Langmuir adsorption, Application of adsorption - Physical Pharmacy Important Questions

Important Questions:

1. What are the assumption of Langmuir adsorption. Give its derivation.
2. Write BET equation.
3. Explain adsorption applications under following heading: A) Toxicity B) Purification C) Poisoning

Solution And Explainations:

1. What are the assumption of Langmuir adsorption. Give its derivation.

Assumptions in Langmuir Adsorption:

1. The surface of solid posses fixed number of active sites for the  adsoption of gases.
2. At maximum adsorption, the gas layer that is found around the solid is of only one molecule thick.
3. The rate of adsorption (condensation) is proportion to number of sites unoccupied.
4. The rate of evaporation (desorption) is proportional to the number of occupied sites.

The adsorption of a gas on the solid surface depends on the pressure on the gas in the experimental conditions. At a particular pressure, P

Fraction of sites occupied: θ

Fraction of sites unoccupied: (1-θ)
Rate of adsorption: r1 = k1/(1-θ)P
Rate of desorption: r2 = k2θ
where k1 and k2 are the proportionality constants for the processes of adsorption and desorption, respectively.
At equilibrium: r1 = r2


in which y is the mass of gas adsorbed per gram of adsorbent and y_m  is the mass of gas that 1 gram of adsorbent can take up when a monolayer is complete. Substituting these terms in previous equation gives

The Equation represents Langmuir isotherm. Inverting previous equation and multiplying by 'P', gives

  Now, plot of P/y against P gives a straight line and 'y_m' and 'b' can be obtained from the slope and intercept. The value of 'y_m' is used to estimate specific surface of the solids, considering that adsorption is monomolecular layer thickness. As proposed in the assumptions, Langmuir adsorption isotherm is applicable until the monomolecular layer is formed. The agreement of experimental data and theory is poor at low pressures and when pressure approach saturation. This has been attributed to the formation of multilayer adsorption.

2. Write BET equation.

3. Explain adsorption applications under following heading: A) Toxicity B) Purification C) Poisoning

• Surface are determination
• Adsorption Chromatography
• Antidote in poisonning
• Decolorizing/Purification
• Reduced adsorption
• Desiccant / Drying agents
• Medical uses
• Taste masking
• Purification & reduced drug roxicity
• Reduced drug content
• Separation of substances in mixture