PRACTICAL 3 : Adsorption from solution

PRACTICAL 3 –ADSORPTION FROM SOLUTION

INTRODUCTION:

Adsorption is the adhesion of atoms, ions, or molecules from a gas, liquid, or dissolved solid to a surface. This process creates a film of the adsorbate (gas or dissolved solutes) on the surface of the adsorbent (solid surface). The bonding between adsorbent and adsorbate can be ionic, covalent, or metallic bond.  During adsorption, a new bond is formed between adsorbent and adsorbate. Therefore adsorptions are generally exothermic (▵H = -ve). But entropy and free energy decreases during adsorption.

One of the adsorption process is physical adsorption (characteristic of weak van der Waals forces). It occurs at low temperature when shaking of thermal molecule is not enough to cause complete evaporation at adsorbed layer on the surface of solid. Physical adsorption can produce multilayer of adsorbate and it is reversible.

Chemisorption (characteristic of covalent bonding) may also occur due to electrostatic attraction. It involves combination of chemical substance adsorbed to the surface of adsorbent occurs at high adsorbent heat and it is irreversible. It generally produces monolayer adsorbate.

In adsorption from solution, physical adsorption is common than chemisorptions. The factors affecting adsorption in solution are solute concentration, temperature, pH and surface area of adsorbent. The relationship between the degree of adsorption and partial pressure or concentration is known as adsorption isotherm.

OBJECTIVES:

To study the adsorption of iodine from the solution and its relationship with the surface area of activated charcoal and the determination of the surface area of activated charcoal by using Langmuir Isotherm Adsorption Theory.

MATERIALS AND APPARATUS:

12 conical flask, 6 centrifuge tubes, measuring cylinders, analytical balance, Beckman J6M/E centrifuge, burettes, retort stand and clamps, Pasteur pipettes, iodine solutions (specified in Table 1), 1% w/v starch solution, 0.1 M sodium thiosulpate solution, distilled water and activated charcoal.

EXPERIMENTAL PROCEDURES:

12 conical flasks (labelled 1 to 12) were filled with 50 mL mixtures of iodine solutions (A and B) as stated in the Table 1.

Solution A: Iodine (0.05 M)

Solution B: Potassium iodide (0.1 M)

Flask	Volume of solution A (ml)	Volume of solution B (ml)
1 and 7	10	40
2 and 8	15	35
3 and 9	20	30
4 and 10	25	25
5 and 11	30	20
6 and 12	50	0

TABLE 1

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

For flasks 1-6

1) 1 to 2 drops of starch solutions were added as the indicator.

2) 0.1 M sodium thiosulphate solution was titrated until the colour of the solution change from dark blue to colourless.

3) The volume of sodium thiosulphate used was recorded.

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

1) 0.1g of activated charcoal was added.

2) The flasks were capped tightly and the flask was shaken every 10 minutes for 2 hours.

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

4)  The solutions were centrifuged at 3000 rpm for 5 minutes and the resulting supernatant were transferred into new conical flasks.  

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

RESULTS AND CALCULATION:

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

Flask	Volume of solution A (mL)	Volume of solution B (mL)	Volume of sodium thiosulphate solution (mL)
1	10.0	40.0	6.2
2	15.0	35.0	9.5
3	20.0	30.0	12.7
4	25.0	25.0	17.2
5	30.0	20.0	20.7
6	50.0	0.0	33.5

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

Flask	Volume of solution A (mL)	Volume of solution B (mL)	Volume of sodium thiosulphate solution (mL)
7	10.0	40.0	1.1
8	15.0	35.0	1.7
9	20.0	30.0	2.3
10	25.0	25.0	3.2
11	30.0	20.0	3.9
12	50.0	0.0	6.3

Titration equation: I₂ + 2Na₂S₂O₃ = Na₂S₄O₆ + 2NaI

Based on the equation:

2 mol Na₂S₂O₃ ≈ 1 mol I₂

1 mole I₂ = 2 x 126.9 g/mol = 253.8 g/mol

1 ml 0.1M Na₂S₂O₃ = 0.01269 g I₂

0.0001 mol Na₂S₂O₃ = 0.01269g I

RESULTS:

For flasks 1-6: X = Calculate the actual concentration of iodine in solution A

Flask 1:                                                                               

1.0 ml 0.1M Na₂S₂O₃ = 0.01269 g I₂

6.2 ml 0.1M Na₂S₂O₃ = 0.0787 g I₂

1 mol I₂ = 2 x 126.9 = 253.8g

n= 0.0787/ 253.8g = 3.1x 10^-4 mole I₂

[X] = No. of mole (mol)/Volume (L)

          = 3.1x 10^-4 mole / 50/1000L

          = 6.2 x 10^-3 M

Flask 2:

