Title
Practical 3: Adsorption from solution
Introduction
Adsorption is the adhesion of molecules, ions or atoms of gas or liquid to a surface of an adsorbent (solid or liquid). This will lead to accumulation of the gas or liquid on the surrounding of the adsorbent which later will form a molecular or atomic film. Adsorption is different from absorption, in which a substance diffuses into a medium.
Adsorption is a consequence of surface energy. Atoms on the clean surface of the adsorbent experience a bond deficiency because they are not wholly surrounded by other atoms. Thus, bond is favourable to be created with anything available surrounding it. Adsorption can be categorized into two groups that is chemical adsorption or physical adsorption.
Chemical adsorption occurs when the adsorption involves only chemical bonds between adsorbent and adsorbate. This type of adsorption acquires activation energy and it is can be very strong and not readily reversible and it forms monolayer film around the adsorbent. Meanwhile, physical adsorption is the term that refers to adsorption that occurs due to van der Waals forces between the adsorbent and adsorbate. This type of adsorption is more reversible and non-specific. Physical adsorption can be monolayer or multi layers.
Langmuir isotherm theory was published in 1916. Following this theory, the Langmuir equation was introduced which depicts a relationship between the number of active sites of the surface undergoing adsorption pressure.
One of the most famous application of adsorption is the adsorption on activated charcoal. In this experiment, adsorption of Iodine from solution onto activated charcoal was studied and Langmuir equation was used to estimate the surface area if activated charcoal.
Material and Apparatus
12 conical flasks, 6 centrifuge tubes,
measuring cylinders, analytical balance, burettes, retort stand and clamps, Pasteur
pipettes, iodine solutions, 1% w/v starch solution, 0.1M sodium thiosulphate
solution, distilled water, activated charcoal
Procedures
Using measuring cylinders, 12 conical
flasks (labelled 1-12) were filled with 50ml mixtures of iodine solutions (A
and B) as stated in 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
as an indicator.
2) The solution was titrated using 0.1M of
sodium thiosulphate solution until the colour of the solution changed from dark
blue to colourless.
3) The volume of the sodium thiosulphate
used was recorded.
Set 2 : Concentration of iodine in
solution A at equilibrium (C)
For flasks 7-12:
1) 0.1g of activated charcoal was added.
2) The flasks were capped tightly. The
flasks were swirled or shook for every 10 minutes for 2 hours.
3) After 2 hours, the solution was
transferred into centrifuge tubes and labelled accordingly.
4) The solutions were centrifuged at 3000
rpm for 5 minutes and the resulting supernatant were transferred into new
conical flasks. Each conical flasks were labelled accordingly.
5) Steps 1, 2 and 3 were repeated as
carried out for flasks 1-6 in Set 1.
Result
Flask
|
Vol. Of sodium Thiosulphate needed
|
flask
|
Vol. Of sodium Thiosulphate needed
|
1
|
10.15mL
|
7
|
7.7mL
|
2
|
14.65 mL
|
8
|
11.7mL
|
3
|
18.7 mL
|
9
|
15.9mL
|
4
|
23.2 mL
|
10
|
19.8mL
