What is an adsorption isotherm? Describe Freundlich adsorption isotherm.
The plot between the extent of adsorption against the pressure of gas (P) at constant temperature (T) is called the adsorption isotherm.
Freundlich adsorption isotherm:
Freundlich adsorption isotherm gives an empirical relationship between the quantity of gas adsorbed by the unit mass of solid adsorbent and pressure at a specific temperature.
From the given plot it is clear that at pressure PS, reaches the maximum valve. Ps is called the saturation pressure. Three cases arise from the graph now.
Case I- At low pressure:
The plot is straight and sloping, indicating that the pressure in directly proportional to i.e.,
Case II- At high pressure:
When pressure exceeds the saturated pressure, becomes independent of P values.
Case III- At intermediate pressure:
At intermediate pressure, depends on P raised to the powers between 0 and 1. This relationship is known as the Freundlich adsorption isotherm.
On plotting the graph between log and log P, a straight line is obtained with the slope equal to and the intercept equal to log k.