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Adsorption: Isotherms

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Adsorption: Isotherms - Lesson Summary

Adsorption is basically an "equilibrium process".

To understand the equilibrium process, consider an adsorbent and a gaseous adsorbate in a closed vessel, placed at a particular pressure P.

It is observed that due to adsorption, the pressure of the gas decreases initially, and becomes constant after some time. This indicates that a state of equilibrium has been attained.

Once equilibrium is reached, the rate of adsorption becomes equal to the rate of desorption.

The extent of adsorption = x/m

Where 'x' is the mass of the gas adsorbed and 'm' is the mass of the adsorbent.

The variation in the amount of gas adsorbed by the adsorbent with variation in the pressure, at constant temperature, can be expressed by means of a curve. This curve, at constant temperature, is called the "adsorption isotherm

Adsorption isotherm:

At constant temperature, a graph between the amount of the gas adsorbed per gram of the adsorbent and the equilibrium pressure of the adsorbate is called the adsorption isotherm.

Freundlich, in 1909, was the first to propose a mathematical relation for an adsorption isotherm.

He gave an empirical relationship between the quantity of gas adsorbed by a unit mass of a solid adsorbent and pressure at a particular temperature.

The relation can be expressed as: x/m = K P1/n

Where x/m is the amount of gas adsorbed by a unit mass of the adsorbent

n and K are constants, and P is the pressure.

This relationship is generally represented in the form of a curve, and is known as the Freundlich adsorption isotherm.

Freundlich adsorption isotherm: A curve where a mass of gas adsorbed per gram of the mass of the adsorbent, which is plotted against pressure.

In this adsorption isotherm, x/m reaches its maximum value at point 'P s.' In other words, the extent of adsorption is the highest at this point.

From the curve even if the pressure is increased beyond 'P s', the extent of adsorption remains the same. This is called the 'saturation state,' and 'P s' is called the 'saturation pressure.'

At low pressure x/m α p1

At a very high pressure is x/m independent of the value of 'P'.

This is represented as x/m α P0

In the intermediate range of pressure x/m depends upon P1/n

Where 'n' is a positive integer.

At a particular temperature, 'n' and 'k' depend upon the nature of the adsorbate and the adsorbent.

Now, when 1/n = 0

x/m = KP0

x/m = K

It is in this part of the curve that adsorption is independent of pressure.

On the other hand, when

1/n= 1

x/m = KP

x/m α P

In other words, adsorption varies directly with pressure in this part of the isotherm.


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