Practical 3: Adsorption from Solution
INTRODUCTION:
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. There are two types of adsorption: (1) chemical
adsorption (chemisorption), and (2) physical adsorption (physisorption).
Chemical adsorption involves the formation of chemical bonds such as in the
surface oxidation of a metal. Physical adsorption involves nonspecific
attraction due to weaker van der Waals or dipole forces and is similar to the
condensation of a gas to a liquid.
There
are several factors that will affect the adsorption process. There are the
solute concentration, temperature, pH and surface area of the adsorbent.
Determination
of the surface area of drug powder is very important in pharmacy field.
Adsorption measurement can be used to determine the surface area of a solid. In
this experiment, we use the adsorption from solution to determine the surface
area of activated charcoal.
OBJECTIVE:
To determine the surface area of
activated charcoal via adsorption from solution.
APPARATUS:
12 conical flask, 6 centrifuge tubes,
measuring cylinder, analytical balance, Beckman J6M/E, retort stand and clamps,
Pasteur pipette
MATERIALS:
0.05 M iodine solution, 0.1 M potassium
iodine, 1 % w/v starch solution, 0.1 M sodium thiosulphate solution, activated
charcoal, distilled water
EXPERIMENTAL
PROCEDURES:
12 conical flaks with labeled 1-12 was
filled with 50 ml mixtures of iodine solutions (A and B) as stated in the Table
1 by using measuring cylinders.
Table 1:
Solution A: Iodine (0.05M) Solution B: Potassium iodide (0.1M)
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
|
Set 1: actual concentration of iodine in
solution A (X)
For
flasks 1-6:
- 1-2 drops of starch solution are added as an indicator.
- The solution is titrated by using 0.1 M sodium thiosulfate solution until the colour of the solution changes from dark blue to colourless.
3. The volume of sodium
thiosulphate used is recorded.
Set 2: Concentration of iodine in
solution A at equilibrium (C)
For
flasks 7-12:
- 0.1g of activated charcoal is added.
- The flasks are capped tightly. The flask is swirl or shook every 10 minutes for 2 hours.
3.
After 2 hours, the solutions
are transferred into centrifuge tubes and they are labeled accordingly.
4.
The solutions are centrifuged
at 3000rpm for 5 minutes and the resulting supernatant is 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.
GENERAL NOTES:
|
Titration equation:
I2 + 2Na2S2O3 ↔ Na2S4O6 +
2NaI
Na2S2O3 =
½ I2
|
Given:
(1 mole Na2S2O3
= ½ mole I2)
1 mole iodine = 2 x 126.9g
1 ml 0.1M Na2S2O3
= 0.01269g I
|
If the amount of the activated
charcoal used is Y gram, therefore the total mole of iodine adsorbed by 1g of
activated charcoal (N) is given by the following equation:
N = (X – C) x 50/1000 x 1/y
|
RESULTS:
The volume of sodium thiosulfate used
Flask
|
Initial
volume/ml
|
Final
volume/ml
|
Volume
used/ml
|
1
|
31.0
|
37.2
|
6.2
|
2
|
0.0
|
9.5
|
9.5
|
3
|
11.5
|
24.2
|
12.7
|
4
|
23.1
|
40.3
|
17.2
|
5
|
8.8
|
29.5
|
20.7
|
6
|
1.8
|
35.3
|
33.5
|
7
|
0.0
|
1.1
|
1.1
|
8
|
2.8
|
4.5
|
1.7
|
9
|
10.8
|
13.1
|
2.3
|
10
|
14.0
|
17.2
|
3.2
|
11
|
17.4
|
21.3
|
3.9
|
12
|
21.9
|
28.7
|
6.8
|
PRACTICE:
- Calculate N for iodine in each flask.
