Date:
25 March 2014
Objectives:
To determine the surface area of activated charcoal via adsorption
from solution.
Introduction
Adsorption
occurs when particles which are ion, atom or molecules on the surface of solids
are capable of attracting other molecules due to the instability of energies
around the particles. The energy here refers to electrostatic, valency or Van
de Waals. Meanwhile, absorption is the process in
which a fluid is dissolved by a liquid or a solid. In general, adsorption is a
surface-based process where a film of adsorbate is created on the surface while
absorption involves the entire volume of the absorbing substance.
The
nature of forces between adsorbent (solid surface) and adsorbate (gas or
dissolved solutes) influence the strength of bonds. Adsorption consists of two
types which are chemisorption and physical adsorption. Chemisorption involves
only chemical bonds between adsorbent and adsorbate which acquire activation
energy that can be strong and not readily reversible. Physical adsorption
occurs when adsorbent and adsorbatereact due to Van de Waals force which is
non-specific and occur at any condition.
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 where 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 flasks, 6 centrifuge tubes, measuring
cylinders, analytical balance, Beckman J6M/E centrifuge, burettes, retort stand
and clamps, paster pipettes, iodine solutions, 1%w/v starch solution, 0.1M
sodium thiosulphate solution, distilled water and activated charcoal.
Procedure:
1. 12 conical
flasks are filled with 50ml mixtures of iodine solutions (A and B) as stated in
Table 1 by using burettes or measuring cylinders.
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
|
Table 1
Set 1:
Actual concentration of iodine in solution A (X)
For
flask 1-6
1. 1-2 drops of
starch solution are added as an indicator.
2. The solution
is then titrated by using 0.1M sodium thiosulphate solution until the colour of
the solution change from dark to colorless.
3. The volume of
the sodium thiosulphate used is recorded
Set 2: Concentration of iodine in solution A at equilibrium
Set 2: Concentration of iodine in solution A at equilibrium
For flask 7-12
1. 0.1 g of activated
charcoal is added into the flasks.
2. The flasks are
capped tightly. Then, every 10 minutes the flask is swirled or shacked for 2
hours.
3. After 2 hours,
the solutions are transferred into centrifuge tubes and labeled.
4. The solution
is centrifuge at 3000 rpm for 5 minutes and the resulting supernatant is
transferred into new conical flask. The conical flask is labeled accordingly.
5. Steps 1.2 and
3 are repeated as carried out for flasks 1-6 in Set1.
Result
QUESTION:
1. Calculate N
for iodine in each flask.
2. Plot amount of iodine adsorbed (N) versus balance concentration of
solution (C) at equilibrium to obtain adsorption isotherm.
Graph of Amount of Iodine Adsorbed (N) versus Balance Concentration
of Solution at Equilibrium (C)
3. According to Langmuir theory, if there is no more than a monolayer of iodine adsorbed on the charchoal,
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 3.2 x 10-19m2.
Avogadro number = 6.023 x 1023 molecule, calculate the
surface area of charcoal in m2g-1.
Answer:
C/N (M/molg-1)
|
C (M)
|
0.1957
|
0.0009
|
0.2034
|
0.0024
|
0.2086
|
0.0034
|
0.1905
|
0.0040
|
0.1862
|
0.0046
|
0.2190
|
0.0090
|
Slope = 1/Nm =
= 2.419
Y - intercept = 0.191
Therefore, to obtain the
value of Nm:
1/Nm = 2.419
Nm = 1 ÷ 2.419
= 0.4134 molg-1
To calculate the iodine
molecule adsorbed on the monomolecular layer:
We know that Nm = 0.4134
molg-1
That’s mean in 0.1434 mol
of iodine, 1g of charcoal is adsorbed. Therefore, the number of mol of iodine
molecules that adsorbed on the monomolecular layer is:
Avogadro number = 6.023 x
1023
Numbers of molecules =
number of moles x Avogadro number
=
0.1434 mol x 6.023 x 1023
= 8.64 x 1022
molecules/g
Area covered by one
adsorbed molecules is 3.2 x 10-19 m2
Therefore, the surface
area of charcoal:
Surface area of charcoal =
(8.64 x 1022 molecules/g) x (3.2 x 10-19
m2)
=27648 m2g-1
4. Discuss the result of the experiment. How do you determine
experimentally that equilibrium has been reached after shaking for 2 hours?
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.
Repeat the experiment and
titrate with sodium thiosulphate. If the volume stays constant then equilibrium
is reached.
DISCCUSION:
Determination of the surface area of
powder drug, which is related to its particles size, is important in pharmacy
area. As stated earlier in the introduction, surface area is one of the factors
that govern the rate of dissolution and bioavailability of the drugs that are
absorbed through the gastrointestinal tract.
This experiment, adsorption of iodine
from solution used to determine the measurement of surface area of the solid.
Langmuir equation is used to estimate the surface area of activated charcoal
sample.
Cf/N = (1/Nm) Cf + (1/kNm)
In the set 1 experiment,to calculate the actual
concentration of iodine in solution A (X), titration method was used to
calculate the concentration of iodine. This is because the iodide ion and
iodine molecule are in equilibrium in the conical flask. Starch is used as an
indicator in the titration. The solution turn dark blue colour when starch is
added as iodine molecules are present. Then, when sodium thiosulfate is added,
the iodine molecule react with sodium thiosulfate to form sodium iodide. When
there is totally no iodine molecule in the solution, the dark colour change to
colourless. The equation below used to calculate the moles of iodine.
I2 + 2Na2S2O3 →
Na2S4O6 + 2 NaI
In the set 2 experiment, 0.1 g of activated charcoal
was added into flask 7-12 and capped tightly. The activated charcoal added is
act as adsorbent to adsorp the iodine molecule. Adsorption of iodine molecule
on the activated charcoal is a result from Van der Waal’s forces which exists
between molecules.
According to Langmuir theory, there is no more than a
monolayer of iodine adsorbed on the charcoal. The surface of charcoal can be
calculated by plotting a graph of C/N versus C with a straight line slope of
1/Nm.
CONCLUSION
The
Nm value is 0.4134 g mol-1. The
number of iodine adsorbed onto the monomolecular layer 8.64 x 1022 molecules. The surface area of charcoal
is 27648 m2/g.
REFERENCES
Patrick J. Sinko,
Lippincott Williams and Wilkins. Martin’s Physical Pharmacy and Pharmaceutical
Sciences, 5th Edition.
Alexander T Florence and David Attwood. (2006).
Physiocochemical Principles of Pharmacy. 4th Ed. Palgrave. USA.
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