27 March 2014
Objective:
To study the adsorption of iodine from solution and
estimate the surface area of activated charcoal sample by using Langmuir
equation.
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 and dissolved solutes).
It is recognised as chemical adsorption or chemisorptions
when adsorption involves only chemical bonds between adsorbent and adsorbate.
It requires activation energy which 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, 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). For a particular adsorbent or adsorbate, the degree of adsorption
at a specified temperature depends on the partial pressure of a gas or on
concentration of adsorbate for adsorption from solution. Adsorption isotherm is
the relationship between the degree of adsorption and partial pressure or
concentration.
In this experiment, we use activated
charcoal sample as adsorbent and iodine as adsorbate. Adsorption of iodine will
determine the surface area of activated charcoal. This method can be applied in
determination of the surface area of powder drug in pharmaceutical industries.
The surface area is very important in field of pharmacy as it is one of factor
that affects the rate of dissolution and bioavailability of drugs that are
absorbed in gastrointestinal tract.
Materials
and apparatus:
12 conical flask, 6 centrifuge tubes,
measuring cylinder, analytical balance, Beckman J6M/E centrifuge, burettes,
retort stand and clamps, pasteur pipettes, 0.05 M iodine solution, 0.1 M
Potassium iodine, 1% w/v starch solution, 0.1 M sodium thiosulphate solution,
distilled water and activated charcoal.
Procedures
12 conical flasks which labeled 1 to 12
was filled with 50 ml mixture of 0.05 M Iodine (solution A) and 0.1 M Potassium
iodine ( solution B) by using measuring cylinder as stated in the table 1.
Table
1:
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.
1-2 drops of starch solution
was added into the conical flask as an indicator.
2.
The solution was then
titrated with 0.1 M sodium thiosulphate solution until the colour of 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).
For flask 7-12:
1.
0.1 g activated charcoal wad
added into the conical flask and the flask was tightly cap and was swirl or
shake every 10 minutes for 2 hours.
2.
The solutions were
transferred into the centrifuge tubes and labeled accordingly after 2 hours.
3.
The solutions were then
centrifuge at 3000 rpm for 5 minutes and the resulting supernatants were
transferred into new conical flask accordingly.
4.
Steps 1, 2, and 3 were
repeated for flask 1-6 in Set 1.
Results:
Flask
|
Volume of solution A (mL)
|
Volume of solution B ( mL)
|
Volume of sodium thiosulphate, Na2S2O3
needed (X mL)
|
1
|
10
|
40
|
11.5
|
2
|
15
|
35
|
14.5
|
3
|
20
|
30
|
19.0
|
4
|
25
|
25
|
24.8
|
5
|
30
|
20
|
29.2
|
6
|
50
|
0
|
46.4
|
Flask
|
Volume of solution A (mL)
|
Volume of solution B( mL)
|
Volume of sodium thiosulphate, Na2S2O3
needed (C mL)
|
7
|
10
|
40
|
1.2
|
8
|
15
|
35
|
1.6
|
9
|
20
|
30
|
2.2
|
10
|
25
|
25
|
3.0
|
11
|
30
|
20
|
3.6
|
12
|
50
|
0
|
8.8
|
Flask
|
Actual Concentration, X (M)
|
Flask
|
Concentration at equilibrium, C(M)
|
1
|
0.0115
|
7
|
0.0012
|
2
|
0.0145
|
8
|
0.0018
|
3
|
0.0190
|
9
|
0.0022
|
4
|
0.0248
|
10
|
0.0030
|
5
|
0.0292
|
11
|
0.0038
|
6
|
0.0464
|
12
|
0.0088
|
Titration Equation:
I₂ + 2Na₂S₂O₃ =
Na₂S₄O₆ + 2 NaI
Na₂S₂O₃= ½ I₂
Na₂S₂O₃= ½ I₂
1 mole Na₂S₂O₃= ½
mole I₂
1 mole iodine = 2 x 126.9g mole I2
1 mL 0.1 M Na₂S₂O₃=0.01269g
I
Flask
1:
From
the result:
Volume of Na2S2O3 used = 11.5 mL
Number of moles of Na2S2O3 =11.5 x 0.1/1000
= 1.15 x 10-3 mol
Number of moles of I2= 1.15 x 10-3
mol ÷ 2
= 5.75 x 10-4 mol
