DEFINITION OF ADSORPTION
Adsorption is a process that occurs when a gas or liquid solute accumulates on the surface
Adsorption is a process that occurs when a gas or liquid solute accumulates on the surface
of a solid or a liquid (adsorbent), forming a molecular or atomic film (the adsorbate). It is
different from absorption, in which a substance diffuses into a liquid or solid to form a
solution. The term sorption encompasses both processes, while desorption is the reverse
process.
OBJECTIVE
In this experiment, adsorption of iodine from solution is studied and Langmuir equation is used to estimate the surface area of activated charcoal sample.
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 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 thiosulphate solution, distilled water and activated charcoal.
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 thiosulphate solution, distilled water and activated charcoal.
Using burettes or
measuring cylinders, fill 12 conical flasks (labeled 1-12) 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)
Flask
|
Volume of solution A (ml)
|
Volume of solution B (ml)
|
1 and 7
|
10
|
40
|
2and 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) Add 1-2 drops of
starch solution as an indicator.
2) Titrate using 0.1 M
sodium thiosulfate solution until the colour of the
The colour of the solution changes from dark blue to colourless.
3) Record the volume of
the sodium thiosulphate used.
Set 2: Concentration of
iodine in solution A at equilibrium (C).
For
flasks 7-12:
1) Add 0.1g activated
charcoal.
2) Cap the flasks
tightly. Swirl or shake the flask every 10 minutes for 2 hours.
3) After 2 hours,
transfer the solutions into centrifuge tubes and label them accordingly.
4) Centrifuge the
solutions at 3000 rpm for 5 minutes and transfer the resulting supernatant into
new conical flasks. Label each conical flask accordingly.
5) Repeat steps 1,2 and
3 as carried out for flasks 1-6 in Set 1.
GENERAL
NOTES:
|
|
Titration equation:
I2 + 2Na2S203
= Na2S4O6 + 2NaI
Na2S2O3
= ½ I2
|
|
Given:
1 mole iodine = 2 x
126.9 g
1 ml 0.1 M Na2S2O3 =
0.01269 gI
|
(1 mole Na2S2O3 =½ mole I2
mole I2)
|
If the amount of
activated charcoal used is Y gram,
therefore the total mole of iodine
|
|
adsorbed by 1 g of
activated charcoal (N) is given by the following equation:
|
|
N
= (X-C) x 50/1000 x 1/y
|
|
Results:
Flask
|
Volume of Na2S2O3 (mL)
|
1
|
8.7
|
2
|
12.6
|
3
|
17.6
|
4
|
21.2
|
5
|
26.1
|
6
|
43.6
|
7
|
7.5
|
8
|
8.3
|
9
|
14.8
|
10
|
18.6
|
11
|
18.6
|
12
|
38.4
|
DISCUSSION:
According to the Free Online Dictionary definition, adsorption is a process that occurs when a gas or liquid solute accumulates on the surface of an adsorbent solid or a liquid, forming a molecular or atomic film which is known as the adsorbate. The adsorption from liquid phase can take place in liquid-solid interface, liquid-liquid interface and liquid-vapour interface. Based on this experiment, the adsorption from liquid phase is carried out in the liquid-solid interface. Activated charcoal is used as the adsorbent in this experiment. The Langmuir isotherm is used to calculate the surface area of the charcoal and the calculations are shown below. The Langmuir isotherm is an isotherm derived from a kinetic mechanism, which is based on four hypotheses. The hypotheses are the surface of the adsorbent is uniform, adsorbed molecules do not interact with one another, all adsorption occurs through the same mechanism and at the maximum adsorption, only a monolayer is formed, whereby the molecules of adsorbate do not deposit on other and only presence on the free surface of the adsorbent.
