Methods Of Purification

Purification Techniques

The purification techniques of organic compounds is a necessary process after their extraction from their natural source or synthesis in the laboratory. Methods of purification of organic compounds mainly depend on the basic nature of the compound and also on the impurities in it.

The purity of an organic compound is detected by its high boiling or melting points. The purification process means removing impurities in any organic compound.
The general purification techniques used are:

For solids: 

• Sublimation
• Crystallisation

For liquids:

• Distillation
• Differential Extraction
• Chromatography

Methods Of Purification Of Organic Compounds

Sublimation

The process of turning solids directly into vapours is called sublimation. This form of purification is useful for separating such compounds from non-sublimable impurities.Here, the substance is heated in a dish over an inverted funnel to extract the sublimable compounds. Keeping the funnel cool speeds up the process. The vapours will soon solidify inside the funnel.

Crystallisation

The principle of crystallisation here is that the impurities and actual compounds have different solubilities in the given solvent. The compound should be sparingly soluble in the solvent at room temperature but highly soluble at a higher temperature. Upon heating, the solution becomes saturated and on cooling, the compounds crystalize and are removed using filtration. For example, crystals of benzoic acid can be separated with water as it is barely soluble in lower temperatures but soluble when the water is hot. Repeated crystallization is used for impurities having similar solubility as the compounds. 

Purification Of Colloidal Solution

Distillation

When the compounds and impurities have different boiling points, a distillation method is used. The temperature at which the vapour pressure of the liquid will be equal to atmospheric pressure. It helps separate the volatile compounds and their non-volatile impurities. 

In a given mixture, the volatile compounds vaporize faster and get collected in a separate container. Later a condenser transforms the vapours back to the liquid state. For example, in a mixture of chloroform with aniline, the former has a lower boiling point and is easily separated.

Types Of Distillation 

Fractional Distillation

When the boiling points between the liquids aren’t much and the vapours may condense together. Here a fractionating column is connected to the opening of the system to collect the compounds.

Vacuum Distillation

A vacuum pump is used to let the liquid boil faster due to the absence of atmospheric pressure. This method is used for faster distillation. 

Steam Distillation

• The steam that is passed is the separating agent. Here, the liquid reaches boiling point faster due to the vapour pressure of water and helps the liquid to atmospheric pressure quickly
• Total pressure = Aqueous tension + vapour pressure of liquid components
• Once again, this method is used for speeding up the purification process
• Differential Extraction
• Immiscible liquids or liquids that do not mix well together are separated using this method. These liquids are poured into a separating funnel and left for a while. Due to their gravity differences, the heavier liquid settles down while the liquid with less density floats at the top. This helps in the easier separation. For example, oil and water. 

Chromatography

Chromatography is the separation technique used for separating constituent elements in a mixture of substances, for purifying compounds and checking the purity levels of organic compounds. Here, a stationary phase is used where the mixture of substances is applied. Then either a liquid or gas medium is passed through this that separates the compounds present in the stationary phase. The compound separates spending upon their affinity towards the medium used. 

Adsorption Chromatography

Here the separating compounds are adsorbed to the surface of the stationary phase and later separated, Silica gel and alumina are the most common stationary phases used. The compounds separated will be at different levels making it easier to extract them. Adsorption Chromatography is further divided into:

• Column Chromatography
• Thin Layer Chromatography

Column Chromatography

Here, a glass column filled with silica gel or alumina is used. The compound that has a high affinity to the fixed phase will adhere to the top, and that with the least affinity will be at the bottom. When the solvent evaporates, it will be easier to separate the compounds. 

Thin Layer Chromatography

Here a thin sheet made of alumina is used. A small dot of the mixture is placed on it, and then it is allowed to be slightly immersed in the solvent. As the solvent moves up, it separates the constituent compounds. Here the adsorption depends on its affinity with the compound having a high affinity at the bottom.