The purification of organic compounds is a necessary, albeit complex, step after its extraction from a natural source or synthesis in the laboratory. The method of purification depends largely on the nature of the compound and the impurities present.

Table of Contents

Sublimation

Crystallisation

Distillation

Fractional Distillation

Vacuum Distillation

Steam Distillation

Differential Extraction

Chromatography

One easy method to check the purity of an organic compound is to melt or boil it, as organic compounds tend to have sharp melting and boiling points.

The methods of purification include:

• Distillation
• Filtration
• Crystallization
• Chromatography

Sublimation

Crystallisation

Distillation

Differential Extraction

Chromatography

Methods of Purification

Sublimation

Some solids can directly pass to the vapour state without going through the liquid phase. This process is known as sublimation and it is used to purify and separate sublimable compounds from non-sublimable ones.

Methods of Purification – Sublimation

An inverted funnel is kept above the china dish containing the substance, and is cooled in order to hasten the process of collecting the sublimable compounds. The vapours of the substance then solidify on the funnel.

Check: Sublimation

Crystallization

The principle here is that the compound and the impurities have different solubilities in a solvent. A solvent is chosen where the compound to be purified is sparingly soluble, that is, it is sparingly soluble at lower temperature and soluble at a higher temperature. The solution is heated to get a saturated solution, and on cooling, the crystals of the compounds are removed via filtration.

Crystals of benzoic acid can be formed when the acid is added to both cold and hot water. Benzoic acid is only slightly soluble in cold water, but more soluble in hot water.

If the mixture contains impurities with the same solubility as the compound to be purified, repeated crystallisation should be carried out.

Check: Crystallisation

Purification of Liquids

Distillation

The underlying principle behind distillation is that the mixture of liquids can be separated by the difference in their boiling points. The boiling point is defined as the temperature at which the vapour pressure of the liquid is equal to the atmospheric pressure. This method separates volatile liquids from non-volatile liquids. The setup is given below.

Purification of Liquids – Distillation

The mixture is taken in an RB flask and boiled. The component with the lower boiling point, which is more volatile, evaporates faster and is collected in a separate container. A condenser is used to hasten the process of condensation.

Distillation can be used to separate a mixture of chloroform and aniline, since the boiling point of chloroform is 60°C and that of aniline is 189°C.

Check Distillation

Fractional Distillation

A fractionating column is used when the difference between the boiling points of two liquids is small, as their vapours may combine together. This column is attached to the top of the RB.

Check out this article on Fractional Distillation

Fractional Distillation Purification of Liquids via Fractional Distillation

Vacuum Distillation

The boiling point of liquids is affected by atmospheric pressure. To reduce the boiling point of the liquids and make the distillation process faster, a vacuum pump can be used to reduce the atmospheric pressure. This will cause the liquids to boil at a temperature lower than their boiling points and boil faster.

Process of Vacuum Distillation

Steam Distillation

The principle employed in this variant is that the liquids will boil faster due to aqueous tension (vapour pressure of water) which helps in equalising the atmospheric pressure, by passing steam into the flask containing the liquids to be separated.

Total pressure = Aqueous tension + Vapour pressure of liquid components

Steam Distillation: Purification of Liquids

Without the presence of aqueous tension, boiling would have taken longer to reach equilibrium with atmospheric pressure. However, with the introduction of steam, this process is accelerated.

Check out this article on Steam Distillation!

Types of Distillation - Video Lesson

Differential Extraction

This method is used for immiscible liquids, that is, liquids that do not mix together. For example, oil and water are immiscible.

The immiscible liquids are taken in a separating funnel and left undisturbed. After a period of time, they will separate into layers based on their specific gravities, with the heavier liquid at the bottom. Afterwards, they can be collected.

Substances can also be separated based on their solubility in a liquid. For example, phenol can be extracted using a NaOH solution as one of the liquids.

Chromatography

Chromatography is an essential separation method used to divide individual particles of a combination of substances, to purify the compounds and assess the purity of organic compounds. In this technique, a mixture of substances is applied to a stationary phase (solid or liquid). The mixture of gas or the pure solvent is allowed to move gradually on the stationary phase, resulting in the components of the mixture beginning to separate from each other.

Chromatography is of two types:

Adsorption Chromatography

Partition Chromatography

Adsorption Chromatography

The adsorption process is based on the principle that constituents of a mixture are absorbed to varying degrees onto an adsorbent, usually silica gel or alumina. When a mobile phase passes over the fixed phase, the different constituents of the mixture are adsorbed at different points along the fixed phase.

Adsorption Chromatography - Check!

Adsorption Chromatography is further classified into:

• Ion Exchange Chromatography
• Affinity Chromatography
• Size Exclusion Chromatography
• Hydrophobic Interaction Chromatography

Column Chromatography

Thin Layer Chromatography

Column Chromatography

A mixture is separated using a column of either silica gel or alumina, packed into a glass column. The constituent with the highest affinity for the fixed phase is adsorbed at the top, followed by the other constituents. An eluant is then used to retrieve them. Finally, the solvent is evaporated to obtain the constituent.

Check out Column Chromatography here!

![Column Chromatography]()

Thin Layer Chromatography

A spot of the mixture is placed on a sheet of alumina (0.2 mm thick). The solvent is then added, which rises due to capillary action. The constituents of the mixture are then separated as they rise with the solvent, depending on their differential adsorption.

Learn more about Thin Layer Chromatography here

Crystallization of Impure Sample: Read More