I2
+ 2Na2S2O3 = Na2S4O6
+ 2NaI
The volume of Na2S2O3 used = 6.2ml
The
no. of moles of Na2S2O3 = 6.2ml x 0.1M
= 0.62 mol
The
no. of moles of I2 = 0.62 mol ÷ 2
= 0.31 mol
The concentration of I2 in solution A = 0.31mol ÷ 50ml
= 0.0062 M
Thus
X = 0.0062 M
For
flask 7:
I2
+ 2Na2S2O3 = Na2S4O6
+ 2NaI
The volume of Na2S2O3 used = 1.1ml
The
no. of moles of Na2S2O3 = 1.1 ml x 0.1M
= 0.11 mol
The no. of moles of I2 = 0.11 mol ÷ 2
= 0.055 mol
The concentration of I2 in solution A = 0.055 mol ÷ 12ml
= 4.58x10-3 M
Thus
C = 4.58x10-3 M
For
flask 2-6, the value of X is found by using the method used for flask 1.
For
flask 8-12, the value of C is found by using the method used for flask 7.
The
value of N is calculated by using the formula:
N
= (X - C) x 50/1000 x 1/y
Where
y = 0.1g
Flask
|
C(x10-3M)
|
N(x10-4)
|
|
1
and 7
|
6.2
|
4.58
|
8.3
|
2 and 8
|
9.5
|
7.08
|
12.1
|
3 and 9
|
12.7
|
9.58
|
17.1
|
4 and 10
|
17.2
|
13.30
|
24.0
|
5 and 11
|
20.7
|
16.25
|
26.25
|
6 and 12
|
33.5
|
28.33
|
28.35
|
2. Plot amount of iodine adsorbed (N) versus
balance concentration of solution (C) at equilibrium to obtain adsorption
isotherm.
Graph N Versus C At Equilibrium
3. According to Langmuir theory, if there
is no more than a monolayer of iodine adsorbed on the charcoal,
C/N = C/Nm
+ I/KNm
Where C = concentration of
solution at equilibrium
Nm
= 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/Nm and intercept of 1/KNm is obtained.
C(x10-3M)
|
N(x10-4)
|
C/N
|
4.58
|
8.3
|
5.54
|
7.08
|
12.1
|
5.70
|
9.58
|
17.1
|
6.12
|
13.30
|
24.0
|
6.80
|
16.25
|
26.25
|
7.32
|
28.33
|
28.35
|
10.90
|
Graph
C/N versus C
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, Avogrado no. = 6.023 x 1023 molecule calculate the
surface area of charcoal in m2g-1.
From the graph
obtained, the gradient of the graph,
No. of molecules = 4.318x10-4 mol x (6.023 x 1023)
Area covered = (2.6x1020) x (3.2 x 10-19)
The surface area of charcoal = 83.22 m2 ÷ 0.1g
= 231.58
Thus, 1/Nm = 231.58
and Nm = 4.318x10-3 mol g-1
No.
of moles charcoal = 4.318x10-3 mol g-1 x 0.1g
= 4.318x10-4 mol
No. of molecules = 4.318x10-4 mol x (6.023 x 1023)
= 2.6x1020
molecules
Area covered = (2.6x1020) x (3.2 x 10-19)
= 83.22 m2
The surface area of charcoal = 83.22 m2 ÷ 0.1g
= 832.2 m2g-1
4. How do you determine experimentally
that equilibrium has been reached after shaking for 2 hours?
We repeat
the experiment and then titrate with sodium thiosulfate. If the volume stays
constant then the equilibrium is reached.
DISCUSSION:
DISCUSSION:
Adsorption occurs 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 such as electrostatic, valency or Van Der Waals around the particles. An example of this phenomenon includes nitrogen and oxygen gases being adsorbed by charcoal cooled in liquid air. There are many types of adsorption such as Physical Adsorption which is van der waals adsorption , Langmuir Isotherm Adsorption, Isotherm BET Adsorption, Chemisorption and Adsorption from solution. Adsorbent is a material that has the ability to extract certain substances from solid, liquid and gas by causing them to adhere to its surface without changing the physical properties of the adsorbent. The most common examples for adsorbent that are being frequently used are activated carbon, silica gel, and activated alumina. Medicated charcoal which is also an example of adsorbent is prepared by carbonisation of wood, cellulose waste and coconut shell without any contact with air. The charcoal obtained is activated by heating with steam or carbon dioxide at 950oC with the aim to extract out any hydrocarbon adsorbed during carbonization and to increase surface area. Some examples of application of adsorption includes discolouration whereby silica and alumina gel are used to adsorp water vapour. For medicinal usage, kaolin is used to adsorp toxin and toxic alkaloids.
