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In this laboratory exercise, we will generate a Langmuir Isotherm for the adsorption of Acetic. Acid on the surface of Norit A Activated Charcoal. The surface area. 3. The Langmuir Isotherm. SUB-MENU. Introduction · Derivation – from Equilibrium Considerations · Derivation – from Kinetic Considerations. The equilibrium sorption data were fitted into Langmuir,. Freundlich, Temkin and Dubinin–Radushkevich (DRK) isotherms. Of the four adsorption isotherm, the.

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Whenever a gas is in contact with a solid there will be an equilibrium established between the molecules in the gasphase and the corresponding adsorbed species molecules or atoms which are bound to the surface of the solid.

The Langmuir isotherm was developed by Irving Langmuir in to describe the dependence of the surface coverage of an adsorbed gas on the pressure of the gas above the surface at a fixed temperature.

There are many other types of isotherm Temkin, Freundlich Whilst the Langmuir isotherme is one of the simples, it still provides a useful insight into the pressure dependence of the extent of surface adsorption.


The basic idea behind the Langmuir model is the coverage of the surface by a monomolecularic layer. The model approach also assumes that only one gas is being adsorbed.

Thus, at constant temperature a part of the surface will be covered with isptherme adsorbed molecule another part not. Between the free gas and the adsorbed gas a dynamic equilibrium will exist. Per time unit there will be as much molecules adsorbing as there will be desorbing.

The rate of adsorption will be proportional with the equilibrium pressure of the gas and the free surface. Total number of adsorption sites: Applying the ideal gas law and assuming a Gas-Solid system If we assume that the energy and the entropy of the lattice do not change significantly during adsorption than we can state that when a molecule is adsorbed the entropy will decrease, deltaS will be negative and therefore deltaH will be negative.

The magnitude of adsorption ethalpy reflects the strengthn of binding of the adsorbate to the substrate. With physical adsorption physisorption there is only an increase in surface concentration during adsorption. Only van der Waals forces play a role. A chemical bond is formed between the molecule and the surface.



Not only the heat of adsorption but also the the activation energy of the adsorption process is higher with chemisorption than with physisorption.

As a consequence physisorption is prelavent at lower temperatures and chemisorption at higher temperatures.

Load factor or coverage or sorption capacity 3: Combining 3 and 4 5. Applying the langmir gas law and assuming a Gas-Solid system 4: The surface will be completely covered.