# Freundlich equation

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## Overview

The 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.

The Freundlich Adsorption Isotherm is mathematically expressed as

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

or

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

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.

There are basically two well established types of adsorption isotherm:

• the Langmuir adsorption isotherm (see Langmuir equation)
• the Freundlich adsorption isotherm

The Langmuir adsorption isotherm describes quantitatively the build up of a layer of molecules on an adsorbent surface as a function of the concentration of the adsorbed material in the liquid in which it is in contact. In a modified form it can also describe a bi-layer deposition. The shape of the isotherm (assuming the (x) axis represents the concentration of adsorbing material in the contacting liquid) is a gradual positive curve that flattens to a constant value. It often represents an initial surface adsorption followed by a condensation effect resulting from extremely strong solute-solute interaction. In chromatography the Freundlich isotherm is not common, most adsorption processes are best described by the Langmuir isotherm.