Determination of Freundlich and Langmuir Constants using Activated charcoal

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BACKGROUND

Adsorption is a surface phenomenon. In adsorption process the adsorbate is adhered to the surface of adsorbent.¹

Freundlich studied adsorption of gas on solid and from the experimental data gave the equation

y = x/m = kP 1/b

where y= ‘x/m’, the amount of adsorbate adsorbed by mass ‘m’ gm of adsorbent at equilibrium pressure ‘P’ and is determined from the experiment at constant temperature.

K and b are constants indicating adsorption affinity and capacity and is dependent on the nature of the adsorbent and adsorbate and on temperature.

Langmuir’s adsorption isotherm

Assumptions:

1.    There are active sites on the solid surface at which adsorption takes place.

2.    The adsorption is monlayer adsorption and not multilayer adsorption.

3.    There is existence of a dynamic equilibrium between adsorbed molecules and free molecules. If adsorption by a solid is from solution, Langmiur adsorption isotherm is given by

Ce = 1+ Ce

(x/m) b.ym ym

Where

where y= ‘x/m’, the amount of adsorbate adsorbed by mass ‘m’ gm of adsorbent at equilibrium concentration ‘C’ and is determined from the experiment at constant temperature.

ym = monolayer capacity = mass of adsorbate adsorbed per gm of adsorbent when a monolayer is complete (on the surface of adsorbent)

From ym and the area of one molecule of adsorbate, the surface area of the adsorbent may be estimated.

The specific surface area of charcoal is given by S = ym. N. A

                                                                  M

Where N = Avogadro number = 6 x 10²³/mole

M = Molecular weight of acetic acid = 60

A = Cross sectional area of acetic acid molecule = 16 x 10-16 cm²

Aim: To find out the monolayer capacity and specific surface area (area per gram) of given powder (activated charcoal) by adsorption method.

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REQUIREMENTS

Chemicals:    Distilled water

                Activated charcoal

                0.5N Acetic acid

                0.2N Sodium hydroxide

                Phenolphthalein

Apparatus:      Reagent bottles

Beakers

Burette

Pipette

Stand

Conical flask

Funnels

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PROCEDURE

Determine the exact normality (N0) of given acetic acid solution by titrating 10ml against 0.2N NaOH solution using phenolphthalein indicator. (End point: Colourless to pink). Prepare following mixtures of acetic acid and distilled water in five separate dry bottles and keep them in a water bath at room temperature.

Add 1gm of the activated charcoal to each of the five different dry bottles and keep them in a water bath at room temperature. Swirl the bottles for 40 mins to attain adsorption equilibrium. Filter the solutions using dry filter paper into five different dry flasks and determine the effective normality of each filtered solution (N₂) by titrating 10ml against standard 0.2N NaOH solution, using phenolphthalein indicator. (End point : Colourless to pink)

Observations

Water bath temperature = ^◦C

Mass of activated charcoal (m) = 1g

10ml of given acetic acid = V ml of 0.2N NaOH

Exact normality of acetic acid (N₀) = 0.2.V

N₁ = Initial normality= Initial concentration in gm equiv./litre; C_e = Equilibrium concentration of acetic acid = N₂. M gm/l

Where M = Molecular weight of acetic acid = 60

x = Amount of acetic acid adsorbed per 50ml of the solution

= (N₁ – N₂) x E x 50

1000

= (N₁ – N₂) x 60 x 50

1000

       = 3000(N₁ – N₂)

      1000

       = 3(N₁ – N₂)

Where N₂ = Normality of the solution after adsorption (after filtration)

E = Equivalent weight of acetic acid (adsorbate)

m = Mass of activated charcoal (adsorbent) in gm

(x/m) = amount of acetic acid in gm. adsorbed per gm. of adsorbent

Graph and calculation

At the adsorption equilibrium,

C_e = 1 + C_e

(x/m)     b.y_m y_m
Where y_m    = monolayer capacity

= mass of adsorbate adsorbed per gm of adsorbent to form a monolayer (on the surface of adsorbent)

Plot C_e vs C_e ; the slope of the line is = 1/ y_m

     (x/m)

y_m = 1/(slope)

The specific surface area of charcoal is given by S = y_m. N. A

                        M

Where N = Avogadro number = 6 x 10²³/mole

M = Molecular weight of acetic acid = 60

A = Cross sectional area of acetic acid molecule = 18 x 10^-16 cm²

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CONCLUSION

Monolayer capacity of activated charcoal was found to be =________
Specific surface area of charcoal was found to be =_________ cm²/g or  ___________m²/g

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REFERENCES

1. More HN, Hajare AA. Practical Physical Pharmacy, Career publications; 2010: 57-59.