Adsorption Isotherms

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. x/m  is the mass of the adsorbate adsorbed per unit mass of the adsorbent. The graph showing variation in x/m with pressure(p) at a constant temperature is called adsorption isotherm. Let us see the variation in extent of adsorption in case of gases and of solutes from their solutions.

(i) Adsorption of Gases

The adsorption isotherm of a gas which is adsorbed on a solid is shown in Fig. 17.5. It shows that 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 (see the dotted line).

Freundlich Adsorption Isotherm

Freundlich gave an empirical mathematical relationship between the extent of adsorption (x/m) and the equilibrium pressure (p) of the gas as :

In this relation k is a constant at a given temperature and depends upon the nature of the adsorbate and adsorbent. The value of n changes with pressure. It is 1 at low pressures and increases with pressure. The relationship is valid at a constant temperature. Therefore, it is called Freundlich Adsorption Isotherm. On taking logarithm of the above equation, we get

This is an equation of a straight line and a plot of log x/m against log p should be a straight line with slope 1/n as depicted in Fig. 17.6. In actual practice, a straight line is obtained provided the data at very low and very high pressures is neglected.