Question

Explain adsorption from solution phase.Write Freundlich’s equation related to it.

Answer 1

When we talk about the Freundlich's equation in solution phase, it's important for us to remember that we are not referring to the pressure of the adsorbent,but rather focusing on the concentration of the solution

Complete step by step:
Let us first understand the definition of adsorption: the tendency of accumulation of molecular species at the surface of the molecule rather than in the bulk of a solid or liquid is known as adsorption.
- The molecular species which accumulates on the surface is called adsorbate
- And the material on whose surface this process of adsorption takes place is known as adsorbent.
- A common example is the water solution of raw sugar which when passed over animal charcoal beds, becomes colourless due to the result of adsorption of the colour by the animal charcoal.
Here,we will try to focus mainly on adsorption in the solution phase.
- We are aware that solids can absorb solutes from a given solution.
- So by taking an example of acetic acid solution, which is taken in water and shaken with charcoal, a part of the acid is adsorbed by the charcoal and concentration of the given acid starts decreasing in the solution.When we check this by litmus paper in charcoal, it becomes colourless.
- Another example is of $\text{Mg}{{\left( \text{OH} \right)}_{\text{2}}}$. It's precipitate attains a blue colour when precipitated in presence of magneson.The colour obtained here is due to the adsorption phenomenon shown Magneson.
Few observations that we find in the case of adsorption from solution phase are stated below:
- The extent of adsorption shows a decrease with increase in temperature.
- The extent of adsorption shows an increase with increase of surface area of adsorbent.
- The extent of absorption depends on the concentration of the solute.
- The extent of adsorption also depends on the nature of adsorbent and adsorbate.
Let's now briefly understand the Freundlich's adsorption isotherm in solution phase:
- This isotherm shows us the variation in the amount of gas adsorbed by the adsorbent with pressure at a constant temperature, by plotting a curve.
The relationship is expressed by: $\dfrac{\text{x}}{\text{m}}\text{=k}\text{.}{{\text{P}}^{{}^{\text{1}}/{}_{\text{n}}}}$( n is always greater than 1)$\text{log}\dfrac{\text{x}}{\text{m}}\text{=logK+}\dfrac{\text{1}}{\text{n}}\text{logC}$ $$$$
- where x is the mass of gas adsorbed on mass m
- P is the pressure
- k and n are constants
- This is the normal Freundlich's adsorption isotherm.
Now,while describing the adsorption in the solution phase,the behaviour slightly differs because instead of pressure, we take the concentration of the solution for expressing the relationship.
The equation is: $\dfrac{\text{x}}{\text{m}}\text{=k}{{\text{C}}^{\dfrac{\text{1}}{\text{n}}}}$
- Here C refers to the equilibrium concentration,when the adsorption is completed,
- On taking logarithm on both sides,we get:
- $\text{log}\dfrac{\text{x}}{\text{m}}\text{=logK+}\dfrac{\text{1}}{\text{n}}\text{logC}$
Now when we try to plot it ,we find a straight line between log x/m against log C which confirms the validity of Freundlich's isotherm.

Note: The precise mechanism of adsorption in a solution phase is not actually known,but we can test the validity of Freundlich's isotherm experimentally by taking solutions of different concentrations of acetic acid.Taking equal volumes of it's solutions and adding it to charcoal ,we can compare it's final and initial concentration,and thus find the value of x, thus establishing the Freundlich's isotherm equation.