Luy ingga I. Waktu Percobaan : Rabu, 04 Desember pukul Tujuan Percobaan :Menentukan isotherm adsorpsi menurut Freundlich pada proses adsorpsi asam oleh karbon aktif IV. Dasar Teori Adsorpsi adalah gejala pengumpulan molekul-molekul suatu zat pada permukaan zat lain, sebagai akibat dari ketidakjenuhan gaya-gaya pada permukaaan zat tersebut.

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Chemistry Langmuir Adsorption Isotherm Langmuir Equation which depicted a relationship between the number of active sites of the surface undergoing adsorption and pressure What is Langmuir Adsorption Isotherm? In , Irving Langmuir proposed another Adsorption Isotherm which explained the variation of Adsorption with pressure. Based on his theory, he derived Langmuir Equation which depicted a relationship between the number of active sites of the surface undergoing adsorption and pressure.

Assumptions of Langmuir Isotherm Langmuir proposed his theory by making following assumptions. Fixed number of vacant or adsorption sites are available on the surface of solid. All the vacant sites are of equal size and shape on the surface of adsorbent. Each site can hold maximum of one gaseous molecule and a constant amount of heat energy is released during this process. Dynamic equilibrium exists between adsorbed gaseous molecules and the free gaseous molecules.

Where A g is unadsorbed gaseous molecule, B s is unoccupied metal surface and AB is Adsorbed gaseous molecule. Adsorption is monolayer or unilayer. Derivations of the Langmuir Adsorption Equation Calculation of Equilibrium Constant Langmuir proposed that dynamic equilibrium exists between adsorbed gaseous molecules and the free gaseous molecules. Using the equilibrium equation, equilibrium constant can be calculated. Where Ka represents equilibrium constant for forward reaction and Kd represents equilibrium constant for backward direction.

Derivation Langmuir Equation which depicts a relationship between the number of active sites of the surface undergoing adsorption i. Therefore rate of forward reaction is directly proportional to both mentioned factors. Similarly, Rate of backward reaction or Rate of Desorption depends upon number of sites occupied by the gaseous molecules on the surface of adsorbent. At equilibrium, rate of adsorption is equal to rate of desorption.

Alternate form of Langmuir Adsorption Equation Langmuir adsorption equation can be written in an alternate form in terms of volume of gas adsorbed. Let V be volume of gas adsorbed under given sets of conditions of temperature and pressure and Vmono be the adsorbed volume of gas at high pressure conditions so as to cover the surface with a unilayer of gaseous molecules.

This condition can be fulfilled at low pressure conditions only. Thus Langmuir Equation is valid under low pressure only. Langmuir Equation assumes that adsorption is monolayer. But, monolayer formation is possible only under low pressure condition.

Under high pressure condition the assumption breaks down as gas molecules attract more and more molecules towards each other. BET theory proposed by Brunauer, Emmett and Teller explained more realistic multilayer adsorption process. Another assumption was that all the sites on the solid surface are equal in size and shape and have equal affinity for adsorbate molecules i.

But we all know that in real solid surfaces are heterogeneous. Langmuir Equation assumed that molecules do not interact with each other. This is impossible as weak force of attraction exists even between molecules of same type. The adsorbed molecules has to be localized i. This is not possible because on adsorption liquefaction of gases taking place, which results into decrease in randomness but the value is not zero.

From above facts we can conclude that, Langmuir equation is valid under low pressure conditions. Therefore, The above equation shows linear variation between extent of adsorption of gas and pressure.

The reaction at this stage becomes zero order Combining the results of equation 4 and 5 , we can conclude that Equation 3 is in agreement with Freundlich adsorption equation. We can say that Freundlich adsorption equation is a special case of Langmuir equation.

Apr 4,


Langmuir Adsorption Isotherm

Perbedaan isoterm ini dengan Langmuir adalah BET berasumsi bahwa molekul-molekul adsorbat bisa membentuk lebih dari satu lapisan adsorbat di permukaannya. Pada isoterm ini, mekanisme adsoprsi untuk setiap proses adsorpsi berbeda-beda. Mekanisme yang diajukan dalam isoterm ini adalah: Isoterm Langmuir biasanya lebih baik apabila diterapkan untuk adsorpsi kimia, sedangkan isoterm BET akan lebih baik daripada isotherm Langmuir bila diterapkan untuk adsoprsi fisik. Perbedaan Adsorpsi kimia dengan adsorpsi fisika 5. Faktor-Faktor yang mempengaruhi adsorpsi a. Ukuran partikel karbon mempengaruhi tingkat adsorbsi; tingkat adsorbsi naik dengan adanya penurunan ukuran partikel. Kapasitas total adsorbsi karbon tergantung pada luas permukaannya.


Langmuir adsorption model

Langmuir was awarded the Nobel Prize in for his work concerning surface chemistry. His theory began when he postulated that gaseous molecules do not rebound elastically from a surface, but are held by it in a similar way to groups of molecules in solid bodies. The first experiment involved observing electron emission from heated filaments in gases. He also noted that generally the attractive strength between the surface and the first layer of adsorbed substance is much greater than the strength between the first and second layer. However, there are instances where the subsequent layers may condense given the right combination of temperature and pressure. The surface containing the adsorbing sites is a perfectly flat plane with no corrugations assume the surface is homogeneous. However, chemically heterogeneous surfaces can be considered to be homogeneous if the adsorbate is attracted to only one type of functional groups on the surface.


How can I plot the Langmuir adsorption isotherm?



3: The Langmuir Isotherm


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