Adsorption Isotherms

The extent of adsorption is measured in terms of the quantity x/m where, x is the mass of the gas (adsorbate) adsorbed at equilibrium on mass m of the adsorbent. The graph showing variation in x/m with pressure (p) at a constant temperature is called adsorption isotherm.

Adsorption of Gases

The extent of adsorption of a gas on a solid increases with the increase in the pressure of the gas, p at three different constant temperatures. The curves also show that the extent of adsorption, decreases at a fixed pressure as the temperature is increased (dotted line).

Adsorption from Solutions

Adsorption occurs from solutions also. The solute gets adsorbed on the surface of a solid adsorbent. Charcoal, a good adsorbent, is often used to adsorb acetic acid, oxalic acid and organic dyestuffs from their aqueous solutions.

The extent of adsorption, x/m depends upon the concentration c of the solute.

Freundlich isotherm is applicable to adsorption from solutions when concentration is used in place of pressure.