TITLE: Adsorption
from solution
DATE: 18 April 2013
OBJECTIVE:
To
study the adsorption process from solution that is important in determination
of the surface area of powder drug, which is related to its particle size, is
important in the field of Pharmacy.
INTRODUCTION:
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. The attachment
or adsorption bonds can be strong or weak, depending on the nature of forces
between adsorbent (solid surface) and adsorbate (gas or dissolved solutes).
When adsorption involves only chemical bonds between adsorbent and adsorbate,
it is recognized as chemical adsorption or chemisorption. Chemical adsorption
or chemisorption acquires activation energy, can be very strong and not readily
reversible.
When
the reaction between adsorbent and adsorbate is due solely to van der Waals
forces, this type of adsorption is known as physical adsorption or van der
Waals adsorption. This process is non-specific and can occur at any condition.
This type of adsorption is reversible, either by increasing the temperature or
reducing the pressure of the gas or concentration of the solute.
Chemical
adsorption generally produces adsorption of a layer of adsorbate (monolayer
adsorption). On the other hand, physical adsorption can produce adsorption of
more than one layer of adsorbate (multilayer adsorption). Nevertheless, it is
possible that chemical adsorption can be followed by physical adsorption on
subsequent layers. For a particular adsorbent/ adsorbate, the degree of
adsorption at a specified temperature depends on the partial pressure of the
gas or on concentration of the adsorbate for adsorption from solution. The
relationship between the degree of adsorption and partial pressure or
concentration is known as adsorption isomerism. The studies of types of
isotherm and changes of isotherm with temperature can provide useful
information on the characteristics of solid and the reactions involved when
adsorption occurs.
In
adsorption from solution, physical adsorption is far more common than
chemisorption. However, chemisorption is sometimes possible, for example, fatty
acids are chemisorbed from benzene solutions on nickel and platinum catalysts.
PROCEDURE:
Material and
apparatus:
12
conical flasks, 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
thiosulfate solution, distilled water and activated charcoal.
Experiment:
12
conical flasks (labeled 1-12) were filled with 50ml mixtures of iodine
solutions (A and B) as was stated in the Table 1 by using burettes or measuring
cylinders.
Table 1: Solution A : Iodine (0.05
M)
:
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 flask 1-6:
1.
1-2 drops of starch
solution was added as an indicator.
2.
0.1 M sodium thiosulfate was titrated into the
flasks until the colour of the solution had changed from dark blue to
colourless.
3.
The volume of
the sodium thiosulfate used was recorded.
Set 2: Concentration of iodine in
solution A at equilibrium (C)
For flask 7-12:
1.
0.1 g activated
charcoal was added.
2.
The flasks were
caped tightly. Every 10 minutes for 2 hours the flask was shaken and swirled.
3.
After 2 hours,
the solutions were transferred into the centrifuge tubes and were labeled
accordingly.
4.
The solutions
were centrifuged at 3000rpm for 5 minutes and the resulting supernatants were
transferred into new conical flasks. Each conical was labeled accordingly.
5.
The steps 1,2
and 3 were repeated as carried out for flasks 1-6 in Set 1.
RESULTS
Flasks
|
Volume of Na2S2O3 (mL)
|
||
Initial
volume
|
Final volume
|
Total volume
|
|
1
|
0.0
|
8.0
|
8.0
|
2
|
8.0
|
21.0
|
13.0
|
3
|
21.0
|
38.0
|
17.0
|
4
|
0.0
|
21.2
|
21.2
|
5
|
0.0
|
28.0
|
28.0
|
6
|
0.0
|
43.5
|
43.5
|
7
|
0.0
|
9.9
|
0.8
|
8
|
6.0
|
19.0
|
1.2
|
9
|
0.0
|
16.1
|
2.2
|
10
|
19.0
|
37.0
|
3.0
|
11
|
10.0
|
31.0
|
4.5
|
12
|
0.0
|
38.0
|
8.0
|
Flasks
|
X (M)
|
Flasks
|
C (M)
|
1
|
8.00x10-3
|
7
|
2.86
x10-3
|
2
|
13.00x10-3
|
8
|
4.39
x10-3
|
3
|
17.00x10-3
|
9
|
7.86
x10-3
|
4
|
21.20x10-3
|
10
|
10.70
x10-3
|
5
|
28.00x10-3
|
11
|
16.10
x10-3
|
6
|
43.50x10-3
|
12
|
28.60
x10-3
|
CALCULATION
Based on the titration equation,
I2 + 2Na2S2O3
= Na2S4O6 + 2NaI
Na2S2O3
= ½ I2
Flask1:
From the results,
The volume of Na2S2O3
used = 8.0ml
The no. of moles of Na2S2O3
= 8.0ml x 0.1M
= 0.80 mol
The no. of moles of I2 = 0.80mol ÷ 2
= 0.40mol
The concentration of I2 in solution A
= 0.40mol
÷ 50ml
=0.008 M
Thus X =
8.0 x10-3 M
|
For flask 7:
From the results,
The volume of Na2S2O3
used = 0.8ml
The no. of moles of Na2S2O3
= 0.8ml x 0.1M
= 0.08 mol
The no. of moles of I2 = 0.08mol ÷ 2
= 0.04mol
The concentration of I2 in solution A
= 0.04mol
÷ 14ml
=0.00286M
Thus C =
2.86 x10-3 M
|
Flask 2:
From the results,
The volume of Na2S2O3
used = 13.0ml
The no. of moles of Na2S2O3
= 13.0ml x 0.1M
= 1.30 mol
The no. of moles of I2 = 1.30mol ÷ 2
=
0.65mol
The concentration of I2 in solution A
= 0.65mol ÷ 50ml
= 0.013 M
Thus X = 13.0 x10-3 M.
