Differential extraction is a purification technique that is used for separating an organic compound present in an aqueous solution with the help of an organic solvent in which that compound is more soluble than water.
Differential extraction is carried out in a separating funnel. The name chromatography is derived from the Greek word ‘chroma’ which means colour and ‘graphein’ which means to write.
Chromatography is used for identifying, separating and purifying the compounds of a mixture. Based on the principle involved, Chromatography is divided into two types
- Adsorption chromatography
- Partition chromatography
Adsorption chromatography is based mainly on the differences between the adsorption affinities of the different compounds in a mixture on the commonly used adsorbents like silica gel and alumina. When the mobile phase is allowed to run over the adsorbent, the components in the mixture move by varying distances over the stationary phase.
The relative adsorption of each component in the mixture is expressed in terms of its retardation factor, Rf. The value of the retardation factor is determined by dividing the distance moved by the substance from the base line by the distance moved by the solvent from the base line.
Based on the principle of differential adsorption, chromatographic techniques are of two types. They are
- Column chromatography
- Thin layer chromatography
In column chromatography, the stationary phase which is the adsorbent is packed in a vertical glass tube. The mixture of components which are to be separated were placed on the top of the stationary phase.
In thin layer chromatography the mixtures are separated using a glass plate which is coated with an adsorbent. The plate is known as thin layer chromatography plate or chromaplate.
Partition chromatography is based on continuous differential partitioning of solutes between the mobile and stationary phases. It includes paper chromatography, where a special quality paper known as chromatography paper is used.