1 ml 0.1M Na₂S₂O₃ = 0.01269 g I₂

9.5ml 0.1M Na₂S₂O₃ = 0.1206 g I₂

1 mol I₂ = 253.8g

n = 0.1206/253.8= 4.8 x 10^-4 mol I₂

[X]= No. of mole (mol)/Volume (L)

          = 4.8 x 10^-4mol / 50/1000L

          = 9.6x10^-3 M

 Flask 3:

1 ml 0.1M Na₂S₂O₃ = 0.01269 g I₂

12.7 ml 0.1M Na₂S₂O₃ = 0.1612 g I₂

1 mole I₂ = 253.8 g

 n=0.1612/253.8 g = 6.4x10^-4 mol I₂

X(M) = No. of mole (mol)/Volume (L)

          = 6.4 x 10^-4 mol / 0.05L

          = 0.0128M

Flask 4:

1 ml 0.1M Na₂S₂O₃ = 0.01269 g I₂

17.2ml 0.1M Na₂S₂O₃ = 0.2183 g I₂

1 mole I₂ = 253.8g

n= 0.2183/253.8 = 8.6x 10^-4 mol I₂

[X]= No. of mole (mol)/Volume (L)

          = 8.6 x 10^-4 mol / 0.05L

          = 0.0172 M

Flask 5:

1 ml 0.1M Na₂S₂O₃ = 0.01269 g I₂

20.7 ml 0.1M Na₂S₂O₃ = 0.2627 g I₂

1 mol I₂ = 253.8 g                 

n=0.2627/253.8 = 1.0 x 10^-3 mol I₂

[X] = No. of mole (mol)/Volume (L)

          = 1.0 x 10^-3 mole / 0.05L

          = 0.0200 M

Flask 6:

1 ml 0.1M Na₂S₂O₃ = 0.01269 g I₂

33.5 ml 0.1M Na₂S₂O₃ = 0.4251g I₂

1 mol I₂ = 253.8 g

n =0.4251/253.8 = 1.7x 10^-3 mol I₂

[X] = No. of mole (mol)/Volume (L)

          = 1.7 x 10^-3 mol / 0.05L

          =0.0340M

For flasks 7-12: C =Calculate the concentration of iodine in solution A at equilibrium

Flask 7:

1 ml 0.1M Na₂S₂O₃ = 0.01269 g I₂

1.1 ml 0.1M Na₂S₂O₃ = 0.0140 g I₂

1 mol I₂ = 253.8 g

n= 0.0140 / 253.8 = 5.5 x 10^-5 mol

[C]= No. of mole (mol)/Volume (L)

          = 5.5 x 10^-5 mol / 50/1000L

          = 1.10x10^-3 M

Flask 8:

1 ml 0.1M Na₂S₂O₃ = 0.01269 g I₂

1.7 ml 0.1M Na₂S₂O₃ = 0.0216 g I₂

1 mol I₂ = 253.8 g

n = 0.0216/253.8 = 8.5 X10^-5 mol

[C] = No. of mole (mol)/Volume (L)

          = 8.5 x 10^-5 mol / 50/1000L

          = 1.70x10^-3 M

Flask 9:

1 ml 0.1M Na₂S₂O₃ = 0.01269 g I₂

2.3 ml 0.1M Na₂S₂O₃ = 0.0292 g I₂

1 mol I₂ = 253.8 g

n = 0.0355 / 253.8 =1.15 x 10^-4 mol

[C] = No. of mole (mol)/Volume (L)

          = 1.15 x 10^-4 mol / 50/1000L

          = 2.30 x 10^-3M

Flask 10:

1 ml 0.1M Na₂S₂O₃ = 0.01269 g I₂

3.2 ml 0.1M Na₂S₂O₃ = 0.0406 g I₂

1 mol I₂ = 253.8 g

n = 0.0406/253.8 = 1.60 x 10^-4 mol I₂

[C] = No. of mole (mol)/Volume (L)

          = 1.60 x 10^-4 mol / 50/1000L

          = 3.20 x 10^-3 M

Flask 11:

1 ml 0.1M Na₂S₂O₃ = 0.01269 g I₂

3.9 ml 0.1M Na₂S₂O₃ = 0.0495 g I₂

1 mol I₂ = 253.8 g

n=0.0495/ 253.8 = 1.95 x 10^-4 mol I₂

[C] = No. of mole (mol)/Volume (L)

          = 1.95 x 10^-4 mol / 50/1000L

          = 3.90 X 10^-3 M

Flask 12:

1 ml 0.1M Na₂S₂O₃ = 0.01269 g I₂

6.8 ml 0.1M Na₂S₂O₃ = 0.0863 g _I2

1 mol I₂ = 253.8 g

n = 0.0863/253.8 = 3.40 x10^-4 mol I₂

[C] = No. of mole (mol)/Volume (L)

