Adsorption isotherms represents the variation of adsorption at constant temperature.
When amount of adsorption is plotted versus temperature at constant pressure it is called adsorption isobar.
Adsorption isobars of physisorption and chemisorption are different as represented in the graphs.
In physical adsorption, x/m decreases with increase in Temprature, But in chemical adsorption, x/m increases with rise in temperature and then decreases. The increase illustrates the requirement of activation of the surface for adsorption is due to fact that formation of activated complex requires certain energy.
The decrease at high temperature is due to desorption, as the kinetic energy of the adsorbate increases.
Adsorption isotherm can be studied quantitatively. A plot between the amount of adsorbate adsorbed and pressure (or concentration of adsorbate) at constant temperature is called adsorption isotherms.
In order to explain these isotherms various equations were suggested as follows:
(i) Freundlich adsorption isotherm.
According to Freundlich,
where x is the amount of adsorbate, adsorbed on ‘m’ gm of adsorbent at a pressure of p. K and n are constant introduced by freundlich.
Value n is always less than unity.
This equation is applicable for adsorption of gases on solid surfaces. The same equation becomes x/m = Kc1/n , when used for adsorption in solutions with c as concentration.
This equation quantitively predicts the effect of pressure(or concentration) on the adsorption of gases(or adsorbates) at constant temperature.
Taking log on both sides of equation x/m =Kp1/n
log x/m = logK + 1/n log p
Hence the intercept represents the value of log k and the slope gives 1/ n.
This equation explains the increase of x/m with increase in pressure. But experimental values show the deviation at low pressure.
This equation is purely empirical and valid over a limited pressure range.
The values of constants k and n also found vary with temperature. No theoretical explanations were given.