Purification by Acid-Base Extraction

Lab Partners:

Purpose

The aim of this lab was to become familiar with process of purification by using acid-base extraction process. This lab was consists of two-part, in first part there was the concept of solubility behavior of organic compounds in neutral, acidic and basic solutions and in second part of experiment was the practical use of first part concept.

Chemicals and Equipment:

Part 1

Weighing paper Ethyl acetate
Lab balance Sat, aq. NaHCO (~1 M)
Graduated cylinder, 10 mL HCI, 3 M
Test tubes, large (16) Universal pH paper
Universal pH paper Separatory funnel with stopper
Organic solids (labeled by number only) Ring stand with ring
Benzoic acid Buchner funnel
Cholesterol Filter paper
4-tert-Butylphenol Side-arm filter flask
Meta-Amino acetophenone

 

Part 2

Ethyl acetate If time
Impure benzoic acid Watch glass
Erlenmeyer flasks, 250 mL (2) Drying oven
Graduated cylinder, 100 mL Melting point tubes
Melting point apparatus

 

Extraction Procedure

Part 1, Solubility Behavior

1) 100 mg of one of the numbered samples was weighed into each of four large test tubes, used for the same compound

2) 5 mL of water were added to a large test tube with the help of graduated cylinder

3) Then added 5 mL of A) water, B) 1 M NaHCO, C) 1 M NaOH, and D) 1 M HCI, each to a different one of the four test tubes containing the compound under investigation

4) The content of each tube was mixed well by swirling it rapidly and agitating properly

5) When significant amounts of the solid dissolved were noted, and then added 3M NaOH to neutralize the acid present. PH paper was used to make sure the pH had gone from acidic to the basic

6) This procedure was repeated (Steps 1-5) with each of the other three samples

7) Correct identity was assigned to each of the four compounds according to the observations and results

Part 2

Purification by Extraction

1) The set up of a ring stand with a ring was made and put a separatory funnel into it

2) The stopcock was closed at the bottom

3) 0.5 g of impure benzoic acid was weighed by using a piece of weighing paper and put the sample into the sep. funnel

4) 25 mL of ethyl acetate was added, using a portion of it to rinse any benzoic acid off the top ground glass joint into the solution

5) Ground glass stopper was put into the sep. funnel, picked it up, inverted it with hand cupped over the stopper to insure it stayed in the sep. funnel, and opened the stopcock to vent any pressure

6) Cautiously alternate shaking and venting was carried out to remove the gas (CO2)

7) Sep. funnel was put back in the ring, removed the stopper, and allowed the liquid phases to separate completely

8) The lower layer was drained into a 250 mL Erlenmeyer flask with careful observation of the meniscus and stopped the draining process when it was near to the stopcock

9) Another 20 mL of sat, sodium bicarbonate was added to the sep. funnel, stopper, inverted, vented, and shaken as before

10) The Sep, funnel was returned to the ring stand, removed the stopper, and after the phases were separated combined the aqueous bicarbonate layer with initially collected

11) Slowly added 25 mL of 3 M HCI with proper mixing (swirl) to the Erlenmeyer flask containing the aqueous. When no more CO₂ was generated, the formation of solid was noticed

12) Final pH of solution was checked with litmus paper to ensure that it was acidic

13) The Erlenmeyer flask was put into the ice-bath and swirled periodically until the crystals were formed and solution was cooled

14) Purified solid benzoic acid was filtered through Buchner funnel containing a piece of filter paper using vacuum

15) The flask was rinsed twice with water to transfer more crystals and to wash the crystals in the Buchner funnel

16) The water was sucked out of the crystals as could be, and then examined them with respect to the appearance of the starting impure benzoic acid

17) The crystals were dried in the oven and then weighed

 

Results

Unknown number H2O NaHCO3 NaOH HCl Identity
1 No Yes Yes No Phenol or carboxylic acid
2 No No No No May be ether soluble
3 No No No Yes Amine
4 No No Yes No Phenols

Weight of crystal + watch glass = 24.71 g

Weight of watch glass = 24.53 g

Weight of crystals = 24.71 – 24.53

= 0.18 g

Amount of crude benzoic acid used = 0.47 g

Percentage recovery = 0.18/0.47 x 100

= 38.28 %

= 38.3 %

 

Discussion

From experiment performance the identification of unknown compound was made according to the solubility chart. As first unknown was insoluble in water and HCl, so it might be carboxylic acid or phenol. Unknown2, that was not soluble in any of the four solvents, so it might be ether soluble. Similar practice had been made to verify the solubility of remaining unknown compounds.

The second part of procedure was based on the 1st procedure. As the benzoic acid was soluble in NaHCO3 which was according to the solubility chart noticed in the experiment. When shaken, the carbon dioxide is removed an on addition of ethyl acetate layers are formed which are further separated by using separatory funnel.

Acid-Base concept was very interesting and successful as the recovery had been made. The recovery was only 38.3%. The possible reasons for loss of benzoic acid might be

  • Loss from separating the layers in separatory funnel
  • Loss from washing the crystals
  • Loss of crystals while filtering
  • Loss of crystals while drying

As far as the overall practical concerned, we came to know that how we can use this method to identify the unknown compound and how an impure compound can be purified. The overall experimental activity was very interesting and informative. We are glad to perform such an amazing practical and purification method.

 

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