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In chemistry, recrystallization is a procedure for purifying compounds. The most typical situation is that a desired "compound A" is contaminated by a small amount of "impurity B". There are various methods of purification that may be attempted (see Separation process), which includes recrystallization. There are also different recrystallization techniques that can be used such as:
Typically the mixture of "compound A" and "impurity B" are dissolved in the minimum amount of solvent to fully dissolve the mixture i.e. a saturated solution. The solution is then allowed to cool. As the solution cools the solubility of compounds in solution drops. This results in the desired compound dropping (recrystallizing) from solution. The slower the rate of cooling, the bigger the crystals formed .
The crystallization process requires an initiation step. Once a small crystal has formed, more crystals can grow from that crystal. Since "Compound A" is in excess this will usually result in these crystals forming first and thus leaves a greater ratio of impurity in solution. Thus the resulting solid is more pure than the original mixture.
The level of purity can then be checked by taking a melting point range of the solid and comparing it to an accepted melting point range if one exists. Compounds have higher melting points when pure, so the melting point will rise when the compound is more pure. Obviously other analytical techniques can be used to assess compound purity such as NMR spectroscopy.
This purification technique results in the inevitable loss of the part of "compound A" that remains in solution. A yield of 80% would be considered quite good. However the impure solution can be concentrated and the procedure repeated to gather a "second crop" of crystals.
Successful recrystallization depends on finding the right solvent. This is usually a combination of prediction/experience and trial/error. The mixture must be soluble at higher temperatures, and must be insoluble (or have low solubility) at lower temperatures.
This method is the same as the above but where two (or more) solvents are used. This relies on both "compound A" and "impurity B" being soluble in a first solvent. A second solvent is slowly added. Either "compound A" or "impurity B" will be insoluble in this solvent and precipitate, whilst the other of "compound A"/"impurity B" will remain in solution. Thus the proportion of first and second solvents is critical. Typically the second solvent is added slowly until one of the compounds begins to crystallize from solution and then the solution is cooled. Heating is not required for this technique but can be used.
The reverse of this method can be used where a mixture of solvent dissolves both A and B. One of the solvents is then removed by distillation or by an applied vacuum. This results in the a change in the proportions of solvent causing either "compound A" or "impurity B" to precipitate.
Hot filtration can be used to separate "compound A" from both "impurity B" and some "insoluble matter C". This technique normally uses a single solvent system as described above. When both "compound A" and "impurity B" are dissolved in the minimum amount of hot solvent, the solution is filtered to remove "insoluble matter C". This matter may be anything from a third impurity compound, to fragments of broken glass. For a successful procedure one needs to ensure that the filtration apparatus is hot to stop the dissolved compounds crystalizing from solution. Often it is simpler to do the filtration and recrystallization as two independent and separate steps. That is dissolve "compound A" and "impurity B" in a suitable solvent at room temperature, filter (to remove insoluble compound/glass), remove the solvent and then recrystallize using any of the methods listed above.
Crystallization requires an initiation step. This can be spontaneous or can be done by adding a small amount of the pure compound a seed crystal to the saturated solution, or can be done by simply scratching the glass surface to generate a seeding surface for crystal growth. It is thought that even dust particles can act as simple seeds.
Single perfect crystals (for X-ray analysis)
Growing crystals for X-ray crystallography can be quite difficult. For X-ray analysis, single perfect crystals are required. Typically a small amount (5-100 mg) of pure compound is used, and crystals are allowed to grow very slowly. Several techniques can be used to grow these perfect crystals:
In geology, solid-state recrystallization is a metamorphic process that occurs under situations of intense temperature and pressure where grains, atoms or molecules of a rock or mineral are packed closer together, creating a new crystal structure. The basic composition remains the same. This process can be illustrated by observing how snow recrystallizes to ice without melting. As opposed to metasomatism, which is a chemical change caused by metamorphism, recrystallization is a physical process. However, recrystallization can occur when a local migration of chemicals results in the chemical change of the rock or mineral with no external addition of materials.
For ice, recrystallization refers to the growth of larger crystals at the expense of smaller ones. Some biological antifreeze proteins have been shown to inhibit this process, and the effect may be relevant in freezing-tolerant organisms.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Recrystallization". A list of authors is available in Wikipedia.|