The solid product is seldom pure when obtained from a chemical reaction, being contaminated with various impurities, reagents and byproducts. For purification, the process of crystallization, sometimes called recrystallization, is generally employed. When dealing with large quantity formulas, the utmost care should be taken to obtain the maximum yield of a pure crystallized compound.
Crystallization by Cooling. The ideal solvent is one in which the compound to be obtained in pure crystalline form is insoluble at cold temperatures, but readily soluble at hot temperatures. Also the impurities should either be insoluble or else very soluble and filtered accordingly to remove. In real life operations, this perfect solvent cannot always be found, so the nearest approach to it should be selected.
The solvents most commonly employed are: water, ethyl and methyl alcohol, ether, benzene, petroleum ether, acetone, glacial acetic acid; also two or three solvents may be mixed to get the desired effect as described later. If you still cannot dissolve the compound, try some of these: chloroform, carbon disulfide, carbon tetrachloride, ethyl acetate, pyridine, hydrochloric acid, sulfuric acid (acids are usually diluted first), nitrobenzene, aniline, phenol, dioxan, ethylene dichloride, di, tri, tetrachloroethylene, tetrachloroethane, dichloroethyl ether, cyclohexane, cyclohexanol, tetralin, decalin, triacetin, ethylene glycol and its esters and ethers, butyl alcohol, diacetone alcohol, ethyl lactate, isopropyl ether, etc.
If unsure of what solvent to use, look in the Merck Index or in a chemistry handbook. This may save you the time and expense of testing for the best solvent.
Choosing a Solvent. In order to select a suitable solvent, place small quantities, (50 to 100 mg) of product into several test tubes and treat with a few drops of single solvents of the above class. If the product dissolves easily in the cold upon shaking or if it does not dissolve appreciably on boiling, the solvent in question may be regarded as unsuitable. Where the product 01 substance dissolves on heating or boiling, and separates out again on cooling, the solvent used is suitable; make sure that you choose the solvent that gives good crystals in the greatest abundance. At times, crystallization will not take place due to cooling or even supercooling; in such a case, the side of the glass container should be rubbed with a glass rod, and/or "seeded" by the addition of a very small amount of crude product, since such operations often induce crystallization. With substances which are sparingly soluble in the common solvents, solvents of high boiling points such as toluene, nitrobenzene, etc., should be used.
Where no single solvent is found suitable, a mixture of two mixable solvents, one of which the product is soluble and the other insoluble, may be used. The substance is dissolved in a small quantity of the solvent that has the strongest dissolving power, then the solvent that does not dissolve the product, is added until complete crystallization occurs. This process can be carried out with or without heat. Let me use an example. Youjust dissolved a few grams of nitrostyrene in a small (always use a small amount of solvent if possible) quantity of boiling ethanol and upon cooling in a freezer no crystals appear. Next, you try "seeding" and another hour in the freezer, but still no luck. By testing small amounts of the styrene with different solvents you find something that will not dissolve it, so you add this solvent slowly to the hot or cold styrene solution and the product crystallizes, if not you must now take much time to evaporate both solvents. Needless to say that this does little purification and may take days. Evaporation is greatly speeded up if done under vacuum conditions.
To Prepare Solutions. If considerable heating is necessary, a reflux condenser should be employed to avoid loss of solvent. Where the resulting solution does not require filtration, a conical flask should always be used. During any heating, the contents of the vessel needs to be frequently shaken or stirred, since the crystals melt to a heavy oil settling on the bottom of the vessel making the vessel liable to crack.
In preparing the solution, an excessive amount need not be employed at first; successive small quantities should be added to the boiling or near boiling solution until the substance just completely dissolves, or until nothing but impurities remain undissolved. With substances of low melting point, care should be taken that concentrated solutions from which the substance commences to separate at temperatures above its melting point are not used,
Crystallization by Evaporation. This method is employed when the substance is so easily soluble in all solvents (hot or cold), that it will only crystallize after, partial or complete evaporation. If complete evaporation must be employed, impurities will remain. So, if possible, filter off the mother liquor (solvent), as this is where the dissolved impurities will be. If you should need to heat the product with an effective solvent until thoroughly dissolved, pour through filter paper to remove solid impurities.
The type of vessel employed depends on volatility of the solvent; obviously the conical flask already recommended for "crystallization by cooling" is not suitable for spontaneous evaporation, while a beaker or shallow 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 so they should be removed carefully with a spatula or spoon before attempting to filter off the crystals.
Another method that can be used, if the above methods fail, is to dissolve the substance in some solvent, then add a second solvent mixable with the first solvent, but in which the substance is not soluble or sparingly soluble. The first solvent is then gradually removed and the substance crystallizes back out. If the first solvent is more volatile than the second, it can be evaporated out of the solution leaving the non-soluble solvent behind to crystallize the substance. If the first (dissolving) solution is not as volatile as the second solution, place the solution in a desiccator over some substance which absorbs the first solvent but not the second; in this way water may be removed from a water-alcohol solution by caustic potash or quicklime.
If a substance can only be crystallized by total evaporation, it can usually be purified by distillation first.
Continue reading here: Filtration
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