PRACTICAL 3 : ADSORPTION FROM SOLUTION

Objectives

To determine the surface area of drug, related to its particle size.

Introduction

Adsorption is the binding process, due to attractive interactions, of a chemical species (the adsorbate) onto a solid surface (the adsorbent). An example is adsorption of water vapor molecules onto the surfaces of silica particles. The adsorbate solid could be in gas or a liquid phase.

The degree of adsorption of a gas by a solid depends on the chemical nature of the adsorbent (the material used to adsorb the gas) and the adsorbate (the substances being adsorbed), the surface area of the adsorbent, the temperature, and the partial pressure of the adsorbed gas. The types of adsorption are generally recognizes as physical or Van der Waals adsorption and chemical adsorption or chemisorption. Physical adsorption, associated with Van der Waals forces, is reversible, the removal of the adsorbate from the adsorbent being known as desorption. A physically adsorbed gas can be desorbed from a solid by increasing the temperature and reducing the pressure. Chemisorption, in which the adsorbate is attached to the adsorbent by primary chemical bonds, is reversible unless the bonds are broken.

Chemical adsorption will produce mono adsorption (a layer of adsorbate) while physical adsorption will produce adsorption of multilayers of adsorbate (more than one layer of adsorbate). It also can happen where the chemical adsorption is followed by the physical adsorption. The partial pressure of the gas or the concentration of adsorbate will affect the degree of adsorption from the solution. Adsorption isotherm is the relationship between degree of the adsorption and the partial pressure of concentration. The characteristic of the solid form and the reaction involved when adsorption occurs will be different if there are changes of the isotherm in the temperature.

In this experiment, we use the adsorption from solution to determine the surface area of activated charcoal. there are several factors will influence the extent of adsorption from solution and is summarized in the table below.

FACTORS AFFECTING ADSORPTION	EFFECT OF ADSORPTION
Solute Concentration	Increase solute concentration will increase the amount of adsorption occurring at equilibrium until a limiting value is reached.
Temperature	Process is usually exothermic, therefore, an increase in temperature will decrease adsorption.
pH	pH influences the rate of ionization of the solute, hence, the effect is dependent on the species that is more strongly adsorbed.
Surface Area of Adsorbent	An increase in surface area will increase the extent of adsorption.

 Determination Of Surface Area Of Activated Charcoal Via Adsorption From Solution

Surface area is one of the factor that govern the rate of dissolution and the bioavailability of drug that will determine the rate of adsorption of the drug through gastrointestinal tract. The surface area of the solid can be determined by the adsorption measurement. In the method of B.E.T (Brunauer, Emmett, and Teller), adsorption of gas was used to measure the surface area. In this experiment, adsorption of iodine from solution is studied and Langmuir equation is used to estimate the surface area of activated charcoal sample.

Materials 

Iodine solutions
1% w/v starch solution
0.1M sodium thiosulphate solution
Distilled water
Activated charcoal

Apparatus

12 conical flask
6 centrifuge tubes
Measuring cylinders
Analytical balance
Beckman J6M/E centrifuge
Burettes
Retort stand and clamps
Pasteur pipettes

Procedures

• Burettes or measuring cylinders is used to fill 12 conical flasks (labelled 1-12) with 50 ml mixtures of iodine solutions (A and B) as stated in the Table 1.

          For flasks 1-6 :

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

2.     Titration process was done by using 0.1M of sodium thiosulfate solution until the colour of the solution changes from the dark blue to colourless.

3.     The volume of the sodium thiosulfate used is recorded.

For flasks 7-12 :

1.     0.1 g of activated charcoal was added.

2.     The flasks were cap tightly. The flasks were swirled or shaken every 10 minutes for 1 hour.

3.     After 2 hours, the solutions were transferred into centrifuge tubes and were labelled accordingly.

4.     The solution was centrifuged at 3000 rpm for 5 minutes and the resulting supernatants were transferred into new conical flasks. Each conical flask was labelled.

