Introduction
Surface
adsorption to a solid falls into two broad categories; physisorption and
chemisorption. Physisorption is a non-specific loose binding of the adsorbate
to the solid via van der Waals type interactions. Multilayered adsorption is
possible and it is easily disrupted by increasing
temperatures.
Chemisorption involves a more specific binding of the absorbate to the solid.
It is a process that is more akin to a chemical reaction and hence, only
monolayer adsorption is possible. “The difference between physical and
chemisorption is typified by the behavior of nitrogen on iron. At the
temperature of liquid nitrogen, -190oC, nitrogen is adsorbed physically on iron
as nitrogen molecules, N2. The amount of N2 adsorbed decreases rapidly as the temperature
rises. At room temperature iron does not adsorb nitrogen at all. At high
temperatures,
~500oC, nitrogen is chemisorbed on the iron surface as nitrogen atoms.” (Castellan
1983) The Langmuir Isotherm best describes chemisorption processes.
The Langmuir Adsorption
Isotherm:
In this laboratory exercise, we will generate
a Langmuir Isotherm for the adsorption of Acetic Acid on the surface of Norit
A Activated Charcoal. The surface area of the Activated Charcoal available
for adsorption of Acetic Acid will be determined from the Isotherm data and
other assumptions. Activated Charcoal is particularly useful in purification
processes involving impurity adsorption because of its high porosity, giving
an very high
surface to mass ratio. It has
been noted: Adsorption by a solid is not a
very important process unless the solid has a very large surface area compared
to its mass. Consequently, charcoal made from bone, blood, or coconut shells is
especially effective because it has a highly
porous structure. Charcoal is activated by being heated to quite high
temperatures in a vacuum or in a stream of dry air. This treatment probably
desorbs the hydrocarbons that are adsorbed when the charcoal is first produced.
1- Langmuir life:
• The
surface of the adsorbant is in contact with a solution containing an
adsorbate
which is strongly attracted to the surface.
• The
surface has a specific number of sites where the solute molecules
can be adsorbed.
• The
adsorption involves the attachment of only one layer of molecules
where AS
represents a solute molecule bound to a surface site on S. The
equilibrium constant Kads for this
reaction is given by:
If we define
Y as the amount of adsorption in units of moles adsorbate per mass
adsorbant, andYmax and the maximal adsorption, then:
where the
Freundlich parameters k and n are empirically determined. A plot
of log Y vs. log C allows for a determination of these parameters.
We will
prepare our Acetic Acid-Charcoal isotherm by allowing Acetic Acid solutions of
various concentrations to equilibrate for about a week with a given mass of Norit
A. The amount of Acetic Acid (HAc) not adsorbed to the Norit A will
be determined by titration with Sodium Hydroxide (NaOH).
This will allow us to easily determine
the amount of Acetic Acid that has adsorbed to a given mass of the Charcoal.
3-Procedure:
Week 1
Prepare
about 1L of 0.1M NaOH from a 6M stock solution and store it in a tightly
stoppered plastic bottle. Standardize the NaOH against Potassium Hydrogen
Phthalate (KHP).
For each
sample above, add 100 mL of the solution to a charcoal sample. Swirl the flasks
vigorously and then place them into the shaker bath at 25oC. Let them agitate
until next week.
Pre-Lab
Preparation
1. Determine
how to prepare the stock 0.4M Acetic Acid solution.
2. Determine
the dilution factor for the preparation of the 0.1M NaOH solution.
3. Determine the
amount of KHP needed for the standardization.
Week 2
Filter your
charcoal solutions (discard the first 10 ml of filtrate to clean the filter
flask of any contaminants and to saturate with acid any adsorption sites which
might be on the filter paper) and titrate a suitable aliquot with the
standardized NaOH. Perform your titrations in triplicate. Be sure to also
titrate your original 0.4M Acetic Acid solution to confirm its concentration exactly.
Data Analysis:
All results must be accompanied by an
appropriate error estimate.
1. Determine
the concentration of the NaOH solution from your standardization data.
2. Using
your titration data, for each sample:
Calculate the number of moles of
acetic acid in the solution before adsorption.
Calculate the number of moles of
acetic acid in the solution after adsorption.
3.
Determine Y for each sample. Include appropriate error estimates for
each Y value.
4. Plot
your isotherm; Y vs. C. Include appropriate error bars for Y.
5. Plot the
isotherm in the form of (Eq. 6) and determine the Langmuir parameters Ymax
and Kads. Include appropriate error estimates for each parameter.
6. Make the
suggested plot to determine the Freundlich parameters k and n.
Again, include appropriate error estimates for each parameter.
7. Re-plot
your isotherm; Y vs. C. Include the Langmuir and Freundlich isotherms,
in each case generated using the parameters determined above. Comment on the
quality of the Langmuir and Freundlich isotherms.
8. Assume
the surface area occupied by one acetic acid molecule on the surface of the charcoal
is 21Å2. Determine the surface area of 1 g of charcoal. Express your result in m2/g.
Is your number reasonable? Compare your result with that of the literature.
conclusion
The Langmuir
isotherm was developed by Irving Langmuir in 1916 to describe the dependence of
the surface coverage of an adsorbed gas on the pressure of the gas above the
surface at a fixed temperature. There are many other types of isotherm (Temkin,
Freundlich ...) which differ in one or more of the assumptions made in deriving
the expression for the surface coverage; in particular, on how they treat the
surface coverage dependence of the enthalpy of adsorption. Whilst the Langmuir
isotherm is one of the simplest, it still provides a useful insight into the
pressure dependence of the extent of surface adsorption.
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