|
5
|
27.6 mL
|
11
|
23.8mL
|
6
|
45.1 mL
|
12
|
40.5mL
|
Questions
1)
calculate N for iodine
Flask
|
Vol. Of sodium Thiosulphate needed
|
Initial concentration of iodine, X
|
flask
|
Vol. Of sodium Thiosulphate needed
|
Concentration of Iodine at equilibrium, C
|
1
|
10.15mL
|
No of mole of Iodine
= 10.15 x 0.01269g/ 2x126.9gmol-1
=5.075x10-4mol
X= 5.075x10-4mol
/ 0.05L
=0.01M
|
7
|
7.7mL
|
No of mole of Iodine
= 7.7 x 0.01269g/ 2x126.9gmol-1
=3.85x10-4mol
C= 3.85x10-4mol
/ 0.05L
=0.0077M
|
2
|
14.65 mL
|
No of mole of Iodine
=14.65 x 0.01269g/ 2x126.9gmol-1
=7.325x10-4mol
X= 7.325x10-4mol
/ 0.05L =0.015M
|
8
|
11.7mL
|
No of mole of Iodine
=11.7 x 0.01269g/ 2x126.9gmol-1
=5.85x10-4mol
C= 5.85x10-4mol
/ 0.05L
=0.0117M
|
3
|
18.7 mL
|
No of mole of Iodine
= 18.7 x 0.01269g/ 2x126.9gmol-1
=9.35x10-4mol
X= 9.35x10-4mol
/ 0.05L =0.019M
|
9
|
15.9mL
|
No of mole of Iodine
=15.9 x 0.01269g/ 2x126.9gmol-1
=7.95x10-4mol
C= 7.95x10-4mol
/ 0.05L
=0.0159M
|
4
|
23.2 mL
|
No of mole of Iodine
= 23.2 x 0.01269g/ 2x126.9gmol-1
=1.16x10-3mol
X= 1.16x10-3mol
/ 0.05L =0.0232M
|
10
|
19.8mL
|
No of mole of Iodine
=19.8 x 0.01269g/ 2x126.9gmol-1
=9.9x10-4mol
C= 9.9x10-4mol
/ 0.05L
=0.0198M
|
5
|
27.6 mL
|
No of mole of Iodine
= 10.15 x 0.01269g/ 2x126.9gmol-1
=1.38x10-3mol
X= 1.38x10-3mol
/ 0.05L =0.0276M
|
11
|
23.8mL
|
No of mole of Iodine
=23.8 x 0.01269g/ 2x126.9gmol-1
=1.19x10-3mol
C= 1.19x10-3mol
/ 0.05L
=0.0238M
|
6
|
45.1 mL
|
No of mole of Iodine
= 10.15 x 0.01269g/ 2x126.9gmol-1
=2.255x10-3mol
X= 2.255x10-3mol
/ 0.05L =0.0451M
|
12
|
40.5mL
|
No of mole of Iodine
=40.5 x 0.01269g/ 2x126.9gmol-1
=2.025x10-3mol
C= 2.025x10-3mol
/ 0.05L
=0.0405M
|
N= (X-C) X 50/1000 x 1/0.1
Flasks
|
Initial concentration of charcoal, X (M)
|
Concentration of Iodine at equilibrium, C (M)
|
Total mole of Iodine adsorbed by 1g of activated
charcoal, N
(mol)
|
1 and 7
|
0.01
|
0.0077
|
3.5x10-4 mol
|
2 and 8
|
0.015
|
0.0117
|
1.8x10-4 mol
|
3 and 9
|
0.019
|
0.0159
|
3.9x10-4 mol
|
4 and 10
|
0.0232
|
0.0198
|
1.5x10-4 mol
|
5 and 11
|
0.0276
|
0.0238
|
3.0x10-4 mol
|
6 and 12
|
0.0451
|
0.0405
|
2.3x10-4 mol
|
1)
plot amount of Iodine adsorbed
(N) versus balance concentration of solution (C) at equilibrium to obtain
adsorption isotherm
Flasks
|
Concentration of Iodine at equilibrium, C (M)
|
Total mole of Iodine adsorbed by 1g of activated
charcoal, N
(mol)
|
1 and 7
|
0.0077
|
3.5x10-4 mol
|
2 and 8
|
0.0117
|
1.8x10-4 mol
|
3 and 9
|
0.0159
|
3.9x10-4 mol
|
4 and 10
|
0.0198
|
1.5x10-4 mol
|
5 and 11
|
0.0238
|
3.0x10-4 mol
|
6 and 12
|
0.0405
|
2.3x10-4 mol
|
1)
according to Langmuir theory,
if there is no more than a monolayer of
iodine adsorbed on the charcoal,
C/N = C/Nm +
1/KN
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 no = 6.023x1023 molecule, calculate the surface area of
charcoal in m2g-1
Flasks
|
Concentration of Iodine at equilibrium, C (M)
|
C/N
|
1 and 7
|
0.0077
|
6.696
|
2 and 8
|
0.0117
|
7.091
|
3 and 9
|
0.0159
|
10.258
|
4 and 10
|
0.0198
|
11.65
|
5 and 11
|
0.0238
|
12.526
|
6 and 12
|
0.0405
|
17.609
|
The value of Nm from
graph,
Gradient 1/Nm =341.7 g mol-1
Nm = 1/341.7 g mol-1
= 2.927 x 10-3 g mol-1
=
0.00297 g mol-1
No. of molecules of charcoal = Nm x Avogadro no.