Concentration of I2 in solution A(X)
=mole/volume
=5.75
x 10-4 mol ÷ 0.05 L
=0.0115M
|
Flask
2:
From
the result:
Volume of Na2S2O3 used = 14.5 mL
Number of moles of Na2S2O3 =14.5 x 0.1/1000
= 1.45 x 10-3
mol
Number of moles of I2= 1.45 x 10-3
mol ÷ 2
=
7.25 x 10-4 mol
Concentration of I2 in solution A(X)
=mole/volume
=7.25x
10-4 mol ÷ 0.05 L
=0.0145M
|
Flask
3:
From
the result:
Volume of Na2S2O3 used = 19.0 mL
Number of moles of Na2S2O3 =19.0 x 0.1/1000
= 1.90 x 10-3
mol
Number of moles of I2= 1.90 x 10-3
mol ÷ 2
=
9.50 x 10-4 mol
Concentration of I2 in solution A(X)
=mole/volume
=9.50
x 10-4 mol ÷ 0.05 L
=0.0190M
|
Flask
4:
From
the result:
Volume of Na2S2O3 used = 24.8 mL
Number of moles of Na2S2O3 =24.8x 0.1/1000
= 2.48 x 10-3
mol
Number of moles of I2= 2.48 x 10-3
mol ÷ 2
=
1.24 x 10-3mol
Concentration of I2 in solution A(X)
=mole/volume
=1.24
x 10-3 mol ÷ 0.05 L
=0.0248M
|
Flask
5:
From
the result:
Volume of Na2S2O3 used = 29.2 mL
Number of moles of Na2S2O3 =29.2 x 0.1/1000
= 2.92 x 10-3
mol
Number of moles of I2= 2.92 x 10-3
mol ÷ 2
=
1.46 x 10-3mol
Concentration of I2 in solution A(X)
=mole/volume
=1.46
x 10-3 mol ÷ 0.05 L
=0.0292M
|
Flask
6:
From
the result:
Volume of Na2S2O3 used = 46.4 mL
Number of moles of Na2S2O3 =46.4 x 0.1/1000
= 4.64 x 10-3
mol
Number of moles of I2= 4.64 x 10-3
mol ÷ 2
=
2.32 x 10-3mol
Concentration of I2 in solution A(X)
=mole/volume
=2.32
x 10-3 mol ÷ 0.05 L
=0.0464M
|
Flask
7:
From
the result:
Volume of Na2S2O3 used = 1.2 mL
Number of moles of Na2S2O3 =1.2 x 0.1/1000
= 1.2 x 10-4
mol
Number of moles of I2= 1.2 x 10-4
mol ÷ 2
=
6.0 x 10-5mol
Concentration of I2 in solution A(C)
=mole/volume
=6.0 x
10-5 mol ÷ 0.05 L
=1.2
x 10-3 M
|
Flask
8:
From
the result:
Volume of Na2S2O3 used = 1.6 mL
Number of moles of Na2S2O3 =1.6 x 0.1/1000
= 1.6 x 10-4
mol
Number of moles of I2= 1.6 x 10-4
mol ÷ 2
=
8.0 x 10-5mol
Concentration of I2 in solution A(C)
=mole/volume
=8.0 x
10-5 mol ÷ 0.05 L
=1.6
x 10-3 M
|
Flask
9:
From
the result:
Volume of Na2S2O3 used = 2.2 mL
Number of moles of Na2S2O3 =2.2 x 0.1/1000
= 2.2x 10-4
mol
Number of moles of I2= 2.2 x 10-4
mol ÷ 2
=
1.1 x 10-5mol
Concentration of I2 in solution A(C)
=mole/volume
=1.1 x
10-5 mol ÷ 0.05 L
=2.2 x
10-3 M
|
Flask
10:
From
the result:
Volume of Na2S2O3 used = 3.0 mL
Number of moles of Na2S2O3 =3.0 x 0.1/1000
= 3.0 x 10-4
mol
Number of moles of I2= 3.0 x 10-4
mol ÷ 2
=
1.5 x 10-4mol
Concentration of I2 in solution A(C)
=mole/volume
=1.5 x
10-4 mol ÷ 0.05 L
=3.0
x 10-3 M
|
Flask
11:
From
the result:
Volume of Na2S2O3 used = 3.6 mL
Number of moles of Na2S2O3 =3.6 x 0.1/1000
= 3.6 x 10-4
mol
Number of moles of I2= 3.6 x 10-4
mol ÷ 2
=
1.8 x 10-4mol
Concentration of I2 in solution A(C)
=mole/volume
=1.8 x
10-4 mol ÷ 0.05 L
=3.6
x 10-3 M
|
Flask
12:
From
the result:
Volume of Na2S2O3 used = 8.8 mL
Number of moles of Na2S2O3 =8.8 x 0.1/1000
=8.8 x 10-4
mol
Number of moles of I2= 8.8 x 10-4
mol ÷ 2
=
4.4 x 10-4mol
Concentration of I2 in solution A(C)
=mole/volume
=4.4 x
10-4 mol ÷ 0.05 L
=8.8
x 10-3 M
|
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 = amount of activated charcoal used
=0.1g
Flask 1 and 7:
N= (0.0115-0.0012) x 50/1000 x 1/0.1
= 5.15 x 10-3 mol/g |
Flask 2 and 8:
N= (0.0145-0.0016) x 50/1000 x 1/0.1
= 6.35 x10-3 mol/g |
Flask 3 and 9:
N= (0.0190-0.0022) x 50/1000 x 1/0.1
=8.40 x 10-3 mol/g |
Flask 4 and 10:
N= (0.0248-0.0030) x 50/1000 x 1/0.1
=1.09 x 10-2 mol/g |
Flask 5 and 11:
N= (0.0292-0.0036) x 50/1000 x 1/0.1
=1.27 x 10-2 mol/g |
Flask 6 and 12:
N= (0.0464-0.0088) x 50/1000 x 1/0.1
= 1.88 x 10-2 mol/g |
2. Plot amount of iodine adsorbed (N) versus balance
concentration of solution (C) at equilibrium to obtain adsorption isotherm.
