adsorbate) at constant temperature is called adsorption isotherms. In order to explain these isotherms various equations were suggested as follows: XII U10-Surface XII U10-Surface - - - - (i) Freundlich adsorption isotherm. According to Freundlich, x m = kp n 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 =K c 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 =Kp n log x m = logK + n log p Plot of log x/m against log p for the adsorption of a gason a solid log p log ( x/m ) slope = n Freundhich adsorption isothermi plot of x/m against p x/m x p m x p m x p /n p m Figure . log x m vs log p graph Hence the intercept represents the value of log k and the slope gives n . This equation explains the increase of x m with increase in pressure. But experimental values show the deviation at low pressure.
Limitations 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. XII U10-Surface XII U10-Surface - - - - .
. Applications of adsorption Though we have innumerable applications for adsorption, we consider few of them . Gas masks: During world war I charcoal gas mask was employed by both the British and American. Activated charcoal was found to be one of the best adsorbents.
. To create high vacuum in vessels, Tail and Dewar used activated charcoal.For dehydration and also purification of