• ADSORPTION


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    • Abstract: ADSORPTIONAdsorption is a process that occurs when a gas or liquid solute accumulates on the surfaceof a solid or a liquid (adsorbent), forming a molecular or atomic film (the adsorbate). It isdifferent from absorption, in which a substance diffuses into a liquid or solid to form a

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ADSORPTION
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.
Adsorption is operative in most natural physical, biological, and chemical systems, and is
widely used in industrial applications such as activated charcoal, synthetic resins and water
purification.
Similar to surface tension, adsorption is a consequence of surface energy. In a bulk
material, all the bonding requirements (be they ionic, covalent or metallic) of the
constituent atoms of the material are filled. But atoms on the (clean) surface experience a
bond deficiency, because they are not wholly surrounded by other atoms. Thus it is
energetically favourable for them to bond with whatever happens to be available. The exact
nature of the bonding depends on the details of the species involved, but the adsorbed
material is generally classified as exhibiting physisorption or chemisorption.
Physisorption or physical adsorption is a type of adsorption in which the adsorbate
adheres to the surface only through Van der Waals (weak intermolecular) interactions,
which are also responsible for the non-ideal behaviour of real gases.
Chemisorption is a type of adsorption whereby a molecule adheres to a surface through
the formation of a chemical bond, as opposed to the Van der Waals forces which cause
physisorption.
Adsorption is usually described through isotherms, that is, functions which connect the
amount of adsorbate on the adsorbent, with its pressure (if gas) or concentration (if liquid).
One can find in literature several models describing process of adsorption, namely
Freundlich isotherm, Langmuir isotherm, BET isotherm, etc. We will deal with Langmuir
isotherm in more details:
Langmuir isotherm
In 1916, Irving Langmuir published an isotherm for gases adsorbed on solids, which
retained his name. It is an empirical isotherm derived from a proposed kinetic mechanism.
It is based on four hypotheses:
1. The surface of the adsorbent is uniform, that is, all the adsorption sites are equal.
2. Adsorbed molecules do not interact.
3. All adsorption occurs through the same mechanism.
4. At the maximum adsorption, only a monolayer is formed: molecules of adsorbate
do not deposit on other, already adsorbed, molecules of adsorbate, only on the free
surface of the adsorbent.
For liquids (adsorbate) adsorbed on solids (adsorbent), the Langmuir isotherm (Fig. 1)
can be expressed by
1
Amax k .c
m= (1)
1 + kc
where m is the substance amount of adsorbate adsorbed per gram (or kg) of the adsorbent,
the unit of m is mol.g-1, resp. mol.kg-1. Amax is the maximal substance amount of adsorbate
per gram (or kg) of the adsorbent. The unit of Amax is mol.g-1, resp. mol.kg-1. k is the
adsorption constant (mol-1.dm3); c (mol.dm-3) is the concentration of adsorbate in liquid.
In practice, activated carbon is used as an adsorbent for the adsorption of mainly organic
compounds along with some larger molecular weight inorganic compounds such as iodine
and mercury.
Activated carbon, also called activated charcoal or activated coal, is a general term that
includes carbon material mostly derived from charcoal. For all three variations of the name,
"activated" is sometimes substituted by "active." By any name, it is a material with an
exceptionally high surface area. Just one gram of activated carbon has a surface area of
approximately 500 m² (for comparison, a tennis court is about 260 m²). The three main
physical carbon types are granular, powder and extruded (pellet). All three types of activated
carbon can have properties tailored to the application. Activated carbon is frequently used in
everyday life, in: industry, food production, medicine, pharmacy, military, etc. In pharmacy,
activated charcoal is considered to be the most effective single agent available as an
emergency decontaminant in the gastrointestinal tract. It is used after a person swallows or
absorbs almost any toxic drug or chemical.
Amax
1.6
m (mol.g-1)
1.2
0.8
0.4
0
0 0.4 0.8 1.2
c (mol.dm-3)
Fig. 1. Langmuir isotherm
2
Objective:
Determination of adsorption isotherm of acetic acid on activated charcoal. Determine
the adsorption constant (k) and the maximal adsorbed substance amount of acetic acid
per gram of charcoal (Amax) of Langmuir isotherm.
Equipment and chemicals:
6 boiling flasks (250 ml), 6 Erlenmayer’s flasks (250 ml), 6 funnels, 3 burettes (50ml), 10
titrimetric flasks, 3 pipettes, holders for funnels, holders for burettes, filtering paper, glazed
paper for weighing, spoon, rubber stoppers,
solution of acetic acid (c=1 mol dm-3), solution of NaOH (c=0.2 mol dm-3), activated
charcoal, phenolphthalein.
Method:
1. Prepare aqueous solutions of acetic acid into numbered flasks following the scheme
given in the table Tab. 1. The total volume of each solution is 60 ml. Use flasks
fitted with stoppers.