QUESTIONS:
According to the Free Online Dictionary definition, adsorption is a process that occurs when a gas or liquid solute accumulates on the surface of an adsorbent solid or a liquid, forming a molecular or atomic film which is known as the adsorbate. The adsorption from liquid phase can take place in liquid-solid interface, liquid-liquid interface and liquid-vapour interface. Based on this experiment, the adsorption from liquid phase is carried out in the liquid-solid interface. Activated charcoal is used as the adsorbent in this experiment. The Langmuir isotherm is used to calculate the surface area of the charcoal and the calculations are shown below. The Langmuir isotherm is an isotherm derived from a kinetic mechanism, which is based on four hypotheses. The hypotheses are the surface of the adsorbent is uniform, adsorbed molecules do not interact with one another, all adsorption occurs through the same mechanism and at the maximum adsorption, only a monolayer is formed, whereby the molecules of adsorbate do not deposit on other and only presence on the free surface of the adsorbent.
QUESTIONS:
Molecular weight of iodine, I2 : 253.8 gmol-1
1. Calculate N for iodine in each flask.
N=(X-C)x50/1000x1/y
(y=0.1g)
ml 0.1M Na2S2O3=
0.01269g I
No. mole = Mass / Molecular weight
Concentration of iodine in solution A,(X)
|
Concentration of iodine in solution A,(C)
|
N=(X-C)x50/1000x1/y (y=0.1g)
|
Flask 1
Mole of
iodine = 8.7 ml x 0.01269 gml-1/253.8 gmol-1
= 4.35 x 10-4 mol
X= 4.35
x 10-4 mol / (50 ml/1000 ml)
= 0.0087 M
|
Flask 7
Mole of
iodine = 7.5 ml x 0.01269 gml-1/253.8 gmol-1
= 3.75 x 10-4 mol
C= 3.75
x 10-4 mol / (50 ml/1000 ml) = 0.0075 M
|
For flask 1 and 7
N=
(0.0087 -0.0075) x 50/1000 x 1/0.1
= 0.0006 molg-1
|
Flask 2
Mole of
iodine = 12.6 ml x 0.01269 gml-1/253.8 gmol-1
= 6.30 x 10-4 mol
X=6.30 x
10-4 mol / (50 ml/1000 ml)
= 0.0126
M
|
Flask 8
Mole of
iodine = 8.3 ml x 0.01269 gml-1/253.8 gmol-1
= 4.15 x 10-4 mol
C= 4.15
x 10-4 mol / (50ml/1000 ml)
= 0.0083 M
|
For flask 2 and 8
N=
(0.0126-0.0083) x 50/1000 x 1/0.1
= 0.00845 molg-1
|
Flask 3
Mole of
iodine = 17.6 ml x 0.01269 gml-1/253.8 gmol-1
= 8.8 x 10-4 mol
X= 8.8 x
10-4 mol / (50 ml/1000 ml)
= 0.0176 M
|
Flask 9
Mole of
iodine = 14.8 ml x 0.01269 gml-1/253.8 gmol-1
= 7.4 x 10-4 mol
C= 7.4 x
10-4 mol / (50 ml/1000 ml)
= 0.0148 M
|
For flask 3 and 9
N=
(0.0176-0.0148) x 50/1000 x 1/0.1
= 1.4 x 10-3 molg-1
|
Flask 4
Mole of
iodine = 21.2 ml x 0.01269 gml-1/253.8 gmol-1
= 1.06 x 10-3 mol
X= 1.06x
10-3 mol / (50 ml/1000 ml)
= 0.0212 M
|
Flask 10
Mole of
iodine = 18.6 ml x 0.01269 gml-1/253.8 gmol-1
= 9.3 x 10-4 mol
C= 9.3 x
10-4 mol / (50 ml/1000 ml)
= 0.0186 M
|
For flask 4 and 10
N=
(0.0212-0.0186) x 50/1000 x 1/0.1
= 1.3 x 10-3 molg-1
|
Flask 5
Mole of
iodine = 26.1 ml x 0.01269 gml-1/253.8 gmol-1
= 1.305 x 10-3 mol
X= 1.305
x 10-3 mol / (50 ml/1000 ml)
= 0.0261 M
|
Flask 11
Mole of
iodine = 18.6 ml x 0.01269 gml-1/253.8 gmol-1
= 9.3 x 10-4 mol
C= 9.3 x
10-4 mol / (50 ml/1000 ml)
= 0.0186 M
|
For flask 5 and 11
N=
(0.0261-0.0186) x 50/1000 x 1/0.1
= 3.75 x 10-3 molg-1
|
Flask 6
Mole of
iodine = 43.6 ml x 0.01269 gml-1/253.8 gmol-1
= 2.18 x 10-3 mol
X= 2.18
x 10-3 mol / (50 ml/1000 ml)
= 0.0436M
|
Flask 12
Mole of
iodine = 38.4 ml x 0.01269 gml-1/253.8 gmol-1
=1.92 x 10-3 mol
C= 1.92
x 10-3 mol / (50 ml/1000 ml)
= 0.0384M
|
For flask 6 and 12
N=
(0.0436-0.0384) x 50/1000 x 1/0.1
= 2.6 x 10-3 molg-1
|
2) Plot amount of
iodine adsorbed (N) versus balance concentration of solution (C) at equilibrium
to obtain adsorption isotherm.
