Experiment 2
Adsorption Isotherm
Aim:
To determine the adsorption isotherm of acetic acid by activated charcoal.
Apparatus/Reagents:
Six stoppered bottles (or 100 ml conical flasks). burette with stands (two sets). pipette,
measuring cylinder, funnel, standard solutions of acetic acid (0.6 M) and sodium hydroxide
(0.1 M), powdered activated charcoal and phenolphthalein indícator.
Theory
Introduction
The molecules or ions present at the surface of a solid do not have all their forces satisfied by
union with other particles. Due to this instauration, solid surfaces tend to satisfy their residual
forces by attracting and retaining on them the molecules of other species which are brought
into contact with them. As the molecules remain only at the surface and do not go deeper into
the bulk, their concentration is more at the surface than in the bulk of the solid. This
phenomenon of higher concentration of molecular species on the surface than in the bulk of a
solid is known as adsorption.
Adsorption is a phenomenon in which particles adhere to the surface of a solid. The solid on
which adsorption takes place is called the adsorbent and the material adsorbed is called
adsorbate. For a given weight of adsorbent, its adsorptive capacity is directly proportional to
the surface area since the surface area presented by a given weight of a solid is very large when
it is in colloidal state, colloids possess very good adsorptive properties. The reverse process of
removal of an adsorbed substance from the surface of a solid is known as desorption.
Procedure
(i)
Take 6 cleaned and dry stoppered bottles (or 100 ml conical flasks) and label them
from 1 to 6.
(ii)
Set up two burettes. Fill one with 0.6 M acetic acid and the other with distilled water.
(iii)
Add different volumes of acetic acid and water in the labelled bottles as given below.
Bottle No.
Vol.
of
0.6
M Vol. of Water (ml)
Total Volume (mL)
CH3COOH (ml)
1
60
0
60
2
50
10
60
3
40
20
60
4
30
30
60
5
20
40
60
6
10
50
60
(iv) Shake the solutions well and add 3.0 g of charcoal in each bottle.
(v) Stopper the bottles and let them stand for about an hour with intermittent shaking.
(v) Now, filter the contents of bottle 1 into a beaker through a filter paper. Pipette out 10
ml of the filtrate in to a 100ml conical flask and titrate it against N/10 sodium hydroxide
solution using phenolphthalein as indicator. Take three reading in each case.
Observations
Room temp. = t °C.
Weight of activated charcoal in each bottle = 2g
Vol. of acetic acid solution for titration in each case = 10 ml
Bottle No.
1
Initial
(i)
Final
Vol. of 0.1 M
Mean
NaOH used
Value
V1
(ii)
2
(i)
V2
Calculations:
() Bottle No. I
Initial conc. of acetic acid =0.6 M
10 ml of M1, acetic acid=V1, ml of 0.1 M NaOH.
M1 =
𝑉1 ×0.1
10
= 0.01 V1=Conc. after adsorption .
Thus change in concentration after adsorption (0,6-0.01 ×V1)M
Also if x1 = Change in conc. in grams. per 60 ml of the solution in Bottle No, 1.
Than
X1 =
=
𝐶ℎ𝑎𝑛𝑔𝑒 𝑖𝑛 𝐶𝑜𝑛𝑐.𝑖𝑛 𝑚𝑜𝑙𝑒𝑠 ×𝑉𝑜𝑙𝑢𝑚𝑒 𝑡𝑎𝑘𝑒𝑛
1000
(0.6−0.01𝑉1)×60
1000
× Mol.wt.
× 60g
(ii) Bottle No.2
Initial Conc. of Acetic acid =
06×50
60
M = 0.5 M
As 10 ml of M2 acetic acid = V2 ml of
𝑀
10
NaOH
0.1
M2 = V1 × 10 = 0.01 V2
= conc. after adsorption
Change in conc. in mples/litre = (0.5-0.01 V2)× M
And x2 = change in conc. in grams per 60 ml of the solution
(0.50−0.01𝑉2)
=
1000
×60×60g
Similarly calculate the change in conc. in gm per 60 ml of the volume of the solution of
acetic acid taken for bottle no. 3, 4, 5 and 6 and let these values be as x3, X4, X5, and x6,
respectively. Now tabulate the values of log C and log
𝑥
𝑥
𝑚
as given in the table l and plot
graph by taking log C along X-axis (abscissa) and log 𝑚 along Y-axis (ordinate).