|
For flask 8:
From the results,
The volume of Na2S2O3
used = 1.2ml
The no. of moles of Na2S2O3
= 1.2ml x 0.1M
= 0.12 mol
The no. of moles of I2 = 0.12mol ÷ 2
= 0.06mol
The concentration of I2 in solution A
= 0.06mol
÷ 14ml
=0.00439 M
Thus C =
4.39x10-3 M
|
Flask 3:
From the results,
The volume of Na2S2O3
used = 17.0 ml
The no. of moles of Na2S2O3
= 17.0ml x 0.1M
=
1.70mol
The no. of moles of I2 = 1.70mol ÷ 2
= 0.85mol
The concentration of I2 in solution A
= 0.85mol
÷ 50ml
= 0.017 M
Thus X =
17.0 x10-3 M.
|
For flask 9:
From the results,
The volume of Na2S2O3
used = 2.2ml
The no. of moles of Na2S2O3
= 2.2ml x 0.1M
= 0.22 mol
The no. of moles of I2 = 0.22mol ÷ 2
= 0.11mol
The concentration of I2 in solution A
= 0.11mol
÷ 14ml
=0.00786M
Thus C =
7.86 x10-3 M
|
Flask 4:
From the results,
The volume of Na2S2O3
used = 21.2ml
The no. of moles of Na2S2O3 = 21.2ml x 0.1M
=
2.12 mol
The no. of moles of I2 = 2.12mol ÷ 2
= 1.06mol
The concentration of I2 in solution A
= 1.06mol ÷ 50ml
= 0.0212 M
Thus X =
21.2 x10-3 M.
|
For flask 10:
From the results,
The volume of Na2S2O3
used = 3.0ml
The no. of moles of Na2S2O3
= 3.0ml x 0.1M
= 0.30 mol
The no. of moles of I2 = 0.30mol ÷ 2
=0.15mol
The concentration of I2 in solution A
= 0.15mol
÷ 14ml
=0.0107 M
Thus C =
10.7 x10-3 M
|
Flask 5:
From the results,
The volume of Na2S2O3
used = 28.0ml
The no. of moles of Na2S2O3
= 28.0ml x 0.1M
=
2.80 mol
The no. of moles of I2 = 2.80mol ÷ 2
=
1.40mol
The concentration of I2 in solution A
= 1.40mol ÷ 50ml
= 0.028M
Thus X = 28.0 x10-3M
|
For flask 11:
From the results,
The volume of Na2S2O3
used = 4.5ml
The no. of moles of Na2S2O3
= 4.5ml x 0.1M
= 0.45mol
The no. of moles of I2 = 0.45mol ÷ 2
= 0.225mol
The concentration of I2 in solution A
= 0.225mol
÷ 14ml
=0.0161M
Thus C =
16.1 x10-3 M
|
Flask 6:
From the results,
The volume of Na2S2O3
used = 43.5ml
The no. of moles of Na2S2O3
= 43.5ml x 0.1M
=
4.35mol
The no. of moles of I2 = 4.35mol ÷ 2
=
2.175mol
The concentration of I2 in solution A
= 2.175mol ÷ 50ml
= 0.0435 M
Thus X =43.5
x10-3M
|
Flask 12:
From the results,
The volume of Na2S2O3
used = 8.0ml
The no. of moles of Na2S2O3
= 8.0ml x 0.1M
=
0.80mol
The no. of moles of I2 = 0.80mol ÷ 2
=
0.40 mol
The concentration of I2 in solution A
= 0.40 mol ÷ 14ml
= 0.0286 M
Thus C =28.6
x10-3M
|
DISCUSSION
Adsorption
is the binding of molecules or particles onto a surface, it is different from absorption which is the filling of
pores in a solid. It is also a process in which a gas, liquid, or
solid adheres to the surface of a solid or (less frequently) a liquid but does
not penetrate it, such as in adsorption of gases by activated carbon (charcoal).
