Freundlich Adsorption Isotherm

What is Freundlich Adsorption Isotherm?

In 1909, Freundlich gave an empirical expression representing the isothermal variation of Adsorption of a quantity of gas adsorbed by unit mass of solid adsorbent with pressure. This equation is known as Freundlich Adsorption Isotherm or Freundlich Adsorption equation.

Freundlich Adsorption Equation

Where x is the mass of the gas adsorbed on mass m of the adsorbent at pressure p and k, n are constants whose values depend upon adsorbent and gas at particular temperature.

Explanation of Freundlich Adsorption equation

At low pressure, extent of adsorption is directly proportional to pressure (raised to power one).

Adsorption is directly proportional to pressure at small values

At high pressure, extent of adsorption is independent of pressure (raised to power zero).

Adsorption is independent of pressure at large values

Therefore at intermediate value of pressure, adsorption is directly proportional to pressure raised to power 1/n .Here n is a variable whose value is greater than one.

Aadsorption is directly proportional to pressure raised to power 1/n

Using constant of proportionality, k, also known as adsorption constant we get

Freundlich Adsorption Equation

The above equation is known as Freundlich adsorption equation.

Plotting of Freundlich Adsorption Isotherm

As per Freundlich adsorption equation

Freundlich Adsorption Equation

Taking log both sides of equation, we get,

Freundlich Adsorption equation in Logarithmic form

The equation above equation is comparable with comparable with equation of straight line, y = m x + c where, m represents slope of the line and c represents intercept on y axis.

Plotting a graph between log(x/m) and log p, we will get a straight line with value of slope equal to 1/n and log k as y-axis intercept.

log(x/m) vs. log p graph

log(x/m) vs. log p graph

Limitation of Freundlich Adsorption Isotherm

Experimentally it was determined that extent of adsorption varies directly with pressure till saturation pressure Ps is reached. Beyond that point rate of adsorption saturates even after applying higher pressure. Thus Freundlich Adsorption Isotherm failed at higher pressure.