The experimental isotherms always seem to approach saturation at high pressure.Why this cannot be explained by Freundlich isotherm?

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Please find the solution to the asked query:

 An isotherm is a graph that is plotted between two variables keeping the temperature constant. Adsorption isotherm is a graphical representation of how the amount of gas adsorbed by the adsorbent varies with pressure at constant temperature. So when the variation of extent of adsorption with pressure is studied and the experimental data obtained is plotted, it is found that at high pressures, the amount of gas that can be adsorbed by the adsorbent becomes constant. That is, at high pressure, the extent of adsorption will not show any variation will pressure. In other word, it will become independent of pressure. This is why we obtain the following graph :



 

As seen in the figure, the curve becomes parallel to the x-axis at higher pressure. This suggests that the extent of adsorption becomes constant at higher pressure.This will only happen when all the sites of the adsorbent have been occupied by the gas, or the adsorbent sites have become saturated and cannot hold any more gas. So no matter how much pressure is increased, it will not affect the extent of adsorption as all sites are fully occupied. 

Extent of adsorption varies directly with pressure of the gas until the point of saturation is reached, where pressure of the gas becomes saturation pressure, as determined experimentally. Further increase in the pressure of the gas has no effect on the extent of adsorption, thus Freundlich adsorption isotherm fails at high pressures.


 
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