In comparison, a gas or liquid taken-in during absorption penetrates
or mixes with the absorbent.Binding of molecules or particles to the
surface is usually weak and reversible, but compounds with color and those that
have taste or odor tend to bind strongly. The most common industrial adsorbents
are activated carbon, silica gel, and alumina because they have enormous
surface areas per unit weight.
According to the theory,
the amount of a substance that can be adsorbed onto activated
charcoal depends on the nature of the substance and its concentration, the
surface structure of the activated carbon, the temperature and pH of the
water. For a treatment system with a specific type of carbon and a known
substance, there is a relationship between the amount of adsorbed matter per
unit of weight of carbon and the equilibrium concentration in the water in
which the temperature and pH are kept constant. This relationship is called an
isotherm. The shape of the isotherm can be described in various mathematical
ways. The most well-known is the Freundlich isotherm, the equation is as below:
where a and n are constants, the form 1/n being used to emphasize
that c is raised to a power less than unity. 1/n is dimensionless parameter and
is related to the intensity of drug adsorption. Method of B.E.T (Brunauer,
Emmett and Teller) has been used to calculate the surface area of charcoal by
using the adsorption of gas.
Throughout
the experiment, the surface area of activated charcoal sample is determined by
Langmuir equation and the interaction with iodine. Iodine is titrated with
sodium thiosulphate solution to determine the amount of iodine present. The
value of X, actual concentration of iodine in solution A and C, concentration
of iodine in solution A at equilibrium of each flask can be calculated by using
number of mole of iodine per volume of solution A.
From the graph of amount
of iodine adsorbed versus balance concentration of iodine, a curve graph is
obtained which shows that the number of iodine adsorbed is gradually increasing
in the solution. This phenomenon can be explained by solubility. For the iodine
to be adsorbed onto the activated charcoal, solute-solvent bonds between the
iodine and water must first be broken. Thus, it can be concluded that the
greater the solubility, the stronger are the solute-solvent bonds and hence the
smaller the extent of the adsorption of iodine onto the activated charcoal. For
the second graph of C/N versus C, a straight line should be obtained in this
graph according to Langmuir equation. The Langmuir isotherm was developed by
Irving Langmuir in 1916 to describe the dependence of the surface the coverage
or adsorption of molecules on a solid surface to gas
pressure or concentration of a medium above the solid surface at
a fixed temperature.
The surface area of charcoal calculated is deviated from the actual
value is due to the there are some errors existed during the experiment. It may
because of the inaccuracy titration of iodine with sodium thiosulphate
solution. We may take out some charcoal accidentally from the centrifuged tube
together with the solution that we needed in the titration. The sodium
thiosulphate may be adsorbed onto the surface of the activated charcoal. This
cause the activated charcoal does not achieved equilibrium with the solution
after shaking for 2 hours and thus will require more sodium thiosulphate
solution in the titration process compare to the actual volume that is needed.
Besides, when the solution is shaking for two hours in every 10 minutes, the
solution is not well swirled and the charcoal is not well distributed all over
the solution, therefore adsorption could not happen completely. In addition, we
may not weigh the 0.1g of activated charcoal accurately for every flask causing
the result of adsorption is not accurate. The solution also may not be
distributed evenly in each test tube which is supposed to be.
Equilibrium that has been reached after shaking for 2 hours can be
determine experimentally by observing and recording the temperature of the
solution. Since most of the adsorption is an exothermic reaction, temperature
will increase as the adsorption of adsorbate onto the adsorbent is occurred.
When the temperature remains the same, we can assume that it is at equilibrium
and adsorption occur at monolayer. At equilibrium, no more iodine molecules can
be adsorbed onto the activated charcoal since all the available surface areas
of the activated charcoal have been used up for the adsorption of the activated
charcoal. So, the equilibrium point is reached and we continue the titration
process.
QUESTIONS
1.
Calculate N for iodine in each flask.