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

Results

Flask	Volume of Na2S203 (mL)
1	7.5
2	12.1
3	17.0
4	21.9
5	26.8
6	45.0
7	1.7
8	1.7
9	3.1
10	3.4
11	4.2
12	8.1

Flask	Number of moles of Na2S2O3 used (mol)	Number of moles of I2 (mol) (1 mole of Na2S2O3 reacts with 1/2 mole of I2)	Actual concentration of iodine in solution A, X (M) (no. of moles of I2 / volume of solution)
1	= (0.1)7.5/1000 = 0.75 x 10-3	0.38 x 10-3	= 0.38 x 10-3/ 0.05 =0.76 x 10-2
2	= (0.1)12.1/1000 = 1.21 x 10-3	0.61 x 10-3	= 0.61 x 10-3/ 0.05 = 1.22 x 10-2
3	= (0.1)17.0/1000 = 1.70 x 10-3	0.85 x 10-3	= 0.85 x 10-3/ 0.05 = 1.70 x 10-2
4	= (0.1)21.9/1000 = 2.19 x 10-3	1.10 x 10-3	= 1.1 x 10-3/ 0.05 = 2.20 x 10-2
5	= (0.1)26.8/1000 = 2.68 x 10-3	3.75 x 10-3	= 3.75 x 10-3/ 0.05 = 7.50 x 10-2
6	= (0.1)45.0/1000 = 4.50 x 10-3	2.25 x 10-3	= 2.25 x 10-3/ 0.05 = 4.50 x 10-2

Flask	Number of moles of Na2S2O3 used (mol)	Number of moles of I2 (mol) (1 mole of Na2S2O3 reacts with 1/2 mole of I2)	Concentration of iodine in solution A at equilibrium, C (M) (no. of moles of I2 / volume of solution)
7	= (0.1)1.0/1000 = 1.0 x 10-4	0.50 x 10-4	= 0.85 x 10-4/0.05 = 1.0 x 10-3
8	= (0.1)1.7/1000 = 1.7 x 10-4	0.85 x 10-4	= 0.85 x 10-4/ 0.05 = 1.7 x 10-3
9	= (0.1)3.1/1000 = 3.1 x 10-4	1.55 x 10-4	= 1.55 x 10-4/ 0.05 = 3.1 x 10-3
10	= (0.1)3.4/1000 = 3.4 x 10-4	1.70 x 10-4	= 1.7 x 10-4/ 0.05 = 3.4 x 10-3
11	= (0.1)4.2/1000 = 4.2 x 10-4	2.10 x 10-4	=  2.1 x 10-4/ 0.05 = 4.2 x 10-3
12	= (0.1)8.1/1000 = 8.1 x 10-4	4.05 x 10-4	= 4.05 x 10-4/ 0.05 = 8.1 x 10-3

Flask	Concentration of solution  at equilibrium, C (M)	Amount of iodine adsorbed, N (mol) ((X-C) x 50/1000 x 1/y)	C/N (M/mol)
1 & 7	1.0 x 10-3	= (0.76 x 10-2 - 1.0 x 10-3) x 50/1000 x 1/0.1 = 0.33 x 10-2	0.303
2 & 8	1.7 x 10-3	= (1.22 x 10-2 - 1.7 x 10-3) x 50/1000 x 1/0.1 = 0.53 x 10-2	0.321
3 & 9	3.1 x 10-3	= (1.70 x 10-2 - 3.1 x 10-3) x 50/1000 x 1/0.1 = 0.70 x 10-2	0.443
4 & 10	3.4 x 10-3	= (2.20 x 10-2 - 3.4 x 10-3) x 50/1000 x 1/0.1 = 0.93 x 10-2	0.366
5 & 11	4.2 x 10-3	= (7.50 x 10-2 - 4.2 x 10-3) x 50/1000 x 1/0.1 = 3.54 x 10-2	0.119
6 & 12	8.1 x 10-3	= (4.50 x 10-2 - 8.1 x 10-3) x 50/1000 x 1/0.1 = 1.85 x 10-2	0.438

