Chemistry · Ch 9 — Organic Chemistry – Some Basic Principles and Techniques
Crystallisation
Crystallisation
The Principle of Crystallisation
Crystallisation is the most widely used method for purifying solid organic compounds. The entire technique rests on a single physical fact: different substances have different solubilities in a given solvent, and those solubilities change with temperature in different ways.
The idea is straightforward. You take the impure solid and dissolve it in a solvent in which the compound itself is only sparingly soluble at room temperature, but becomes appreciably soluble when the solvent is heated. The impurities, ideally, are either much more soluble or much less soluble than the compound under these conditions. You heat the solvent, dissolve as much of the impure solid as you can, and then concentrate the solution until it is nearly saturated. When you let this hot, concentrated solution cool, the pure compound can no longer stay dissolved — it comes out of solution as crystals. The impurities, being either too soluble to crystallise or already removed, stay behind in the liquid that remains, which is called the mother liquor. The crystals are then separated by filtration.
The mother liquor is not pure waste — it still contains a small amount of the dissolved compound along with the impurities. In a laboratory, you might recover more product by concentrating the mother liquor further, but the second crop of crystals will be less pure.
Choice of Solvent
The solvent is the key to success. A good solvent for crystallisation must satisfy two conditions at once: it must dissolve very little of the compound when cold, but a great deal of it when hot. This large difference in solubility with temperature is what drives the compound out of solution as pure crystals upon cooling.
If no single solvent meets both conditions, you can use a mixture of solvents. Suppose the compound is highly soluble in one solvent (say, ethanol) and almost insoluble in another (say, water). You dissolve the compound in the hot, good solvent, then add the poor solvent drop by drop until the solution becomes just slightly turbid — that is, until it is on the verge of precipitation. A little more of the good solvent clears it, and on cooling, the compound crystallises out. The two solvents must be miscible with each other.
Removing Coloured Impurities
Sometimes the impurities themselves are not the only problem — they may also impart an unwanted colour to the solution. In such cases, you add a small amount of activated charcoal to the hot solution. The charcoal adsorbs the coloured impurities onto its surface. You then filter the hot solution through a fluted filter paper to remove the charcoal along with the adsorbed colour, and then allow the clear filtrate to cool and crystallise.
Never add activated charcoal to a boiling solution — it can cause violent bumping and the solution may boil over. Always cool the solution slightly before adding the charcoal, then reheat to boiling before filtration.