BSc/MSc/BTech Chemistry Lab Expt: Verify the Freundlich and Langmuir Isotherms for the Adsorption of Oxalic Acid on Activated Carbon

VERIFY THE FREUNDLICH AND LANGMUIR ISOTHERMS FOR THE ADSORPTION OF OXALIC ACID ON ACTIVATED CARBON

 

Aim:

(i) To study the adsorption of oxalic acid on activated charcoal and to Verify (a) Freundlich adsorption isotherm and (b) Langmuir adsorption isotherm.

(ii) To determine the strength of unknown oxalic acid using Freundlich

adsorption.

Principle:

Freundlich adsorption isotherm gives the relationship between the amount of adsorbate (x) adsorbed by a particular mass (m) of the adsorbent and the equilibrium concentration (c) of the adsorbate at a particular temperature.

Freundlich isotherm is x/m = Kc^1/n

log x/m = log K + (1/n) log c where, K and n are constants.

If the plot of log (x/m) against log c is a straight line, it means that the adsorption follows Freundlich adsorption isotherm.

Since (x/m) and c are fractions (<1), log (x/m) and log c will be negative values. To make it easier, plot with the positive quantities.

- log (x/m) = -log K + (1/n) (-log c)

Plot the graph between -log (x/m) versus -log c. This will be a straight line with slope (1/n) and intercept log K.

Langmuir derived another relationship between the amount adsorbed and the equilibrium concentration of the solute.

x/m=αc/(1+βc)

1/((x/m))=(1+βc)/αc

c/((x/m))=1/α+βc/α

This equation can be verified by plotting c/((x/m)) versus c. It should be a straight line with slope β/α and intercept 1/α. Therefore, α and β can be determined experimentally.

In this experiment, a known mass (m) of activated charcoal is added to an aqueous solution of oxalic acid of known initial concentration (c_i)

After thorough shaking, some oxalic acid is adsorbed on activated charcoal. The solution is filtered and the equilibrium concentration of oxalic acid (c) is determined by titrating the filtrate against standard KMnO₄.

2KMnO₄ + 3H₂SO₄ + 5 (COOH)₂   ----------->      K₂SO₄ + 2MnSO₄ + 10 CO₂ + 8H₂O

2 M_oxalicV_oxalic = 5 M_permanganate V_permanganate

Decrease in concentration of oxalic acid = c_i – c

Amount of oxalic acid adsorbed (x) = (c_i – c) × Volume of the solution in L

                                                          = (c_i – c) × (100/1000)

-log(x/m) versus -log c to verify Freundlich isotherm

c/(x/m) versus c to verify Langmuir isotherm

Chemicals and apparatus

Charcoal (12 g for one student), 6 stoppered bottles or conical flask with stopper, Oxalic acid (0.2 M), KMnO₄ (0.02 M) H₂SO₄ (2M), funnel, pipette, burette, thermostat.

Procedure:

Fill up the burette with the KMnO₄ solution and mount the burette on a stand. Take 5 ml of 0.2 M oxalic acid solution into a conical flask and add to it one test tube full of 2 M H₂SO₄ warmed to 60 °C approximately. Add KMnO₄ solution drop by drop and shake the conical flask continuously. The end point will be marked by the appearance of a light pink colour. Note down the volume of KMnO4 required for the titration. Repeat the titration till you get two concordant readings. Calculate the exact molarity of the KMnO₄ solution using

2 M_oxalicV_oxalic = 5 M_permanganate V_permanganate

Use this exact molarity of KMnO₄ in the remaining calculations.

Take six 250 mL dry stoppered bottles or corked conical flasks. Label them as 1, 2, 3, 4, 5, 6. With the help of burettes, measure and add oxalic acid and distilled water in the following combinations.

Flask number	Oxalic acid (mL)	Distilled water (mL)
1	50	50
2	40	60
3	30	70
4	20	80
5	10	90

     

The total volume in each case becomes 100 mL.

Place all the five bottles in a thermostat or water throughout the experiment and set them aside to acquire the temperature of the bath. Measure the temperature of the bath.

Weigh 2g charcoal powder for each of the bottles. Carefully pour weighed charcoal powder into solutions of bottles 1 to 5 and close each of them. Shake each of the bottles for two minutes and place it back in the thermostat. Do it repeatedly for 45 minutes. Shorter intervals will not be enough for the adsorption equilibrium state to be reached due to the heterogeneous porosity of the charcoal powder.

Label another 6 bottles or conical flasks correspond to the bottles 1 to 5. Place one funnel on each bottle. When all this is set, shake the bottles 1 to 5 for the last time and filter their contents into the corresponding new bottles. Each funnel should be covered with a watch glass to prevent evaporation of solution during filtration.

After filtration, remove the funnels along with the watch glasses, filter papers and used charcoal. Close the bottles. Pipette out 10 ml of this solution and titrate each bottle against the standardized KMnO₄ solution.

For each titration add one test tube full of 2 M H₂SO₄ warmed to 60 °C approximately to the flask and titrate against standard KMnO₄ solution from the burette. End point will be marked by the appearance of a pink colour. Repeat the titration to get the concordant values.

Observation:

Total mass of charcoal powder added to each bottle = 2 g

Molarity of stock solution of KMnO4 = 0.02 M

Bottle number	Volume taken (mL)	Initial burette reading (ml)	Final burette reading (mL)	Volume of KMnO4 (mL)
1	10   10
2	10   10
3	10   10
4	10   10
5	10   10

 

You can calculate the initial concentrations (c,) of solutions 1 to 5 as follows:

Solution 1: (50/100) × 0.2 M = 0.1 M

Solution 2: (40/100) × 0.2 M = 0.08 M

Solution 3: (30/100) × 0.2 M = 0.06 M

Solution 4: (20/100) × 0.2 M = 0.04 M

Solution 5: (10/100) × 0.2 M = 0.02 M

After filtration, Calculate the equilibrium concentrations (c) for the solutions 1 to 5 using;

2 M_oxalicV_oxalic = 5 M_permanganate V_permanganate

Solution number	ci	c (M)	x (mol-1)	m (g)	x/m (mol/g)
1
2
3
4
5

 

Solution number	c (M)	log c	-log c	log (x/m) (g/mol)	-log (x/m) (g/mol)	c/(x/m) mol/g. M

 

RESULT

(i) Graph plotted between -log(x/m) and -log c comes out to be a ......................Therefore, Freundlich adsorption isotherm is -----------

(ii) Graph plotted between c/(x/m) and c comes out to be a............................Therefore, Langmuir adsorption isotherm is verified.