Gradient = (1.75 x 10^-2 - 0.125 x 10^-2)/ (8.0 x 10^-3)

                  = 2.03 M/mol

Thus, adsorption isotherm is = 2.03 M/mol

Intercept = 1/KN_m = 0.295 M/mol

Gradient = 1/N_m = (0.438 - 0.295) /(8.1 x 10^-3)

                                 = 17.65 mol

So, N_m = 0.057 mol^-1

Number of iodine adsorbed on monomolecular layer

= 0.057 x 0.1 x 6.023 x 10²³

= 3.433 x 10²¹ molecules

Surface area of charcoal

= 3.433 x 10²¹ x 3.2 x 10^-19 /0.1

= 1.099 x 10⁴ m²g^-1

Questions

Discuss the results of the experiment. How do you determine experimentally that equilibrium has been reached after shaking for 2 hours?

The whole idea in this experiment is to determine the amount of the iodine (i.e. adsorbate) adsorbed on the charcoal (i.e. adsorbent). When we are talking about adsorption of solute on the surface of solid, we are dealing with Langmuir equation. The equation relates the adsorption of molecules on a solid surface to gas pressure or concentration of a medium above the solid surface at a fixed temperature and in this experiment, we varied the concentration.

The Langmuir theory has some assumptions that are;

1. Fixed number of vacant or adsorption sites are available on the surface of solid.
2. All the vacant sites are of equal size and shape on the surface of adsorbent.
3.  Each site can hold maximum of one gaseous molecule and a constant amount of heat energy is released during this process.
4. 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.

    5.  Adsorption is monolayer or unilayer.

In this experiment, the actual concentration of iodine in solution A (X) is already known as we are given molarity of solution A which is 0.05M and volume of the solution as stated in the table. The titration results give concentration of iodine in solution A at equilibrium (C) remaining in the solution. Since

    

                              iodine total (X) = iodine adsorbed + iodine liquid (C)

Hence, it is possible to determine total mole of iodine adsorbed by 1 gram of activated charcoal (N) in conical flask 7 until conical flask 12. Now, we only focus on those flasks because only those flasks have charcoal inside, therefore the adsorption process occurred whereas the other 6 flasks are only for control experiment.

From the table also, we can say that increase volume of Solution A means increase concentration of iodine in solution A (X) will increased the amount of iodine being adsorbed by the charcoal, therefore will increased the total mole of iodine adsorbed by 1 gram of activated charcoal (N) until certain limit is reached. This is because more iodine can filled up the monolayer or unilayer adsorption on the surface of the charcoal. Although the number of adsorption sites available on the surface of solid is fixed, there are sites that still empty with iodine, therefore the adsorption process still does not reach its maximum limit. It has been proven in graph 1 where the plots only produce a straight line with positive slope or adsorption isotherm of 2.03 and not ends with horizontal line.

Based on the table, the adsorption occurred in monolayer and it has been proven when plotting C/N = C/Nm + I/KNm graph, it produce a straight line. On the other hand, if the plots do not produce a straight line graph, it means the adsorption occurred in unilayer. Next, from the graph we can gain information about the amount of iodine required to fill the absorption sites per gram charcoal (Nm) and the constant value to complete a monolayer (K). Then, we can calculate the number of iodine molecule absorbed on the monomolecular layer of charcoal. Based on the table too, the more the amount of iodine, the higher the number of iodine molecule absorbed on the monomolecular layer of charcoal but the result is vice versa for conical flask 6 and conical flask 12. Perhaps there are errors while carrying the experiment for the flasks such as the trituration process has been stopped before the solutions in the flasks are not totally iodize by Sodium Thiosulfate.

Next, we can determine the surface area of charcoal after we gained all the data. This is the last and required data that pharmacist want as it help to determine the surface area of powder drug. Based on the data gained, the surface area of charcoal is 1.099 x 10⁴ m²g^-1.