2. Transfer 10 ml of the solution from each flask into numbered titrimetric flask, so
final volume of acetic acid solution is VA=50 ml per flask.
3. Determine the actual concentration of acetic acid in flasks by titration in this way:
4. For titration, modify the volume in each titrimetric flask. Take away defined
volume of the solution, to obtain in each flask the volume as given in the table
Tab.2
5. Add 2-3 drops of phenolphthalein and titrate by NaOH.
6. Once the endpoint has been reached, read the burette. The volume of the base Xi0
(ml) that was required to reach the endpoint write down to the Tab.3.
7. Calculate the actual concentration of acetic acid ci0 in the flask No. 1 – 6,
respectively, and write it down to the table Tab. 3.
8. Using practical balance and glazed paper, weigh 6 portions of activated charcoal,
each portion 5 g. The accuracy of weighing must be 0.01 g.
9. Put activated charcoal into numbered flasks with stoppers (1 portion per flask).
Plug up the flasks, and shake them. Wait for 20 minutes, the process of adsorption
is in progress. Mix the mixtures for several times by flasks shaking within this
period. (Remark: The process of adsorption is a function of time too. It is important
to put charcoal into flasks at the same time, to provide adsorption for the same
period in each flask).
10. Filter the mixtures into clean and dry flasks. To avoid disturbing effect of
adsorption of acetic acid into filtering paper, remove away the first portion of
filtration, app. 5 ml.
11. Determine the final concentration of acetic acid ci in each of the flasks after
adsorption: From each solution, transfer the asked volume into clean and dry
titrimetric flask, again following Tab. 2.
12. Repeat points 5-7, and from the consumed base Xi (ml) determine the concentration
of acetic acid ci after adsorption. Write them down to the Tab. 3.
13. Finishing experiment, wash carefully used flasks, pipettes, etc.
3
Data treatment:
1. Determination of the concentration of acetic acid before (ci0) and after (ci)
adsorption:
X i0 cT
ci =
0
, (2)
V
where Xi0 is the volume of the titrant (NaOH), cT is the concentration of the titrant,
V is the volume of the analyte (acetic acid according to Tab. 2), i=1-6 is the number
of flask. Calculate the concentration of acetic acid after adsorption (ci), using the
Eq. 2 and data form Tab. 3 after adsorption.
2. Determination of the substance amount of acetic acid adsorbed per gram of the
charcoal m (mol.g-1) in individual flask:
(ci0 − ci )V A
mi = , (3)
g
where ci0 , ci are the concentrations of acetic acid before and after adsorption,
respectively. VA is the volume of the liquid phase in the mixture charcoal – acetic
acid, g is the mass of the adsorbent – charcoal (in grams), i=1-6 is the number of
flask. Eq. 3 supposes that VA is the same for i=1-6, and also the mass of the
charcoal (g). Write down the obtained values of mi to the Tab. 3.
3. Determination of k and Amax: The Eq. 1 one can rearrange into a form:
1 1 1 1
= + , (4)
m Amax k c Amax
1 1
thus = f ( ) should be a straight line.
m c
1 1
Use MS Excell to create the dependence = f ( ) , where c is the concentration of
m c
acetic acid after adsorption. Fit the experimental points with a linear function. The
1 1
slope represents the value of , and the intercept corresponds to .
Amax k Amax
Calculate Amax and k from the slope and the intercept.
Tab. 1. Scheme for acetic acid dilution.
Flask No. 1 2 3 4 5 6
Acetic acid (ml) 6 12 18 30 42 60
Distilled water (ml) 54 48 42 30 18 0
Total volume (ml) 60 60 60 60 60 60
4
Tab. 2. Volumes of the acetic acid solutions used for titration before and after the
adsorption.
Titrimetric flask No. 1 2 3 4 5 6
Volume V (ml) 10 10 5 5 5 2
Tab. 3. Experimental data
Flask Xi0 c i0 Xi ci mi 1/ci 1/mi
No. (ml) (mol/dm3) (ml) (mol/dm3) (mmol/g) (dm3/mol) (g/mmol)
1
2
3
4
5
6
The report must include:
• Theory (adsorption, Langmuir isotherm, etc. )
• Equipment and chemicals
• Experimental procedure
• Tables of results, calculations and graphs: m=f(c), 1/m=f(1/c).
• Conclusion with parameters characterizing Langmuir isotherm (k and Amax ).
Used literature:
Kopecký F., Kaclík P., Fazekaš T.: Laboratory manual for physical chemistry,
Farmaceutical faculty of Comenius University, Bratislava, 1996
http://en.wikipedia.org/wiki/Adsorption
http://www.chemvironcarbon.com
J. Oremusová, Manual for laboratory practice in physical chemistry for students of
pharmacy, Department of Physical Chemistry, Faculty of Pharmacy, Comenius
University, Bratislava, 2007, in Slovak
Manual written by Doc. RNDr. D. Uhríková, CSc.
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