Table-1
Bottle
Initial
No.1
conc.
of Conc.
acetic
acetic
acid
after
solution
adsorption
1.
0.6 M
2.
0.5 M
3.
0.4M
Equilibrium
Amount(moles)
of Of
acetic
acid adsorbed (x)
Weight of
acid Adsorbent
(m)
𝑥
𝑚
Log
𝑥
𝑚
Result
A straight line obtained in the plot shows the validity of the Freundlich isotherm over the
concentration range studied.
Log C
Quiz:
1. What is adsorption isotherm? (Chemical Engineering Kinetics, n.d.; robert_treybal, n.d.)
2. Discuss difference between physical and chemical adsorption.
Suggested Reference:
Smith J. M. (1981). Chemical Engineering Kinetics (3rd ed.). McGraw-Hill chemical engineering series.
robert_treybal. (1980). mass_transfer_operations. McGraw-Hill chemical engineering series.
References used by the students:
Rubric wise marks obtained:
Rubrics
Marks
1
2
3
4
5
Total
Experiment 3
Adsorption Isotherm
Aim: Study the adsorption of oxalic acid aqueous solution on charcoal and prove the validity
of Freundlich's adsorption isotherm and Langmuir adsorption isotherm.
Chemicals
Standard NaOH
𝑀
10
,standard oxalic acid solution
𝑀
4
, Powdered activated charcoal., Thermostat
25°C and Sìx stoppered bottles.
Indicator
Phenolphthalein.
End Point
Appearance of permanent light pink color (NaOH in burette)
Theory
We Know that the amount of adsorbate on the adsorbed is dependent on pressure as well as
temperature.
So, we can say that the amount (x) adsorbed is a function of pressure (P) ín the case of gas or
concentration in case of a solution and temperature (T) i.e. x= f(P. T), A plot of P and x.
keeping temperature constant is known as adsorption isotherm.
Freundlich adsorption isotherm: Freundlich proposed an empirical equation to represent, in
general, the adsorption relationship and which is known as Freundlich adsorption isotherm.
According to it,
𝑥
𝑚
= kc1/n
……..(i)
where, x is the amount of solute adsorbed, m is the amount of adsorbent, c is equilibrium
concentration of adsorbate in the solution, k is a constant depending upon the nature of both
adsorbent and adsorbate, while n is another constant which is dependent on the nature of the
adsorbate. The value of 1/n is generally less than unity. On taking logarithms of equation (i),
we get,
𝑥
1
log 𝑚 = log k 𝑛 logc
…….(ii)
If the values of log x/m are plotted as ordinate against log c as abscissa, we get a straight line,
with a slope and 1/n is an intercept on the ordinate log k.
Freundlich adsorption isotherm
At a given temperature, the mass of solute (or gas) adsorbed by a solid adsorbent at various
concentration (pressure in case of gases) is given by the following empirical relation:
𝑥
𝑚
= k. C1/n
(1)
Where x is the mass of solute9 (or gas) adsorbed by m gram of the solid at various
concentration, k and n are constant at a given temperature for the given solid adsorbent and
solute.
Equation(1) can also be written in the following manner:
𝑥
1
Log 𝑚 = log k + 𝑛 log C
𝑥
A plot of log𝑚 versus log C (concentration) gives a straight line with slope equal to
1
𝑛
Langmuir adsorption isotherm: Langmuir gave a relation between the amount adsorbed and the
con-centration for a unimolecular layer and is known as Langmuir adsorption isotherm.
According to it
𝑥
𝑚
𝑘1𝑘2𝑐
=1+𝑘1 𝑐
…...(iii)
where, k1 and k2, are constants.
