Adsorption isotherm is a graph between the amount of the gas adsorbed by an adsorbent and equilibrium pressure of the adsorbate at constant temperature.
Freundlich obtained an empirical relationship between the quantity of gas adsorbed by unit massof solid adsorbent and pressure at a particular temperature. It is mathematically represented in the following way :

x

m

=kP

1

n

where k and n are constants
is pressure,
m is mass of the adsorbent,
x is mass of the adsorbate.
Taking log on both sides
log‌

x

m

=log‌k+

1

n

‌logP
A graph between

x

m

and log P is a straight line with slope equal to

1

n

and ordinate intercept equal to log k