Determine experimentally that equilibrium has been reach after shaking for 2 hours.

We can determine the equilibrium when the solution in the test tube starting producing gas which is carbon dioxide gas due to their reaction.

Discussion

      Adsorption is the sticking of molecules from the gas or liquid phase onto the surface of a solid. A molecule that undergoes adsorption is referred to as the adsorbate, and the solid is the adsorbent. Adsorption occur when particles such as ion, atom, or molecules on the surface of solids are capable of attracting other molecules due to the instability of energies around the particles resulting to the adsorption phenomena. For example, nitrogen and oxygen gas being adsorbed by charcoal cooled in liquid air.

Besides, adsorption is a consequences of surface energy, just like the surface tension. In a bulk material, all the bonding requirement of the constituents atoms are filled. But the atom on the clan surface may experience a bond deficiency. Hence, it is more favourable for them to bond with whatever happen to be available. However, the exact nature of the bonding depends on the details of species involved. The adsorption can be physical adsorption or chemisorptions generally.

Physical adsorption or van der waals adsorption may occur at low temperature when shaking of thermal molecule is not enough to cause complete evaporation at adorbed layer on the surface of solid. It may occur depending on surface area of the adsorbed substance and the properties of adsorbent and adsorbate as well.

On the other hand, chemisorption involves the combination of chemical substances adsorbed to the surface of adsorbent. Chemisorption also occur as opposed to the Van der Waals forces which cause physisorption.

Activated charcoal is a general term that includes carbon material mostly derived from charcoal. For all three variations of the name, "activated" is sometimes substituted by "active." By any name, it is a material with an exceptionally high surface area. Just one gram of activated carbon has a surface area of approximately 500 m² (for comparison, a tennis court is about 260 m²). The three main physical carbon types are granular, powder and extruded (pellet). All three types of activated carbon can have properties tailored to the application. Activated carbon is frequently used in everyday life, in: industry, food production, medicine, pharmacy, military, etc. In pharmacy, activated charcoal is considered to be the most effective single agent available as an emergency decontaminant in the gastrointestinal tract. It is used after a person swallows or absorbs almost any toxic drug or chemical.

If the adsorption of the adsorbate leads to a maximum of a single monomolecular layer when the adsorption is complete, it is possible to calculate the area of the adsorbent. When a monomolecular layer is adsorbed, it may be assumed that the area of an adsorbent equals the total area of the adsorbed molecules.

Solid surfaces can adsorb dissolved substances from solution. When a solution of iodine is shaken with activated charcoal, part of the iodine is removed by the charcoal and the concentration of the solution decreased. From the results gathered, it is realized that K increases as the concentration of iodine is decreased with respect to time. Hence, the degree to which a solid will adsorb material depends on a number of things including temperature, nature of molecule being adsorbed, degree of surface pore structure, and, solute concentration & solvent. Other factors are important factors dealing with the process of adsorption of solutes from aqueous solution by highly porous solids.

Regarding the result, the number of molecules adsorbed per gram of solid, N (mol/g), depends on the specific surface area of the solid, S(m2/g), the final liquid phase concentration Cf (mol/L) or equilibrium gas phase pressure p (atm or kPa), and the specific molecules undergoing adsorption. A plot of N versus Cf or N versus p, where the temperature is held constant, is referred to as an adsorption isotherm. There are a variety of equations used to relate the moles adsorbed to the concentration of adsorbate molecules actually.

Conclusion

As a conclusion, the adsorption process is important in the field of pharmacy as it is a method to determined the surface area of powder drug. In this experiment, the adsorption process follows the Langmuir equation and undergo monolayer adsorption.The temperature and concentration of diffusing molecules will affect the value of D ( diffusion coefficient ) . 

References

Physicochemical Principles of Pharmacy , 2^nd Edition ( 1988 ) A.T Florence and D.ettwood

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