Adsorption Of Acetic Acid

AIM: To study the adsorption of acetic acid on activated charcoal and to estimate the constants of Langmuir and Freundlich isotherms.
INTRODUCTION: Adsorption is a surface phenomenon of a solid due to the existence of unbalanced or residual forces on the surface of a solid. The solid attracts molecules of other species in contact either from the gas phase or from solution on its surface. There is a greater concentration of the adsorbed molecules at the surface of the solid than in the gas phase or in the bulk solution. This phenomenon is known as ADSORPTION. Adsorption depends on the specific area of the solid, the equilibrium solute concentration in solution or pressure in case of gaseous adsorption, the nature of adsorbent and temperature.

1. The Freundlich isotherm may be written as;
(or) ln(X/m)=lnk+alnc
Therefore ln(X/m) Vs lnc is a straight line.
2. Langmuir isotherm may be written as ;
Where X is the mass of the solute adsorbed on mass ‘m’ of the adsorbent and c is the equilibrium concentration of the adsorbate in the solution.
Hence, a plot of c/(X/m) Vs c will be a straight line from which the constants k1 and k2 can be calculated.

Acetic acid and sodium hydroxide solutions are prepared in the usual manner. Five stoppered reagent bottles (250ml) are taken and are cleaned and dried. The bottles are numbered properly. One gram each of activated charcoal or silica gel is taken in each of them and the following solution in the bottle is prepared according to the following table:

Bottle No. 1 2 3 4 5
Vol. Of acetic acid, soln., ml 50 40 30 20 10
Vol. Of distilled water, ml 0 10 20 30 40
Total Volume, ml 50 50 50 50 50

Stopper the bottles tightly and shake them for 40 minutes. After shaking the bottles, keep them in a thermostat (or leave them at room temperature) maintained at 250c for at least one hour. The bottle No.1 is taken and filtered through a clean and dry filter paper. The filtrate is collected after rejecting the first 5ml of it. 10ml of the filtrate is pipetted out into a conical flask and titrate against 0.1N solution of sodium hydroxide using phenolphthalein as indicator until end point is reached. Repeat the same process to the rest of the mixtures.
The time for which charcoal remains in contact with Acetic acid in different bottles is kept constant. (Absorbent Charcoal or Silica gel)

The weight X of acetic acid adsorbed per unit mass of charcoal can be computed as X = (c0-c)MV.
Where, c0 = initial concentration in mol/lt. of acetic acid before adsorption
c = equilibrium concentration in mol/lt. of acetic acid after adsorption
M = molecular weight of acetic acid
V = volume in litres of solution.
c0,c,X are calculated for each sample and are tabulated.
A graph may be plotted against lnX and lnc to test the Freundlich adsorption isotherm and k values are calculated from the straight-line plot as slope = a, and intercept = lnk.
A graph may be plotted between c/(X/m) and c. A straight line indicates the validity of Langmuir adsorption isotherm. k1 and k2 are obtained from the graphs as;
Slope = 1/k1 and intercept = 1/(k1k2).

RESULT: The adsorption of acetic acid on activated charcoal is studied and the constants of the equations of Langmuir and Freundlich isotherms are estimated and are:
Freundlich isotherm: Langmuir isotherm:
a = k1 = k = k2 =

1.Standardisation of Na0H:
Normality of Oxalic acid =
Volume of Oxalic acid pipetted out =
Volume of Na0H run down =
Normality of Na0H =
2.Standardisation of Acetic Acid:
Volume of Acetic Acid taken =
Volume of Na0H run down =
Normality of Acetic acid =
Bottle No. 1 2 3 4 5
Volume of acetic acid taken, ml. 50 40 30 20 10
Volume of distilled water, ml. 0 10 20 30 40
Total volume, ml. 50 50 50 50 50
Volume of sample taken, ml. 10 10 10 10 10
Volumes of Na0H run down, ml.

The Freundlich adsorption isotherm is,
ln(X/m) = lnk + alnc.

From the graph of ln(X/m) Vs lnc
Slope, a =
Intercept, k =
The Langmuir adsorption isotherm is,

c/(X/m) = 1/(k1k2) + c/k2
From the graph of c/(X/m) Vs c,

Slope, k1 =
y-intercept, k2 =

For bottle No.2,
X = (c0-c) MV

lnc =

ln(X/m) =

c/(X/m) =

ln (X/m) c(X/M)

lnc c

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