The value of N is calculated by using the formula
N = (X
- C) x 50/1000 x 1/y
Where y = 0.1g
Flasks 1 and 7:
N = (X - C) x 50/1000 x 1/y
= (8.00x10-3- 2.86x10-3)
x (50/1000) x (1/0.1)
=
25.7 x 10-4mol
|
Flask 4 and 10:
N = (X - C) x 50/1000 x 1/y
= (21.20x10-3-10.70x10-3)
x (50/1000) x (1/0.1)
= 52.5 x 10-4mol
|
Flasks 2 and 8:
N = (X - C) x 50/1000 x 1/y
= (13.00x10-3-4.39x10-3)
x (50/1000) x (1/0.1)
= 43.1 x 10-4mol
|
Flask 5 and 11:
N = (X - C) x 50/1000 x 1/y
= (28.00x10-3-16.10x10-3)
x (50/1000) x (1/0.1)
= 59.5 x10-4 mol
|
Flasks 3 and 9:
N = (X - C) x 50/1000 x 1/y
|
Flask 6 and 12:
N = (X - C) x 50/1000 x 1/y
= (43.50x10-3-28.60x10-3)
x (50/1000) x (1/0.1)
= 74.5 x 10-4mol
|
Flasks
|
X (x10-3M)
|
C(x10-3M)
|
N(x10-4)
|
1 and 7
|
8.00
|
2.86
|
25.7
|
2 and 8
|
13.00
|
4.39
|
43.1
|
3 and 9
|
17.00
|
7.86
|
45.7
|
4 and 10
|
21.20
|
10.70
|
52.5
|
5 and 11
|
28.00
|
16.10
|
59.5
|
6 and 12
|
43.50
|
28.60
|
74.5
|
1.
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,
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/KNmis
obtained.
Graph N Versus C At Equilibrium
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.
CALCULATION
C (M)
|
N (mol)
|
C/N (M/mol)
|
2.86
x10-3
|
25.7
x10-4
|
1.113
|
4.39
x10-3
|
43.1
x10-4
|
1.019
|
7.86
x10-3
|
45.7
x10-4
|
1.720
|
10.70
x10-3
|
52.5
x10-4
|
2.038
|
16.10
x10-3
|
59.5
x10-4
|
2.706
|
28.60
x10-3
|
74.5
x10-4
|
3.839
|
C(x10-3M)
|
C/N (M/mol)
|
2.86
|
1.113
|
4.39
|
1.019
|
7.86
|
1.720
|
10.70
|
2.038
|
16.10
|
2.706
|
28.60
|
3.839
|
From the graph obtained, the gradient of the graph
=
= 112
Thus, 1/Nm = 112 and Nm
= 8.93x10-3mol g-1
No. of moles charcoal = 8.93x10-3mol g-1 x 0.1g
= 8.93x10-4 mol
No. of molecules = 8.93x10-4mol
x (6.023 x 1023)
= 5.378 x1020 molecules
Area covered = (5.378 x1020 )x (3.2 x 10-19)
= 172 m2
The surface area of charcoal = 172m2
÷ 0.1g
= 1720m2g-1
4. Discuss the results of
the experiment. How do you determine experimentally
that equilibrium has been reached after shaking for 2 hours?
From
the results of the experiment, the number of iodine adsorbed is gradually
increasing in the solution due to its solubility as the concentration of iodine
in solution at equilibrium keep on increasing. For the iodine to be adsorbed
onto the activated charcoal, solute-solvent bonds between the iodine and water
must first be broken. The greater the solubility, the stronger are the
solute-solvent bonds and hence the smaller the extent of the adsorption of
iodine onto the activated charcoal. The experiment is repeated and then it will
be titrated with sodium thiosulfate. If the volume obtained stay constant then
the equilibrium is reached.
CONCLUSION
The surface area of charcoal is 1720m2g-1
REFERENCES
1.
Florence A. T.
and Attwood D., Physicochemical Principles of Pharmacy, 1998 3rd
ed., Macmillan Press Ltd, London. Pages 204-236.
2.
Sinko P. J.,
2011, Martin’s Physical Pharmacy and Pharmaceutical Sciences 6th
ed., Lippincott Williams & Wilkins, a Wolters Kluwer Bussiness,
Philadelphia. Pages 85-91.
3.
Conner C.W., 1998,
Physical Adsorption : Experiment, Theory, and Application., Kluwer Academic
Publisher, Netherlands. Pages 98-123.
4.
A. Zangwill,
1988, Physics at surfaces,
Cambridge University Press. Pages 220-238.
5.
http://www.businessdictionary.com/definition/adsorption.html
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