Cristalizacion

SELECTING A SOLVENT
A solvent the dissolves little of the material to be crystallized when it is cold but a great deal of the material when it is hot is a good solvent for the crystallization. With compounds that are well known, such as the compounds that are either isolated or prepared in this textbook, the correct crystallization solvent is already known through the experiments of earlierresearchers. In such cases, the chemical literature can be consulted to determine which solvent should be used. Sources such as handbooks or tables frequently provide this information. Quite often, the correct crystallization solvents are indicated in the experimental procedures in this textbook.
When the appropriate solvent is not known, select a solvent for crystallization by experimenting withvarious solvents and a very small amount of the material to e crystallized. Experiments are conducted on a small test-tube scale before the entire quantity of material is committed to a particular solvent. Such trial-and-error methods are common when one is trying to purify a solid material that has not been previously studied
When choosing a crystallization solvent, tae care not to pick one whoseboiling point is higher that the melting point of the substance to be crystallization. If the boiling point of the solvent is high, the solid may melt in the solvent rather that dissolve. In the boiling point of the solvent is high, the solid may melt in the solvent rather that dissolve. In such a case, the solid may oil out. Oiling out occurs when the solid substance melts and forms a liquid thatis not soluble in the solvent. On cooling, the liquid refuses to crystallize; rather, it becomes a supercooled liquid, or oil. Oils may solidify it the temperature is lowered, but often they will not crystallize. A solidified oil becomes an amorphous solid or a hardened mass – a condition that does not result in the purification of the substance. It can be very difficult to deal with oils whentrying to obtain a pure substance. You must try to redis-solve them and hope that they will precipitate as crystals with slow, careful cooling.
During the cooling period, it may be helpful to scratch the glass container where the oil is present with a glass stirring rod that has not been fire-polished. Seeding the oil as it cools with a small sample of the original solid is another techniquesometimes helpful in working with difficult oils. Other methods of inducing crystallization are discussed in Section 5.7
One additional criterion for selecting the correct crystallization solvent is the volatility of that solvent. Volatile solvents are those that have low boiling points or evaporate easily. A solvent with a low boiling point may be removed from the crystals through evaporation withoutmuch difficulty. It will e difficult to remove a solvent with a high boiling point from the crystals without heating them under vacuum.

DECOLORIZATION
Small amounts of highly colored impurities may make the original crystallization solution appeared colored; this color can often be removed by decolorization either by using activated charcoal (often called Norit) or by passing the solutionthrough a column packed with alumina or silica gel. A decolorizing step should be performed only if the color is due to impurities, not to the color of the desired product, and if the color is significant. Small amounts of colored impurities will remain in solution during crystallization, making the decolorizing step unnecessary. The use of activated charcoal is described separately for semi-microscaleand microscale crystallization, and then the column technique, which can be used with both crystallization techniques, is described.
A. Semi-microscale Powdered Charcoal
As soon as the solute is dissolved n the minimum amount of boiling solvent, the solution is allowed to cool slightly and a small amount of Norit (powdered charcoal) is added to the mixture. The Norit adsorbs the impurities....