0.00297 mol g-1 x 6.023x1023 molecules per mole
= 1.788831 x 10 21 molecule g-1
Surface area of charcoal = 3.2 x 10-19 m2 x 1.788831 x 10 21 molecule g-1
= 572.42592 m2 g-1
No. of molecules of charcoal = Nm x Avogadro no.
0.00297 mol g-1 x 6.023x1023 molecules per mole
= 1.788831 x 10 21 molecule g-1
Surface area of charcoal = 3.2 x 10-19 m2 x 1.788831 x 10 21 molecule g-1
= 572.42592 m2 g-1
Adsorption
is the binding of free moving molecules of a gaseous or solutes of a solution
onto the surface of the solid. In general, one uses solid adsorbents of small
size and often with surface imperfections such as cracks and holes which serve
to increase the surface area per unit mass greatly over the apparent
geometrical area. Such small, porous particles may have specific areas in the
range from 10 to 1000 m2g-1. The adsorption bond can be
strong or weak which depends on the nature of forces between the adsorbent and adsorbate. Adsorbent
refers to the solid whereas adsorbate refers to the molecules that undergo
adsorption such as gas or dissolved solutes.
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 around the
particles resulting to the adsorption phenomena. The type of interaction
between the adsorbed molecule and the solid surface varies over a wide range from
weak nonpolar van der Waals’ forces to strong chemical bonding. Examples of
adsorption where ionic or covalent bonding occurs are the adsorption of
chloride ions and silver chloride (ionic) or of oxygen gas on metals where
oxygen-metal bonds are formed (covalent). In these cases, the process is called
chemisorption, and it is generally characterized by high heats of adsorption
(from 10 to 100 kcal mol-1 of gas adsorbed).
When the reaction between adsorbent
and adsorbate is caused by van der Waal’s forces of attraction, it is known as
physical adsorption. Physical adsorption forces are similar to those which
cause condensation of gases into liquid or solids.
The nature of adsorbent and
adsorbate, surface area of adsorbent and physical conditions such as
temperature and pH determines the extent of adsorption from the solution. Adsorption process is usually studied through
graphs known as adsorption isotherm. That is the amount of adsorbate on the
adsorbent as a function if its pressure or concentration at constant
temperature. Example of isotherm is Langmuir equation and it is most common
isotherm equation to use due to its simplicity and its ability to fit a variety
of adsorption data.
In this experiment, Langmuir
equation is used to estimate the surface area of activated charcoal sample. This
theory is restricted to cases where only one layer of molecules can be adsorbed
at the surface. Monolayer adsorption is usually observed in the case of
chemisorption from the gas phase or adsorption from solution. Monolayer
adsorption is distinguished by the fact that the amount adsorbed reaches a
maximum value at moderate concentrations and remains constant with further
increase in concentration.
Solid surfaces can adsorb dissolved
substances from solution. When a solution of iodine is shaken with activated
charcoal, some of the iodine will be removed by the charcoal and the
concentration of the solution decreases. From the results obtained, it is found
that K increases as the concentration of iodine is decreases with respect to
time. Therefore, the degree to which a solid will adsorb material depends on a
number of factors such as temperature, nature of molecule being adsorbed,
degree of surface pore structure, and solute and solvent concentration.
The number of molecules adsorbed
per gram of solid, N (mol/g), depends on the specific surface area of the
solid, S(m2/g), the final liquid phase concentration Cf (mol/L) or equilibrium
gas phase pressure p (atm or kPa), and the specific molecules undergoing
adsorption. A plot of N versus Cf or N versus p, where the temperature is held
constant, is referred to as an adsorption isotherm. Graph amount of iodine
adsorbed (N) versus balance concentration of solution (C) at equilibrium is a
non-linear graph and shows that the number of iodine adsorbed gradually
increases 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.
The
adsorption of iodine solution in charcoal follows the Langmuir theory of
adsorption isotherm. The adsorption rate increases with increasing
concentration of iodine. From the result obtained, the surface area of charcoal
is.
Reference
Alexander T
Florence, D. A. (2006). In Physicochemical Principles of Pharmacy (4th
ed., pp. 194-201). London: Pharmaceutical Press.
Patrick J. Sinko,
L. W. (n.d.). In Martin’s Physical Pharmacy and Pharmaceutical Sciences
(5th ed., pp. 39-40).