Flask
|
Concentration of
solution, C (M)
|
Amount of iodine
adsorbed (mol/g)
|
1
and 7
|
0.0012
|
0.0052
|
2
and 8
|
0.0018
|
0.0064
|
3
and 9
|
0.0022
|
0.0084
|
4
and 10
|
0.0030
|
0.0109
|
5
and 11
|
0.0036
|
0.0127
|
6
and 12
|
0.0058
|
0.0188
|
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. 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 3.2x10-19 m2, Avogadro number= 6.023x1023 molecule, calculate the surface area of charcoal in m2g-1.
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. 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 3.2x10-19 m2, Avogadro number= 6.023x1023 molecule, calculate the surface area of charcoal in m2g-1.
Flask
|
Concentration of
solution, C (M)
|
Amount of iodine
adsorbed (mol/g)
|
C/N (1/L)
|
1
and 7
|
0.0012
|
0.0052
|
0.2308
|
2
and 8
|
0.0016
|
0.0064
|
0.2500
|
3
and 9
|
0.0022
|
0.0084
|
0.2619
|
4
and 10
|
0.0030
|
0.0109
|
0.2752
|
5
and 11
|
0.0036
|
0.0127
|
0.2834
|
6
and 12
|
0.0058
|
0.0188
|
0.3085
|
Value of 1/KNm (y-int.) = 0.22
Value of Nm :
1/Nm
= gradient of graph
=(y2-y1)/(x2-x1)
=(0.2834-0.2500) / (0.0036-0.0016)
=(0.2834-0.2500) / (0.0036-0.0016)
=16.7
Nm =0.05988 mol g-1
No. of molecules of charcoal
= Nm x Avogadro no.
= 0.05988 mol g-1 x (6.023x1023 molecules per mole)
= 3.607 x1021molecules g-1
= 0.05988 mol g-1 x (6.023x1023 molecules per mole)
= 3.607 x1021molecules g-1
Surface area of charcoal
= (3.2 x 10-19 m2 molecules-1) x (3.607 x1021
molecules g-1)
= 1154.24 m2 g-1
4. How do you determine experimentally that equilibrium has been reached after shaking for two hours?
After been shaken for two hours, the solution has been titrated by using starch solution as indicator. By referred to the flask labelled 1 to 6, the volume of Sodium thiosulphate, Na2S2O3 used can be compared with the flask labelled 7 to 12. The solution will change colour from dark blue to colourless. Equilibrium was reached when the solution becomes homogenous and there is no more colour changed.
= 1154.24 m2 g-1
4. How do you determine experimentally that equilibrium has been reached after shaking for two hours?
After been shaken for two hours, the solution has been titrated by using starch solution as indicator. By referred to the flask labelled 1 to 6, the volume of Sodium thiosulphate, Na2S2O3 used can be compared with the flask labelled 7 to 12. The solution will change colour from dark blue to colourless. Equilibrium was reached when the solution becomes homogenous and there is no more colour changed.
DISCUSSION
Adsorption is the sticking of molecules from the gas or liquid phase onto the surface of a solid and it is different from absorption which is the filling of pores in a solid. A molecule that undergoes adsorption is referred to as the adsorbate, and the solid is the adsorbent.
