Question

How pigment separation using paper chromatography works?

Answer 1

This technique was developed in 1941 and separation of molecules occurs due to difference in polarity. When one end of the paper is dipped into the solvent mixture, the solvent migrates up due to the capillary action. Cellulose layers in the paper binds with the aqueous component of the solvent and act as stationary phase, while organic component being non polar, migrates and forms the mobile phase.

Migration rate of pigments being separated depends on their relative solubility in the stationary phase (polar) and mobile phase (non polar) i.e. according to its partition coefficient (K_p).

Kp = concentration of molecule in stationary phase / concentration in mobile phase

As a result, non polar molecules move faster than polar molecules.

Each analyte has a characteristic R_f value (Retention factor) for a given solvent and paper type. R_f value is a unitless number and is calculated using the formula-

Rf = distance traveled by the substance / distance traveled by solvent.

If R_f = 0 ; solute is immobile and remains in stationary phase.

If R_f = 1 ; solute has no affinity for stationary phase and migrates with the mobile phase.