaporation. — If a compound is so easily soluble in all solvents that it will only crystallise out on partial evaporation, then, in order to get good crystals, a solution, not too dilute, is made, by the aid of heat if necessary, and filtered from the impurities remaining undissolved. In this case, as a crystallisation vessel, one of the various forms of shallow dishes— the so-called crystallising dishes — is used, in which the solution is allowed partially to evaporate. In order to protect the vessel from dust, it is covered with a funnel or watch-glass, in the manner indicated under Drying of Crystals. fn crystallising... 

For water insoluble aldehydes or ketones, the following alternative procedure may be used. Reflux a mixture of 0-6 g. of the aldehyde or ketone, 0 5 g. of hydroxylamine hydrochloride, 5 ml. of ethanol and 0 5 ml. of pyridine on a water bath for 15-60 minutes. Remove the alcohol either by distillation (water bath) or by evaporation of the hot solution in a stream of air (water pump). Add 5 ml. of water to the cooled residue, cool in an ice bath and stir until the oxime crystallises Filter off the solid, wash it with a little water and dry. Recrystallise from alcohol (95 per cent, or more dilute), benzene, or benzene - light petroleum (b.p. 60-80°). 

Hexamethylenetetramine. Pure hexamethylenetetramine [100-97-0] (also called hexamine and HMTA) is a colorless, odorless, crystalline sohd of adamantane-like stmcture (141). It sublimes with decomposition at >200° C but does not melt. Its solubiUty in water varies Htde with temperature, and at 25°C it is 46.5% in the saturated solution. It is a weak monobase aqueous solutions are in the pH 8—8.5 range (142). Hexamethylenetetramine is readily prepared by treating aqueous formaldehyde with ammonia followed by evaporation and crystallisation of the soHd product. The reaction is fast and essentially quantitative (142). 

Small quantities of pure sodium bromide can be prepared by neutralising solutions of either sodium hydroxide [1310-73-2] or sodium carbonate [497-19-8] usiug hydrobromic acid [10035-10-6] which is free of bromine, followed by evaporation and crystallisation. 

Stannous Chloride Dihydrate. A white crystalline soHd, stannous chloride dihydrate is prepared either by treatment of granulated tin with hydrochloric acid followed by evaporation and crystallisation or by reduction of a stannic chloride solution with a cathode or tin metal followed by crystallisation. It is soluble in methanol, ethyl acetate, glacial acetic acid, sodium hydroxide solution, and dilute or concentrated hydrochloric acid. It is soluble in less than its own weight of water, but with much water it forms an insoluble basic salt. 

Sodium dichromate [7789-12-01 M 298.0, m 84.6" (IHiO), 356" (anhydr) b 400"(dec), 64 2.348. Crystd from small volumes of H2O by evaporation to crystallisation. Solubility in H2O is 238% at 0° and 508% at boiling. Red dihydrate is slowly dehydrated by heating at 100° for long periods. It is deliquescent, a powerful oxidising agent-contact with skin- wash immediately as it is caustic. (Possible carcinogen). 

The fi-compound is dissolved in 50 c c. pure dry ether, and dry hydiogen chloride is passed in with constant shaking to prevent the delivery tube from becoming blocked. Colourless crystals of the hydrochloride of the /3-o ime separate and aie filtered and washed with dry ether and then placed in a separating funnel and covered with a layer of ether. A. concentrated solution of sodium carbonate is gradually added with constant shaking until no further effervescence is observed. Sodium chloride is precipitated and the /3-oxime dissolves in the ether. The ether extract is sepaiated, dehydrated over sodium sulphate, and the ether remoi ed as rapidly as possible at the ordinary temperature by evaporation in vacuo. The residue crystallises, and when pressed on a porous plate leaves a mass of small silky needles, m. p. 126—130A It may be re-... 

The filtrate following carbon treatment is evaporated, allowed to cool and crystallised. This yields refined grade itaconic add. The solution may be further evaporated to yield a second sample - this time designated industrial grade. Industrial grade samples can also be made directly by evaporating the fermentative filtrate and thus avoiding the activated carbon step. 

If the filter paper becomes choked by crystallisation of the substance, it is best not to push a hole through it. The paper should rather be held upright in a small beaker in which fresh solvent is kept boiling, and the more dilute solution thus obtained is poured through the same paper. In such cases the whole solution must generally be concentrated by evaporation. 

The practice, which may often be observed, of purifying substances by evaporating their solutions to dryness in a crystallising basin, or of leaving them till the solvent has evaporated, naturally does not achieve its purpose because, of course, the impurities are not removed in this way. 

Preparation of Anhydrous Sodium Acetate.—The hydrated salt (3 H20) is heated over a naked flame in a shallow iron or nickel dish. After the water of crystallisation has evaporated the material solidifies. By careful heating the anhydrous salt is now also fused. After resolidification the still warm substance is powdered and immediately transferred to an air-tight container. Commercial anhydrous acetate should also be fused before use. 

The problem of selecting the most appropriate operation will be further complicated by such factors as the concentration of liquid solution at which crystals start to form. Thus, in the separation of a mixture of ortho-, meta-, and para-mononitrotoluenes, the decision must be made as to whether it is better to carry out the separation by distillation followed by crystallisation, or in the reverse order. The same kind of consideration will arise when concentrating a solution of a solid then it must be decided whether to stop the evaporation process when a certain concentration of solid has been reached and then to proceed with filtration followed by drying, or whether to continue to concentration by evaporation to such an extent that the filtration stage can be omitted before moving on to drying. 

On account of the insolubility of these compounds and the difficulty of filtering and completely washing out the barium sulphate precipitate, in order to abstract from it the whole of the tyrosine, Abderhalden and Teruuchi, in the case of silk, have hydrolysed the protein with hydrochloric acid, the greater part of which was then removed by evaporation in vacuo the remainder of the hydrochloric acid was then estimated in a small aliquot portion, and then separated quantitatively by neutralising with the calculated amount of caustic soda. The tyrosine then crystallised out, and was purified by recrystallisation from water. 

When mixed with valine, leucine and proline, the two former separate in the first fractions on crystallisation when their solution is evaporated if present in large amounts it crystallises out in an almost pure state. It is separated from proline by evaporating to dryness and boiling with... 

Other derivtives can be prepared by reaction of the alcohol with an acid anhydride. For example, phthalic or 3-nitrophthalic anhydride (1 mol) and the alcohol (Imol) are refluxed for half to one hour in a non-hydroxylic solvent, e.g. toluene or alcohol-free chloroform, and then cooled. The phthalate ester crystallises out, is precipitated by the addition of light petroleum or is isolated by evaporation of the solvent. It is recrysullised from water, 50% aqueous ethanol, toluene or light petroleum. Such an ester has a characteristic melting point and the alcohol can be recovered by acid or alkaline hydrolysis. 

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