          = 3.40 x 10^-4 mol / 0/1000L

          = 6.80 X10^-3 M

QUESTIONS:

1.      Calculate N for iodine in each flask.

N = (X – C) x 50/1000 x 1/y

Where y = Amount of activated charcoal used in gram

               = 0.1g

           N = Total mole of iodine adsorbed by 1g of activated charcoal

Flask 1 and 7:

X = 6.20 x 10^-3 M

C = 1.10 x 10^-3 M

N = (X – C) x 50/1000 x 1/y

= (6.20 x 10^-3 – 1.10 x 10^-3)M x 50/1000 x 1/0.1g

= 2.55 x 10^-3 mol/g

Flask 2 and 8:

X = 9.6 x 10^-3 M

C = 1.7 x 10^-3 M

N = (X – C) x 50/1000 x 1/y

= (9.6x10^-3 – 1.7x10^-3) M x 50/1000 x 1/0.1g

= 3.95 x 10^-3 mol/g

Flask 3 and 9:

X = 0.0128 M

C = 2.3 x 10^-3 M

N = (X – C) x 50/1000 x 1/y

= (0.0128– 2.3x10^-3) M x 50/1000 x 1/0.1g

= 5.25 x 10^-3 mol/g

Flask 4 and 10:

X = 0.0172 M

C = 3.2 x 10^-3 M

N = (X – C) x 50/1000 x 1/y

= (0.0172 – 3.2x10^-3) M x 50/1000 x 1/0.1g

= 7.10 x 10^-3 mol/g

Flask 5 and 11:

X = 0.0200 M

C = 3.9 x 10^-3M

N = (X – C) x 50/1000 x 1/y

= (0.0200 – 3.9 x 10^-3) M x 50/1000 x 1/0.1g

= 8.05 x 10^-3 mol/g

Flask 6 and 12:

X = 0.0340 M

C = 6.80 x 10^-3M

N = (X – C) x 50/1000 x 1/y

= (0.0340 – 6.80x10^-3) M x 50/1000 x 1/0.1g

= 0.0136 mol/g

2. Plot amount of iodine adsorbed (N) versus balance concentration of solution (C) at equilibrium to obtain adsorption isotherm.

Flask	X	C	N
1 and 7	6.20 x 10-3	1.10x10-3	2.55x 10-3
2 and 8	9.60 x 10-3	1.70x10-3	3.95x10-3
3 and 9	0.0128	2.30x10-3	5.25x10-3
4 and 10	0.0172	3.20 x 10-3	7.00x10-3
5 and 11	0.0200	3.90x 10-3	8.05 x 10-3
6 and 12	0.0340	6.8 x 10-3	0.0136

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

                                 C/N = C/N_m + 1/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.

C (M)	C/N (g/L)
1.10 x10-3	0.43
1.70 x 10-3	0.43
2.30 x 10-3	0.44
3.20 x 10-3	0.45
3.90 x 10-3	0.48
6.80 x 10-3	0.50

From the graph obtained, the gradient of the graph

=  (0.496-0.434) / (6.2-1.6)

= 0.013

1/ Nm = Gradient of the graph

Thus, 1/N_m =0.013 and N_m =76.92 mole / g

Avogadro number = 6.023 x 10²³ molecule

Number of molecules = Number of moles x Avogadro Number

                                    = 76.92 moles x 6.023 x 10²³ molecule

                                    = 4.63 x 10²⁵ molecules / g

Area covered by one adsorbed molecule is 3.2 x 10^-19 m²

Surface area of charcoal = 4.63 x 10²⁵ molecules / g x 3.2 x 10^-19 m² / molecule

                                        = 1.48 X 10⁷m²g^-1

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

We repeat the experiment and titrate with sodium thiosulphate. If the volume stays constant then equilibrium is reached.

DISCUSSION:

        Common charcoal is made from coal, wood, coconut shell, or petroleum. Activated charcoal is used in water filters, medicines that selectively remove toxins, and chemical purification processes. In which the activated charcoal is carbon that has been treated with oxygen. The treatment results in a highly porous charcoal. These tiny holes give the charcoal a surface area of 300-2,000 m2/g, allowing liquids or gases to pass through the charcoal and interact with the exposed carbon. The carbon adsorbs a wide range of impurities and contaminants, including chlorine, odours, and pigments. Other substances, like sodium, fluoride, and nitrates, are not as attracted to the carbon and are not filtered out. Because adsorption works by chemically binding the impurities to the carbon, the active sites in the charcoal eventually become filled. Activated charcoal filters become less effective with use and have to be recharged or replaced.