Flasks
|
X (M)
|
C (M)
|
Y (g)
|
N (mol)
|
1 and 7
|
0.0087
|
0.0075
|
0.1
|
0.0006
|
2 and 8
|
0.0126
|
0.0083
|
0.1
|
0.00845
|
3 and 9
|
0.0176
|
0.0148
|
0.1
|
1.4 x 10-3
|
4 and 10
|
0.0212
|
0.0186
|
0.1
|
1.3 x 10-3
|
5 and 11
|
0.0261
|
0.0186
|
0.1
|
3.75 x 10-3
|
6 and 12
|
0.0436
|
0.0384
|
0.1
|
2.6 x 10-3
|
 |
| click to enlarge |
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
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.
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.
Concentration Of Solution,
C (M)
|
Amount Of Iodine Adsorbed,
N (molg-1)
|
C/N
(M/molg-1)
|
0.0075
|
0.0006
|
12.500
|
0.0083
|
0.00845
|
0.982
|
0.0148
|
1.4 x 10-3
|
10.571
|
0.0186
|
1.3 x 10-3
|
14.307
|
0.0186
|
3.75 x 10-3
|
4.96
|
0.0384
|
2.6 x 10-3
|
14.770
|
 |
| click to enlarge |
Calculate the surface area of
charcoal in m2g-1.
From the graph the gradient
is used to determine the number of mole per gram charcoal required (Nm)
Gradient, m
= (10.571-0) M/molg-1 ÷ (0.0148-0) M
=
714.26 gmol-1
In general the gradient is
equal to 1/ Nm, therefore
1/Nm = 714.26
Nm = 1/714.26
= 1.4x10-3molg-1
From the Nm 1.4x10-3molof iodine is adsorbed in 1
g of charcoal. Therefore, the number of mol of iodine molecules that adsorbed
on the monomolecular layer is
= 1.4x10-3mol/1g =1.4x103mol.
= 1.4x10-3mol/1g =1.4x103mol.
The number of
molecules of the iodine molecules
= number of
mole of iodine molecules x Avogadro no.
=1.4x10-3mol
x 6.023x 1023
=8.4325 x 1020
molecules of iodine
Assume the surface covered by
one adsorbed molecules is 3.2x 10-19 m2
1 molecules of iodine = 3.2x
10-19 m2 adsorbed on the charcoal layer.
Therefore, 8.4325
1020 molecules of iodine
= (3.2x 10-19
m2) x (8.4325 x 1020)
= 269.84 m2g-1
adsorbed on the charcoal layer
4) Discuss the results
of the experiment. How do you determine experimentally that equilibrium has
been reached after shaking for 2 hours?
