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.
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).
At high pressure, extent of adsorption is independent of pressure (raised to power zero).
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.
Using constant of proportionality, k, also known as adsorption constant we get
The above equation is known as Freundlich adsorption equation.
Plotting of Freundlich Adsorption Isotherm
As per Freundlich adsorption equation
Taking log both sides of equation, we get,
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
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.