PRACTICAL 3: Absorption of solution

DATE: 16^th April 2013

OBJECTIVE:  

1.      To study the adsorption of iodine from solution.

2.      To estimate the surface area of activated charcoal sample by Langmuir equation.

INTRODUCTION:

            Adsorption is the accumulation process of gases, liquids, or solutes on the surface of a solid or liquid, forming a molecular or atomic film.

Principles of Adsorption Adsorption is described as the enrichment of one or more components in the interfacial layer. One of the examples is an excess of molecules at the adsorbent interface which is upon exposure of an adsorbing solid to gas or vapour. The solid surfaces of certain molecules contained in a vapour have selective collection and concentration. Hence, when adsorbates are adsorbed, they may be captured selectively and removed from the effluent stream using an adsorbents even though they are of mixed systems and are at low concentrations, Adsorption is divided into the two sub-categories which are physical adsorption (physisorption) and chemical adsorption (chemisorption). Physical adsorption (physisorption) can also been called as van der Waals adsorption. The adsorption process can be used to determine which type of chemical bonds is formed during the process. Physisorption is applicable to all adsorbate-adsorbent systems if the pressure and temperature are suitable whereas chemisorption can only occur if the system is capable of making a chemical bond. Physical Adsorption (Physisorption) This is a dynamic process where an equilibrium state exists between molecules and the interaction between the adsorbate and adsorbent. No chemical bonds are formed during physical adsorption while the attraction between the adsorbate and adsorbent exists by the formation of intermolecular electrostatic, such as London dispersion forces or van der Waals forces from induced dipole-dipole interactions. This may be dependent on the physical configuration of the adsorbent such as the porosity of activated carbons. Dispersion force is the result of rapid fluctuations in the electronic density of one adsorbent molecule inducing an electrical moment in a second atom. If the adsorbate possesses a permanent dipole, or even a multipole, then additional interactions may occur. This is due to the  charge distributions are induced in the adsorbent and interactions of these moments with any permanent field of the solid. Chemical Adsorption (Chemisorption) Chemisorption involves the transfer of electrons between the adsorbent and the adsorbate with the formation of chemical bonds. Through chemical reaction, adhesion of the adsorbate molecules is exit among the two species. According to a research done, chemical adsorption is less common than physical adsorption. Due to the chemical bonds formed, regeneration of the adsorbent for subsequent re-use is often difficult or impossible. Due to the fact that chemical bonds are formed during the adsorption process, desorption of the adsorbed phase may yield products which are chemically different to the original adsorbate. For example oxygen may chemically bond to the surface of a carbon, which upon desorption may evolve CO and CO2 as products.

PROCEDURE:

Using measuring cylinders, 12 conical flasks (labeled 1-12) are filled with 50 ml mixtures of iodine solutions (A and B) as stated in the Table 1.

Table 1: Solution A: Iodine (0.05 M) 
Solution B: Potassium iodide (0.1 M)

Set 1: Actual concentration of iodine in solution A (X)

For flasks 1-6:

1) 1-2 drops of starch solution are added as an indicator.

2) 0.1 M sodium thiosulfate is used to titrate solution until the colour of the

solution changes from dark blue to colourless.

3) The volume of the sodium thiosulphate used is recorded.

Set 2: Concentration of iodine in solution A at equilibrium (C).

For flasks 7-12:

1) 0.1g activated charcoal is added.

2) The flasks are capped tightly. The flasks are shaken and swirled every 10 minutes for 2 hours.

3) After 2 hours, the solutions are transferred into centrifuge tubes and labeled accordingly.

4) The solutions are centrifuged at 3000 rpm for 5 minutes and the resulting supernatants are transferred into new conical flasks. Each conical flask is labeled accordingly.

5) Steps 1, 2 and 3 are repeated as carried out for flasks 1-6 in Set 1.

  

 















 
3.

According to Langmuir theory, if there is no more than a monolayer of iodine adsorbed on the charcoal,

C/N = C/Nm + I/KN_m

Where C=        concentration of solution at equilibrium

N_m =    number of mole per gram charcoal required

K=       constant to complete a monolayer

Plot C/N versus C, if Langmuir equation is followed, a straight line with slope of 1/N_m and intercept of 1/KN_m is obtained.

Obtain the value of Nm, and then calculate the number of iodine molecule adsorbed on the monomolecular layer. Assume that the area covered by one adsorbed molecule is 3.2 x 10^-19 m², Avogadro no. = 6.023 x 10²³ molecule, calculate the surface area of charcoal in m²g^-1.

4 ) Discuss the results of the experiment. How do you determine experimentally that equilibrium has been reached after shaking for two hours?

Adsorption is a process where free moving molecules of a gaseous or solutes of a solution come close and attach themselves onto the surface of the solid. Activated carbon is produced specifically so as to achieve a very big internal surface (between 500 - 1500 m2/g). This big internal surface makes active carbon ideal for adsorption.

From this experiment , it shows that the solute concentration can influence the extent of adsorption from solution that is the higher the actual concentration of iodine in solution A, the higher the mole of iodine adsorbed by 1 g of activated charcoal. The higher the solute concentration, the higher the amount of the adsorption occuring at equilibrium until a limiting value is reached . In this experiment, when  the concentration of solution increases, the amount of iodine adsorbed is also increased. This can be proven from the graph above which show the amount of iodine adsorbed (N) is proportional to the balance concentration of solution(C). When equilibrium (C) increases from flasks 7 to 12, the total mole of iodine adsorbed by 1 g of activated charcoal (N) also increases. This is proved by the shape of the  graph. The shape of the graph indicates that the higher the balance concentration of solution (C) at equilibrium, the larger the amount of iodine adsorbed (N).
The gradient obtained from the graph is 4533.33.

When activated charcoal is added to the flasks 7-12 to produce the concentration of iodine in solution A at equilibrium, iodine is being adsorbed by the activated charcoal. Therefore, the total amount of iodine remains in the solution decreases. That explains that the actual concentration of iodine in solution A (X) is larger than the concentration of iodine in solution A at equilibrium (C) as shown by the results of the experiment. Thus, the total mole of iodine adsorbed by 1 g of activated charcoal (N) is always positive as X is always larger than C.

We can determine that the equilibrium has been reached after shaking for 2 hours by observing that there is no colour change of the solution, the solution remains in one colour. This shows that the solution is homogeneous and has reached equilibrium.

Conclusion :

The surface area of charcoal is 42.4941 m² g^. The higher the  balance concentration of solution (C) at equilibrium, the larger the amount of iodine adsorbed (N).

Reference :

1. Martin’s Physical Pharmacy and Pharmaceutical Sciences, 5^th Edition, Patrick J. Sinko, 

Lippincott Williams and Wilkins, page 39, 40

2. E.A.Moelwyn- Hughes. (1961). Physical Chemistry, 2nd Ed. Pergamon, New York.
 3.www.rpi.edu/dept/chem-eng/Biotech-Environ/Adsorb/adsorb.htm

4. http://www.lenntech.com

5. www.diffen.com