Chemical Sciences: A Manual for CSIR-UGC National Eligibility Test for Lectureship and JRF/Freundlich equation

The Freundlich equation or Freundlich adsorption isotherm is an adsorption isotherm, which is a curve relating the concentration of a solute on the surface of an adsorbent, to the concentration of the solute in the liquid with which it is in contact. There are basically two well established types of adsorption isotherm: the Freundlich adsorption isotherm and the Langmuir adsorption isotherm.

Freundlich adsorption isotherm

Example of the Freundlich isotherm, showing the amount adsorbed, q (e.g., in mol/kg), as a function of equilibrium concentration in the solution, c (e.g., in mol/L). The graph is for the values of the constants of K=4 and 1/n=0.6.

The Freundlich Adsorption Isotherm is mathematically expressed as

$\ x/m=Kp^{1/n}$

or

$\ x/m=Kc^{1/n}$

where

x = mass of adsorbate

m = mass of adsorbent

p = Equilibrium pressure of adsorbate

c = Equilibrium concentration of adsorbate in solution.

K and 1/n are constants for a given adsorbate and adsorbent at a particular temperature.