REFERENCES
The way to determine that the
equilibrium has been reached after shaking for 2 hours is by observing the
colour changes in the flask. The iodine will undergo colour change from dark
brown to light brown at certain period of time after the flask is shook for
every 10 minutes for 2 hours. The light brown colour of the iodine will not
change until the end of the reaction and this marks the equilibrium point of
the reaction. There is a change in the colour of the iodine throughout the
reaction due to the adsorption of iodine by the activated charcoal. The equilibrium
of the reaction can also be determined by observing the colour change of the sodium
thiosulfate solution using titration method. The sodium thiosulfate solution
will change colour from dark blue to colourless to mark the point of
equilibrium of the reaction.
Based
on results obtained from the experiment, there are many errors that cause the
results of the experiment to be lack of accuracy. Firstly, there are parallax
error that occurs when recording the readings from the burette. This is due to
the wrong position of the eye to the meniscus level of the solution. Secondly, the
time taken for the flask to be shaken for every 10 minutes for two hours is
shorten to one hour and 30 minutes only. This will definitely affect the
accuracy of the readings obtained in the experiment. Moreover, there a few
person who are in charge of shaking all the flasks at all time which have
different speed and strength in shaking the flasks. This will cause the consistency
of the readings to be affected. The activated charcoal maybe accidentally
spilled when the charcoal is added to the flask. This will affect the readings
obtained from this experiment. The centrifuge tubes and the flasks may not be
capped tightly and this also causes the spillage of the solution.
PRECAUTIONS
1. We must make sure that our eyes are
directly perpendicular on the meniscus of the solution when we are recording
our readings. This is to prevent parallax errors from occurring.
2. The flasks have to be shaken for every 10 minutes for 2 hours instead of shaking the flasks for 1 hour and 30 minutes only. This is to improve the accuracy of readings obtained in the experiment.
2. The flasks have to be shaken for every 10 minutes for 2 hours instead of shaking the flasks for 1 hour and 30 minutes only. This is to improve the accuracy of readings obtained in the experiment.
3. The same person must be in charge to
shake all the flasks at all time so that the speed and the strength used to
shake the flasks are the same. This will improve the consistency of the
readings obtained.
4. The flasks have to be capped tightly
to prevent the spillage of solution. The spillage of solution will affect the
accuracy of readings obtained.
5. The same person must be in charge of
titrating the volume of thiosulphate solution in each of the conical flask so
that the change in the colour of the solution can be determined by the same person.
This is to ensure that accuracy of readings is improved.
6. A filter funnel must be used to add
the activated charcoal into the flask respectively. This is to prevent the
activated charcoal from spill and increases the accuracy of the results obtained.
CONCLUSION
The purpose of carrying out this experiment is to identify the surface area of the charcoal by studying the adsorption of iodine from solution process. The Langmuir equation is used to calculate the surface area of the charcoal. The Nm value is 1.4x103 g mol-1. The number of molecules of iodine adsorbed onto the monomolecular layer is 8.4325 x 1020 molecules of iodine. The surface area of charcoal is 269.84 m2g-1.
The purpose of carrying out this experiment is to identify the surface area of the charcoal by studying the adsorption of iodine from solution process. The Langmuir equation is used to calculate the surface area of the charcoal. The Nm value is 1.4x103 g mol-1. The number of molecules of iodine adsorbed onto the monomolecular layer is 8.4325 x 1020 molecules of iodine. The surface area of charcoal is 269.84 m2g-1.
REFERENCES
- Florence Alexander T and Attwood David. 2006. Physicochemical Principles of Pharmacy. Fourth Edition. MacMillan Press Ltd. Great Britain. Page 194 until 200
- Aulton M. E. 2002. Pharmaceutics The Science Of Dosage Form Design. Second Edition. Churchhill Livingstone Press. Spain. Page 65 until 68
- http://en.wikipedia.org/wiki/Adsorption
- http://www.rpi.edu/dept/chem-eng/Biotech-Environ/Adsorb/adsorb.htm
- http://www.fpharm.uniba.sk/fileadmin/user_upload/english/Physical_Chemistry/5-Adsorption.pdf
- http://www.le.ac.uk/chemistry/thermodynamics/pdfs/500/Topic0140.pdf
- http://www.scribd.com/doc/33000065/Adsorption-in-Physical-Pharmacy
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