         Several factors influence the effectiveness of activated charcoal. The pore size and distribution varies depending on the source of the carbon and the manufacturing process.  Large organic molecules are absorbed better than smaller ones. Adsorption tends to increase as pH and temperature decrease. Contaminants are also removed more effectively if they are in contact with the activated charcoal for a longer time, so flow rate through the charcoal affects filtration.

From the first graph drawn, the relationship between the amount of iodine adsorbed and the concentration of iodine at equilibrium is shown clearly. According to the graph, the amount of iodine adsorbed is proportional to the concentration of iodine at equilibrium. As the amount of iodine at equilibrium is increase, there will be more collision between the iodine and the adsorbent. Adsorption occurs based on the collision between the adsorbent and the adsorbate. Thus, as there are more collision, the adsorption will be more

            Adsorption isotherm describes the equilibrium of the adsorption of particles at a surface at constant temperature. It shows the amount of particles attached at the surface as a function of the particles present in the solution. In this experiment, adsorption of iodine from solution is studied and Langmuir equation is used to determine the surface area of activated charcoal.

            Langmuir states that the rate of adsorption and the rate of adsorbate evaporation were equal at constant and unchanging temperature. The assumptions of Langmuir Isotherm are adsorption cannot exceed monolayer coverage, all surface sites are uniform and equivalent, and the ability of a particle to adsorb at specific site is independent to the occupation of neighbouring sites.

From the result obtained, the volume of sodium thiosulphate that is used flasks 7 to 12 (with activated charcoal) are lesser than the volume of sodium thiosulphate used for flasks 1-6 (without activated charcoal). In the beginning, for each of the flasks 7-12, there are mixture of iodine and potassium iodide. When adding starch, iodine will tend to bind with starch and forming starch iodide complex which contribute to deep blue colouration. But for the flasks 7-12, the activated charcoal was added first and only then starch indicator was added. Activated charcoal is very useful in attracting non polar adsorbates. Activated charcoal (adsorbent) is extremely porous, and thus has an enourmous surface area. Its huge surface serves as gigantic target for the iodine molecules (adsorbate) to be attracted and holding them within its pores by a process called adsorption.  This will reduce the amount of iodine present in the solution that will react with sodium thiosulphate.  Thus, the amount of sodium thiosulphate used for flask 7 to 12 was smaller than for flask 1 to 7.

Based on the calculation, the surface area of charcoal is 1.48 X 10⁷m²g^-1. This value is too big if compared to the general actual value of 1g of charcoal which is 500m². This may be due to the error occurred during the experiment. The significant error in this experiment is the volume of the sodium thiosulphate used for each titration is not accurate as the real end point of the titration is not reached thus resulting in inaccurate calculation of the whole experiment. Moreover, the shaking of the flasks every 10 minute interval is not done properly and cause the inconstant volume of iodine adsorbed on the surface of the activated charcoal. 

Starch solutions are widely used in the detection of the end-point of iodine - thiosulfate titrations. The starch gives a very definite colour change at the end-point. Without the starch indicator, the colour of the iodine solution in the conical flask near the end-point fades slowly from pale yellow to colourless. With the starch indicator added, the colour of the solution in the conical flask at the end-point changes suddenly from blue black to colourless. The starch solution should actually added close to the end-point to give a sharp end-point, while avoiding the formation of excess starch-iodine complex, which would be difficult to decompose.

Centrifugation is done to these flasks before being titrated with sodium thiosulphate. Fine particles suspended in a liquid can be separated by. After the solution of in the flasks in centrifuge the higher densities of activated charcoal with bounded iodine will move down. The lesser amount of free iodine present in the solution will reduce the volume of sodium thiosulphate used to reduce the iodine.

PRECAUTION:

1) Wear goggle, lab coat and gloves when carrying out the experiment.

2) Avoid direct contact of sodium thiosulphate with skin as it may irritates the skin. If do, wash your hand thoroughly with water.

3) Avoid inhaling the iodine as it is harmful for the respiratory system.

4) Make sure the end point of the reaction is reached accurately by confirming the changes of blue colour of the solution to colourless.

CONCLUSION:

The surface area of the charcoal is 1.48 X 10⁷m²g^-1.  It is proven that the surface area of the charcoal can be calculated by using Langmuir theory.

REFERENCES:

1) Alfonso R. Gennaro al.1995. Remington: The Science & Practice of Pharmacy.19^th Edition. Easton, Pennsylavania: Mack Publishing Company.

2) Alexander T. Florence, David Attwood. 2006. Physiochemical Principles of Pharmacy. Fourth Edition. London: Pharmaceutical Press

3) http://chemistry.about.com/od/chemistryfaqs/f/charcoal.htm

4)http://uotechnology.edu.iq/petrol_tech/images/new%20index/mohadrat/najem/2%20nd%20class/Chapter%204-2%20adsorption.pdf