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Paper chromatography

Introduction to paper chromatography

Paper chromatography is a chromatography technique used to separate mixture of chemical substances into its individual compounds. Paper chromatography is used to teach TLC or other chromatography as it is very similar to TLC.

Principles of paper chromatography

All chromatography follow the same principle. Paper Chromatography consists of two phases: one mobile phase and one contiguous stationery phase. The stationery phase a paper and the mobile gas is solvent. The compound mixture moves along with the mobile phase through stationery phase and separates depending on the different degree of adhesion (on the paper) of each component in the sample or the compound mixture.

Explanation

The stationery phase

The paper chromatography is very similar to Thin layer chromatography. Difference is, instead of using a thin layer of silica on metal, it uses a special type of chromatography paper as stationery phase. This paper is made of cellulose. Cellulose is a polymer of simple sugar, glucose.

By NEUROtiker (Own work) [Public domain], via Wikimedia Commons

Cellulose contains -OH group similar to the silica or alumina on the TLC plate. The surface of cellulose is thus very polar. So the compounds can form hydrogen bond or can interact by van der waals dispersion forces and dipole dipole forces.

Process

Paper chromatography works in few steps:

Step 1: A horizontal line is drawn near one end (about 1.5 cm from the bottom edge) of the paper. In figure below 6 is the horizontal line.

By No machine-readable author provided. Dubaj~commonswiki assumed (based on copyright claims). [Public domain], via Wikimedia Commons

Step 2: The sample needs to be separated is placed as a small drop or line on to the paper using capillary tube. Labelling the drop by a pencil with an alphabet or number help to identify the compound later. In figure above 3 and 4 are the drops labelled. The drops are then soaked on the paper and dried.

Step 3: The paper is then placed into a sealed container with a swallow layer of  suitable solvent. The solvent level must be lower than the pencil line or drop on it. The container need to be covered to stop the solvent to evaporate.

Step 4: The solvent rises up the paper chromatography taking each component of the sample with it. The components travel with the solvent depends on three things:

  • The polarity of the sample molecule. The non polar components travel faster than the polar component.
  • The attraction between the sample molecule and the solvent or solvent mixture.
  • The attraction between the sample and the silica.

Suppose any sample compound mixture contains three colored molecules  green, blue and red. According to their polarity, the order of these compounds is green<blue<red. Thus the most non polar green will travel first along with the mobile phase. Then blue and at last most polar compound the red one.

Step 5: When the solvent rises near the end of the paper then the paper should be taken out from sealed container and air dried. The paper with separated bands of components are then observed under UV-light.

By L26 (Own work) [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)], via Wikimedia Commons

Rf value

The compounds in the sample travels along with solvent to give separate bands on the paper. The distance travelled by same compound with respect to the solvent is always constant. Thus the ratio of the distance that the compound travelled and the distance that the solvent travelled is denoted as Rf. And mathematically expressed as:

\({ R }_{ f }=\frac { distance/quad travelled/quad by/quad compound }{ distance/quad travelled/quad by/quad solvent }\)

Summary

  • Paper chromatography is an chromatography technique used to separate mixture of chemical substances into its individual compounds.
  • Paper chromatography consists of two phases: one mobile phase and one contiguous stationery phase.
  • Paper used in paper chromatography is made of cellulose.
  • A suitable solvent (mobile phase) is moved along with a compound mixture through the paper according to the polarity and the degree of adhesion of each component on the stationery phase.
  • The ratio of the distance that the compound travelled and the distance that the solvent travelled is denoted as Rf.

 

 

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