Recrystallization

Recrystallization (see also crystallization) is a physical process that has meanings in chemistry, metallurgy and geology.

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Chemistry

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:

Single-solvent recrystallization

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.

Multi-solvent recrystallization

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-recrystallization

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.

Seeding

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:

• Slow evaporation of a single solvent - typically the compound is dissolved in a suitable solvent and the solvent is allowed to slowly evaporate. Once the solution is saturated crystals can form.

• Slow evaporation of a multi-solvent system - the same as above, however as the solvent composition changes due to evaporation of the more volatile solvent. The compound is more soluble in the volatile solvent, and so the compound becomes increasingly insoluble in solution and crystallizes.

• Slow diffusion - similar to the above. However, a second solvent is allowed to evaporates from one container into a container holding the compound solution (gas-diffusion). As the solvent composition changes due to an increase in solvent that is has gas-diffused into solution, the compound become increasingly insoluble in solution and crystallizes.

• Interface/slow mixing (often performed in an NMR tube). Similar to the above, but instead of one solvent gas-diffusing into another, the two solvents mix (diffuse) by liquid-liquid diffusion. Typically a second solvent is "layered" carefully on top of the solution containing the compound. Overtime the two solution mix. As the solvent composition changes due to diffusion, the compound becomes increasingly insoluble in solution and crystallizes, usually at the interface.

• Specialized equipment can be used in the shape of a "H" to perform the above, where one of the vertical line of the "H" is a tube containing a solution of the compound, and the other vertical line of the "H" is a tube containing a solvent which the compound is not soluble in, and the horizontal line of the "H" is a tube which joins the two vertical tubes, which also has a fine glass sinter that restricts the mixing of the two solvents.

• Once single perfect crystals have been obtained, it is recommended that the crystals are kept in a sealed vessel with some of the liquid of crystallisation to prevent the crystal from 'drying out'. Single perfect crystals may contain solvent of crystallisation in the crystal lattice. Loss of this internal solvent from the crystals can result in the crystal lattice breaking down, and the crystals turning to powder.

Geology

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.

Limestone is a sedimentary rock that undergoes metamorphic recrystallization to form marble, and clays can recrystallize to muscovite mica.

Metallurgy

In metallurgy, recrystallization is the nucleation and growth of new undeformed grains in a deformed metal.

Ice

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.

See also

• Crystal
• Crystal structure
• Crystallite
• Crystallization and engineering aspects
• Fractional crystallization
• Seed crystal
• Single crystal

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