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INTRODUCTION
Crystallization is a technique used to purify solid compounds. Crystallization is used to separate a dissolved heat-liable ( that will decompose upon heating and hence can sublime) solid (solute) from a solution.
Crystallization is based on the principles of solubility: compounds (solutes) tend to be more soluble in hot liquids (solvents) than they are in cold liquids. If a saturated hot solution is allowed to cool, the solute is no longer soluble in the solvent and forms crystals of pure compound. Impurities are excluded from the growing crystals and the pure solid crystals can be separated from the dissolved impurities by filtration.
Crystallization occurs in two major steps.
- Nucleation step in which the appearance of a crystalline phase from either a supercooled liquid or a supersaturated solvent starts to form.
- Crystal growthstep which is the crystal increase in the size of particles and leads to a crystal state.
The majority of minerals and organic molecules crystallize easily, and the resulting crystals are generally of good quality, i.e. without visible defects. However, larger biochemical particles, like proteins, are often difficult to crystallize.
The ease with which molecules will crystallize strongly depends on the intensity of either atomic forces (in the case of mineral substances), intermolecular forces (organic and biochemical substances) or intramolecular forces (biochemical substances).
What Happens During Crystallization?
To crystallize an impure, solid compound, add just enough hot solvent to it to completely dissolve it. The flask then contains a hot solution, in which solute molecules – both the desired compound and impurities – move freely among the hot solvent molecules. As the solution cools, the solvent can no longer hold all of the solute molecules, and they begin to leave the solution and form solid crystals.
During this cooling, each solute molecule in turn approaches a growing crystal and rests on the crystal surface. If the geometry of the molecule fits that of the crystal, it will be more likely to remain on the crystal than it is to go back into the solution. Therefore, each growing crystal consists of only one type of molecule, the solute. After the solution has come to room temperature, it is carefully set in an ice bath to complete the crystallization process. The chilled solution is then filtered to isolate the pure crystals and the crystals are rinsed with chilled solvent.
After which, allow the hot solution to cool naturally. The solubility of the solute decreases as the solution is cooled, and the excess solute which can no longer be dissolved in the saturated solution crystallizes out of the solution. The crystals which are formed can be separated from the remaining solution by filtration.
Cooling the solution too quickly will cause impurities to be trapped inside the crystals being formed inside the crystals thus, the crystals isolated are impure. Slow crystallization gives larger crystals than fast crystallization. Small crystals have a large surface area to volume ratio and impurities are located on the surface of the crystals as well as trapped inside the matrix.
Recrystallization
Recrystallization also known as fractional crystallization, is a procedure for purifying an impure compound in a solvent. It is the most important method of purifying nonvolatile organic solids.Recrystallization involves dissolving the material to be purified (the solute) in an appropriate hot solvent.
As the solvent cools, the solution becomes saturated with the solute and the solute crystallizes out (reforms a solid). As the crystal develops, impurities are excluded from the crystal lattice, thereby completing the purification process. The crystals can then be collected, washed, and dried.
The principle behind recrystallization is that the amount of solute that can be dissolved by a solvent increases with temperature. In recrystallization, a solution is created by dissolving a solute in a solvent at or near its boiling point. At this high temperature, the solute has a greatly increased solubility in the solvent, so a much smaller quantity of hot solvent is needed than when the solvent is at room temperature.
When the solution is later cooled, after filtering out insoluble impurities, the amount of solute that remains dissolved drops precipitously. At the cooler temperature, the solution is saturated at a much lower concentration of solute. The solute that can no longer be held in solution forms purified crystals of solute, which can later be collected.
An impure compound is dissolved (the impurities must also be soluble in the solvent), to prepare a highly concentrated solution at a high temperature. The solution is cooled. Decreasing the temperature causes the solubility of the impurities in the solution and the substance being purified to decrease.
The impure substance then crystallizes before the impurities- assuming that there was more impure substance than there were impurities. The impure substance will crystallize in a purer form because the impurities won’t crystallize yet, therefore leaving the impurities behind in the solution. The slower the rate of cooling, the larger the crystals are that form.
A filtration process must be used to separate the more pure crystals at this point. The procedure can be repeated. Solubility curves can be used to predict the outcome of a recrystallization procedure.
Recrystallization works best when
- The quantity of impurities is small
- The solubility curve of the desired solute rises rapidly with temperature
The disadvantage of recrystallization is that it takes a long time. Also, it is very important that the proper solvent is used. The solution must be soluble at high temperatures and insoluble at low temperatures. The advantage or recrystallization is that, when carried out correctly, it is a very effective way of obtaining a pure sample of some product, or precipitate.
Procedure For Recrystallization
The following is an outline of the recrystallization process.
- Pick the solvent.
The criteria used to choose an appropriate recrystallization solvent includes:
a.) finding a solvent with a high temperature coefficient. The solvent must not dissolve the compound at low temperatures, but must dissolve the compound at high temperatures. The solute must dissolve in order to rid its lattice of impurities, but must not remain dissolved at room temperature.
b.) using a solvent that dissolves impurities readily or not at all. If the solvent dissolves the impurities readily (even at room temperature) then the impurities will not become trapped in the developing crystal lattice, but will remain dissolved in the solvent. If the impurities do not dissolve (even at elevated temperatures) then they can be easily removed by gravity filtration.
c.) ensuring the solvent will not react with the solute. As mentioned earlier, recrystallization does not chemically alter a molecule. No chemical bonds must be broken in the solute molecule. The crystal lattice is dissolved at elevated temperatures, but this only involves overcoming the intermolecular attractive forces.