Describe Freundlich 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, x/m 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 x/m i.e., `x/m alpha P`
`x/m = kP` (k is constant)
Case II- At high pressure:
When pressure exceeds the saturated pressure, x/m becomes independent of P values
`x/m alpha P^@`
`x/m = kP^@`
Case III- At intermediate pressure:
At intermediate pressure, x/m depends on P raised to the powers between 0 and 1. This relationship is known as the Freundlich adsorption isotherm.
`x/m alpha P^(1/n)`
`x/m = kP^(1/n)` n > 1
Now taking log: `log x/m =log k + 1/n log P`
On plotting the graph between log (x/m) and log P, a straight line is obtained with the slope equal to 1/n and the intercept equal to log k
Freundlich Adsorption isotherm:
Freundlich, in 1909, gave an empirical relationship between the quantity, of gas adsorbed by unit mass of solid adsorbent and pressure at a particular temperature. The relationship can be expressed by the following equation:
where x is the mass of the gas adsorbed by mass ‘m’ of the adsorbent at pressure P, k and n are constants which depend on the nature of the adsorbent and the gas at a particular temperature. The relationship is generally represented in the form of a curve where mass of the gas adsorbed per gram by the adsorbent is plotted against pressure. These curves indicate that at a fixed pressure, there is a decrease in physical adsorption with increase in temperature. These curves always seem to approach saturation at high pressure.
Taking log of equation (i), we get
`log x/m =log k + 1/n log P`