FREUNDLICH’S ADSORPTION ISOTHERM
The relationship between the magnitude of adsorption (x/m) and p can be expressed mathematically by an empirical equation known as freundlich adsorption isotherm.
x = KP1/xm
x – amount of gas adsorbed
m – mob of adsorbent
P – equilibrium pressure
t,n – constants (depends on nature of adsorbate, adsorbent and T)
Derivations of Freundlich's adsorption isotherm
This equation may be derived from the result observed from the graph .
i) At low pressure adsorption increasesX x p
ii) At high pressure adsorption is almost constant X = constant
iii) At intermediate (normal) pressure adsorption depends on 0 to 1 power of pressure.
X x p 1/n (or) X = KP1/n m
where, n = whole number.
The above equation is called freundlich’s adsorption is other.
Testing of Freundlich adsorption isotherm
Taking log on both sides, the equation
x = KP 1/n becomes
log x = log k + 1 log p m/ n
on plotting log x/m Vs log p, a straight line is obtained with a scope of 1/n and intercepts log k.
Disadvantages or limitation of Freundlich's adsorption isotherm
1. Freundlich equation is purely empirical and has no theoretical basis.
2. The equation is valid only upto a certain pressure and invalid at higher pressure.
3. The constants K and n are not temperature independent, they vary with temperature.
4. Frendilich’s adsorption isotherm fails when the concentration of the adsorbate is very high.