Equation (iii) can also be written as
𝐶
𝑥/𝑚
1
=
𝑘1𝑘2
𝑐
+ 𝑘2
Let K1, K2, be taken as another constant α and k is taken as ß. The equation (iv) becomes:
𝑥
𝛼𝑐
= 1+𝛽𝑐
𝑚
𝑐
𝑥/𝑚
1
𝛽
= 𝛼 + 𝛼c
Where a & B are constants. Their equation may be verified by plotting
which should result in a straight line. The slope
𝑐
𝑥/𝑚
of the straight line will give
against, C
𝛽
𝛼
and the
intercept along y-axis will give Hence the value of the constants a& B can be evaluated
experimentally
Procedure
(i) Standardize NaOH with oxalic acid by titration using phenolphthalein as indicator.
(ii) Put exactly 2 g of finely powdered charcoal into six well cleaned and dried stoppered
reagent bottle and label them from 1 to 6.
(iii) with the help of burette and 50, 40,30, 20, 10 and 5 ml of standard Oxalic acid solution
and 0, 10, 20,30 ,40 and 45 ml. of distilled water in bottle nos. I,2, 3,4,5 and 6 respectively.
Stopper the bottles and shake them vigorously for about 20 minutes. Place the bottles in a
thermostat for about 30 minutes with intermittent shaking.
(iv) Filter the supernatent liquid of each of the bottles through filter paper.
(v) Titrate the each filtrate by pipetting out 10 ml of each with standard NaOH by using
Phenolphthalein as indicator till the appearance of permanent light pink colour.
Observations
Room Temperature = .... t °C
Amount of charcoal added into each bottle (m) = 2 g
Volume of filtrate take for each titration = 10 ml
10 ml of original acid required v ml of
𝑀
10
NaOH.
Table
Initial conc. Vol. of 𝑀
10
of
Oxalic
Used (ml)
acid
Bottle
No.1
Equilibrium
Oxalic
𝑥
𝑚
acid
Conc. of acid a Adsorbent
Ce (mol/l)
𝑥
Log 𝑚
xg
solution(C0)
(mol/l)
1.
2.
3.
And
so
on
Calculation
First calculate the initial conc. (C0) of oxalic acid in moles/ litre for each flask. From the
titration data, calculate the value of equilibrium concentration (Ce) for each flask. The value
of oxalic and adsorbed (x) for each flaskcan be calculated from the change in concentration ie
(C0-Ce) and the initial volume of the solution (50 ml) as under:
X=
(𝐶𝑜−𝐶𝑒)×50
1000
(𝐶𝑜−𝐶𝑒)
=
20
moles
From this, calculate the amount of acid adsorbed per gram of the adsorbent
𝑥
𝑚
in moles or
grams.
1. Plot a graph between log
𝑥
𝑚
values (ordinate) vs. log Ce, (abscissa). If the plot obtained is
straight line, this means Freundlich isotherm is valid here.
1
Evaluate the value of𝑛 from the slope of the line and log K from the intercept on the ordinate
by the straight line for a value of Log Ce equal to zero.
2. Similarly test Langmuir's adsorption isotherm by plotting
the contant α and β.
Precautions
𝐶𝑒
𝑥/𝑚
Vs. Ce Obtain the values of
Log C
1. A wet filter paper is avoided so that the filtrate is not diluted by the wetting solvent.
2. To avoid change in concentration of filtrate due to adsorption by the filter paper, a small
initial volume of the filtrate is rejected.
3.Conical flasks should b thoroughly cleaned and dried.
Quiz:
1. What is Langmuir adsorption isotherm?
2.
Discuss Freundlich’s adsorption in brief.
Suggested Reference:
Smith J. M. (1981). Chemical Engineering Kinetics (3rd ed.). McGraw-Hill chemical engineering series.
robert_treybal. (1980). mass_transfer_operations. McGraw-Hill chemical engineering series.
References used by the students:
Rubric wise marks obtained:
Rubrics
Marks
1
2
3
4
5
Total