What is adsorption isotherm
An adsorption isotherm is a graph that represents the variation in the amount of adsorbate(x) adsorbed on the surface of the adsorbent with the change in pressure at a constant temperature.
As we know from Le Chatelier’s principle, the direction of equilibrium in a reaction shifts in the direction in which stress is relieved. So, here we can see that upon application of excess pressure on the system, the equilibrium shifts in the direction where the number of molecules decreases so that the pressure in the system decreases.
From the graph, we also observe that after attaining a pressure Ps, that is the saturation pressure, the variation in the amount of adsorbent adhering to the adsorbate becomes zero. This happens because the surface area available for adsorption is limited and as all the sites are occupied, a further increase in pressure does not cause any difference.
Different adsorption isotherms have been proposed by different scientists namely,
- Langmuir isotherm
- Freundlich isotherm
- BET theory.
The graph shown above shows the isotherm proposed by Freundlich.
Freundlich adsorption isotherm
Freundlich adsorption gives the variation in the quantity of gas adsorbed by a unit mass of solid adsorbent with the change in pressure of the system for a given temperature. The expression for the Freundlich isotherm can be represented by the following equation:
, where n>1
Where x is the mass of the gas adsorbed, m is the mass of the adsorbent, P is the pressure and n is a constant which depends upon the nature of adsorbent and the gas at a given temperature. Taking the logarithm on both the sides of the equation, we get,
The plot of this equation is a straight line as represented by the following curve.
Applications of adsorption
Following are the applications of adsorption:
- Gas masks: Poisonous gases get adsorbed to at the surface of the mask and prevent its encounter when used by coal miners.
- Production of vacuum: Traces of air are adsorbed on charcoal and removed from devices undergoing the process of evacuation.
- Removal of moisture: Silica gel pellets are used for the adsorption of moisture in medicines and new plastic bottles in order to control humidity.
- Removal of color: The juice extracted from cane is treated with animal charcoal for the removal of the coloring agent in order to get a clear liquid solution.
- As Catalysts: Suitable materials are used as a catalyst such that reactants get adhered to its surface, thus enabling the reaction to proceed at a faster rate and increasing the rate of reaction.
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