The adsorption capacity (mg/g) is affected by the physical and chemical characteristics of the adsorbent such as surface area, pore size, diameter, and chemical composition. The characteristics of adsorbate which are molecular size, vapour pressure and boiling point, liquid density, molecular polarity, chemical composition and the concentration of the adsorbate in the liquid phase (solution), as well as the characteristics of the phase such as pH, temperature, pressure, humidity and the contact time also can affect the adsorption capacity.
Based on the experiment, in order to estimate the surface area of activated charcoal, Langmuir equation is used. The relationship between the number of active sites of the surface undergoing adsorption and pressure is explained through Langmuir isotherm equation which is: Cf/N = (1/Nm) Cf + (1/kNm)
where (1/Nm) is the slope, and (1/kNm) is the intercept, when Cf/N is plotted versus the concentration Cf. A linear plot is obtained when graph of C/N versus C is plotted.
The actual concentration of iodine in solution A (X) and concentration of iodine in solution A at equilibrium (C) are calculated based on the result that is obtained through this experiment. The amount of iodine adsorped (N) is calculated based on the equation N = (X - C) x 50/1000 x 1/y where y is 0.1 g.
There are some errors that may occur throughout this experiment and might affect the result of the experiment a little bit. The amount of activated charcoal that is being weighed might be wrong due to experimenter’s error and this will affect our data. Another error is the amount of solution A might be incorrect due to the parallax error and resulting in our data will be slightly altered.
CONCLUSION:
REFERENCES:
The adsorption capacity (mg/g) is affected by the physical and chemical characteristics of the adsorbent such as surface area, pore size, diameter, and chemical composition. The characteristics of adsorbate which are molecular size, vapour pressure and boiling point, liquid density, molecular polarity, chemical composition and the concentration of the adsorbate in the liquid phase (solution), as well as the characteristics of the phase such as pH, temperature, pressure, humidity and the contact time also can affect the adsorption capacity.
Based on the experiment, in order to estimate the surface area of activated charcoal, Langmuir equation is used. The relationship between the number of active sites of the surface undergoing adsorption and pressure is explained through Langmuir isotherm equation which is: Cf/N = (1/Nm) Cf + (1/kNm)
where (1/Nm) is the slope, and (1/kNm) is the intercept, when Cf/N is plotted versus the concentration Cf. A linear plot is obtained when graph of C/N versus C is plotted.
The actual concentration of iodine in solution A (X) and concentration of iodine in solution A at equilibrium (C) are calculated based on the result that is obtained through this experiment. The amount of iodine adsorped (N) is calculated based on the equation N = (X - C) x 50/1000 x 1/y where y is 0.1 g.
There are some errors that may occur throughout this experiment and might affect the result of the experiment a little bit. The amount of activated charcoal that is being weighed might be wrong due to experimenter’s error and this will affect our data. Another error is the amount of solution A might be incorrect due to the parallax error and resulting in our data will be slightly altered.
CONCLUSION:
A linear plot is obtained when C/N versus C graph is plotted. Langmuir equation is obeyed and followed. Adsorption Adsorption occurs 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 such as electrostatic, valency or Van Der Waals around the particles.
REFERENCES:
1) Amrita Virtual Lab. 2014. Adsorption Isotherm. http://amrita.vlab.co.in/?sub=2&brch=190&sim=606&cnt=1 [ 29 April 2014].
2) Xamplified. 2010. Adsorption Isotherm. http://www.chemistrylearning.com/adsorption-isotherm/ [29 April 2014].
3) The Chemical Company. 2010. Adsorbents and Adsorption. http://www.catalysts.basf.com/p02/USWeb-Internet/catalysts/en/content/microsites/catalysts/prods-inds/adsorbents/adsorbents-adsorption [29 April 2014].
4) Cabot Norit Activated Carbon. 2014. What is adsorption.http://www.norit.com/carbon-academy/what-is-adsorption/ [29 April 2014].
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