What is chromatography?

What is chromatography

The word chromatography originates from the Greek word ‘khromatos’ meaning color writing.

Definition:

A technique used for separating the components of a mixture by their distribution between a stationary and a mobile phase is called chromatography.

Chromatography Principle:

Chromatography is a method used primarily for the separation of a sample mixture and involves the distribution of solute between stationary and mobile phases.

The stationary phase may be a solid or a liquid supported as a thin film on the surface of a solid, or a gel.

The gas or liquid can constitute a mobile phase.

The ‘term chromatography bed’ is used as a general term to denote any of the different forms in which the stationary phase is used. The distribution of the component between two phases is governed by distribution coefficient K.

A component with small value of K mostly remains in the stationary phase as the moving phase flows over it. The component of a mixture with a high value of K remains largely dissolved in the mobile phase and passes over the stationary phase quickly.

Chromatography types:

Types of chromatography are based on the nature of the fixed and moving phase. Following are the types of chromatography:

 (i) Adsorption chromatography

In this type of chromatography fixed phase is a solid e.g, alumina, magnesium oxide, silica gel etc. Column chromatography and thin layer chromatography (TLC) are the examples of this type.

(ii) Partition Chromatography

In this type of chromatography fixed phase is a liquid e.g. water. Paper chromatography is the example of this type.

(iii) Gas Chromatography

In this type of chromatography moving phase is mixture of gases.

Paper chromatography definition:

Chromatography technique in which paper composed of cellulose is used as a support for the stationary phase, i.e., water, is called paper chromatography.

The absorbed water behaves as an immiscible liquid towards another liquid, which passes over the paper. The mobile phase is usually an organic liquid. There are three commonly ways of carrying out paper chromatography, namely:

(a) Ascending paper chromatography

(b) Descending paper chromatography

(c) Radial/Circular paper chromatography

Ascending Paper Chromatography:

In this technique the solvent is in a pool at the bottom of a vessel in which the paper is supported and the solvent travels upwards by capillary action.

Procedure:

  1. Take a solvent mixture specially composed in accordance with the sample to be separated, in chromatographic tank. Cover the tank to homogenize its inner atmosphere.
  2.  Take about 20 cm long strip of Whatmann’s chromatographic paper No.1 and draw on it a thin pencil line about 2.5 cm from an end.
  3. Spot a point, on the pencil line, with the sample mixture solution. To facilitate identification of the components of the mixture, spots of the known compounds may also be placed alongside.
  4. Suspend the dried paper with the clips so that the impregnated end dips into solvent mixture to a depth of 5-6 mm.
  5. Cover the tank. As the solvent front passes the spots; the solutes begin to move toward. The rate at which they move depends on their partition coefficients.
  6. When the solvent front has risen to about 3/4 of the length of the paper, remove the strip, mark the solvent front with a pencil and allow the strip to dry.
  7. The different components of the mixture, if colored, can visually be identified. If colorless, the chromatogram has to be developed by some spraying agents. Physical techniques are also used to identify the spots.

Uses of Chromatography:

  1. The techniques of chromatography are very useful in organic synthesis for separation, purification and isolation of the products.
  2. These techniques are used for the identification of substances and also for the determination of percentage purity of substances. These are equally important in qualitative and quantitative analysis.

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