Adsorption from Solution Phase

Solids adsorb not only gases but also solutes from solution.

• Shaking a solution of acetic acid in water with charcoal removes part of the acid, lowering its concentration.
• Litmus solution shaken with charcoal turns colourless.
• A precipitate of $Mg(OH)_2$ formed in the presence of magneson reagent turns blue because it adsorbs the magneson dye.

Factors governing adsorption from solution

• Extent of adsorption decreases as temperature rises.
• Extent of adsorption increases with the surface area of the adsorbent.
• It depends on the concentration of the solute.
• It depends on the nature of both adsorbent and adsorbate.

Freundlich isotherm for solutions

The precise mechanism is not fully known, but Freundlich's relation describes the behaviour well if concentration replaces pressure:

where $C$ is the equilibrium concentration once adsorption is complete. Taking logarithms:

$\log \frac{x}{m} = \log k + \frac{1}{n}\log C \qquad(5.4)$ …