On the basis of experimental studies, Freundlich gave the following relationship between the extent of adsorption (x/m) and Pressure (P).

At low pressure- The graph is nearly straight and sloping indicating that ‘x/m‘ is directly proportional to pressure, i.e.

x/m ∝ P or x/m = kP, where ‘k’ is constant of proportionality

At high pressure i.e. pressure beyond saturation pressure (Ps)-x/m‘ becomes independent of pressure i.e.

x/m = constant or x/m ∝ P0 or x/m = kP0

For intermediate ranges of Pressure- The term ‘x/m‘ is proportional to the gas pressure ‘P‘ raised to some power lying between one and zero.

x/m ∝ P1/n or x/m = kP1/n, n>1 and ‘k’ is constant of Proportionality and their values depend upon nature of adsorbate and adsorbent. This relation represents Freundlich adsorption isotherm. The constants ‘n‘ and ‘k‘ can be determined as follows-

Taking logarithm of the above equation,we get-

log x/m = log (kP1/n) or log x/m = log k + 1/n log P

This is an equation of a straight line and the graph of log x/m against log P will give a straight line as shown in the figure. The intercept ‘log k’ is represented along ‘log x/m’ axis and the slope of the line is represented by ‘1/n’.