Question

What is the Freundlich adsorption isotherm? Suggest a method to evaluate the constants k and n present in the equation involved. What form does this isotherm assume when applied to adsorption of solute from solutions?

Answer 1

The Freundlich adsorption isotherm is an empirical relationship that describes the adsorption of a gas on the surface of a solid at a constant temperature. It is expressed by the equation:

`x/m = kP^(1//n)`

Where:

`x/m`​ is the amount of gas adsorbed per unit mass of the adsorbent.

P is the equilibrium pressure of the gas.

k and n are constants that depend on the adsorbent, gas, and temperature.

Taking logarithms of both sides:

`log(x/m) = log k + 1/n log P`

This is the equation of a straight line of the form y = mx + c.

Method to evaluate constants k and n:

To determine the values of k and n, the following steps are taken:

Experimental values of `x/m`​ are obtained for different pressures P.

A graph is plotted between `log⁡(x/m)` and log ⁡P.

The resulting straight line has:

Slope = `1/n`

Intercept = log⁡ k

On the graph, values of k and n can be calculated.

Application to adsorption from solutions:

When applied to the adsorption of solutes from solutions, the Freundlich isotherm takes the form:

`x/m = kP^(1//n)`

Where C is the equilibrium concentration of the solute in the solution.

Taking logarithms:

`log(x/m) = log k + 1/n log C`

Again, plotting `log⁡(x/m)` versus log ⁡C gives a straight line. The constants k and n can be evaluated from the slope and intercept of this graph.