Adsorbents are used usually in the form of spherical pellets, rods, moldings, or monoliths. They must have high abrasion resistance, high thermal stability and small pore diameters, which results in higher exposed surface area and hence high surface capacity for adsorption. The adsorbents must also have a distinct pore structure that enables fast transport of the gaseous vapours. The most common industrial adsorbents are activated carbon, silica gel, and alumina, because they present enormous surface areas per unit weight.
The amount of a substance that can be adsorbed onto activated charcoal depends on nature of adsorbate and adsorbent, the surface area of adsorbent, activation of adsorbent and experimental conditions such as temperature. 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. Adsorption process is usually studied through graphs known as adsorption isotherm.
In this experiment, Langmuir equation is used to estimate the surface area of activated charcoal sample. Langmuir derived an equation which explained the relationship between the number of active sites of the surface undergoing adsorption and pressure. This equation is called Langmuir isotherm equation.
C/N = (1/Nm) C + (1/kNm)
Where (1/Nm) is the slope, and (1/kNm) is the intercept, when C/N is plotted versus the concentration C. The inverse of the slope is Nm and this represents the moles adsorbed at monolayer coverage. Nm can be used to determine the specific surface area of a solid. The basic limitation of Langmuir adsorption equation is that it is valid at low pressure only.
The actual concentration in solution A (X) and concentration of iodine in solution A at equilibrium was recorded and amount of iodine adsorb (N) was calculated from the data that was collected. Graph amount of iodine adsorb (N) versus balance concentration of solution (C) at equilibrium is not linear graph and shows that the number of iodine adsorb gradually increase in the solution. This is because 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 linear plot graph with slope of 1/Nm and intercept of 1/KNm was obtained from graph C/N versus C. This shows that the Langmuir equation is followed.
Based
on the results, the volume of sodium thiosulphate that used for flask 7 to 12 (with
activated charcoal) are less than flask 1 to 6 (without activated charcoal). Activated
charcoal is useful in attracting non-polar adsorbates. Activated charcoal has
enormous surface area because it is extremely porous. A huge surface area of
activated charcoal will be a target for the iodine molecules to be attracted
and holding it within its pores by a process called adsorption. Therefore, this
will reduce the amount of iodine present in the solution which will react with
added sodium thiosulphate.
The possible errors that occur while conducting this experiment is the amount of sodium thiosulphate titrated recorded are inaccurate because the position of eyes are not perpendicular to the meniscus so it will alter the results of the experiment. In addition, the amount of charcoal added is more than needed and not distributed equally over the solution. This causes the solution does not achieved equilibrium. Next, the charcoal maybe included in supernatant and that will affect the amount of sodium thiosulphate needed to change colour from dark blue to colourless.
The possible errors that occur while conducting this experiment is the amount of sodium thiosulphate titrated recorded are inaccurate because the position of eyes are not perpendicular to the meniscus so it will alter the results of the experiment. In addition, the amount of charcoal added is more than needed and not distributed equally over the solution. This causes the solution does not achieved equilibrium. Next, the charcoal maybe included in supernatant and that will affect the amount of sodium thiosulphate needed to change colour from dark blue to colourless.
There
are a few precaution steps that need to be taken. Firstly, we must avoid the
parallax error to ensure the accuracy of results. The conical flask need to be
swirled continuously when titrated with sodium thiosulphate solution to ensure
complete reaction occur. The starch indicator should be added close to the end
point to give a sharp end point to avoid the formation of excess starch-iodine
complex, which would be difficult to decompose. In addition, we need to avoid direct contact of sodium thiosulphate with skin
as it may irritate the skin and avoid inhaling the iodine as it is harmful for
the respiratory system.
Conclusion
Conclusion
From this experiment, the adsorption of iodine solution in charcoal follows the Langmuir theory of adsorption isotherm. The result shows that the adsorption increase as the concentration of the iodine solution increase. From the experimental result, the surface area of charcoal is 1154.24 m2 g-1.
References
1. Patrick J. Sinko, Lippincott Williams and Wilkins. Martin’s Physical Pharmacy and Pharmaceutical Sciences (page 39-40), 5th Edition.
2. Alexander T Florence and David Attwood. (2006). Physiocochemical Principles of Pharmacy (page 194-197). 4th Ed. Palgrave. USA. 3.
3.Alfonso
R.Gennaro al. 1995; Remington: The
Science & Practice of Pharmacy; 19th edition (page 246); Mack Publishing Company
Easton, Pennsylavania.
Appendices
 |
| During the experiment |
 |
| Charcoal mixture after centrifuge |
 |
| Working hard to obtain the best results |
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