1. To recover the benzoic acid and p-dichlorobenzene from its mixture from its mixture by using acid-alkali extraction. 2. To determine the percentage recovery and melting point of the recovered benzoic acid and p-dichlorobenzene. Introduction
Acid-base extraction is a process which purifying the acids and bases from mixtures based on their chemical properties. Acid-base extraction is performed to isolate the compounds and natural products from crude extracts. One of the salt of the mixture should be ionic and tend to be water soluble while the another neutral molecules tend not to be water soluble. Laurence M. Harwood, Christopher J. Moody (13 June 1989) said that the addition of an acid to a mixture of an organic base and acid will result in the acid remaining uncharged, while the base will be protonated. If the organic acid, such as a carboxylic acid is sufficiently strong to self-ionization that can be suppressed by adding the acid.
Conversely, the addition of a base to a mixture of an organic acid and base will result in the base remaining uncharged, while the acid is deprotonated to give the corresponding salt. Once again, the self-ionization of a strong base is suppressed by the added base. The acid-base extraction can also be used to separate very-weak acids from stronger acids and very-weak bases from stronger bases as long as the difference of their pKa or pKb constants is large enough. In this experiment, we would like to carry out the extraction to extract out the benzoic acid p-dichlorobenzene. Benzoic acid is a colourless crystalline solid and the simplest aromatic carboxylic acid. While, the d-dichlorobenzene is an organic compound which is colourless. Its melting point is in the range of 52-54 °C
1. 40ml of dichloromethane with contain 1g of p-dichlorobenzene and 1g of benzoic acid are pour into a dry 100ml conical flask.
2. Mix and dissolve the solid mixture by using a glass rod.
3. Dichloromethane is then transferred into a separating funnel and add20ml of 10%NaOH solution.
4. Stopper the separating funnel and shake well.
5. Separating funnel is allowed to stand on a ring clamp.
6. Remove the stopper and run off the lower organic (dichloromethane) into a conical flask.
7. Remove the aqueous layer into a 250ml beaker and keep it aside.
8. The dichloromethane solution is then transferred again into the separating funnel and added another portion of 20ml 10%NaOH.
9. Stopper the separating funnel and shake well.
10. Separating funnel is allowed to stand on a ring clamp.
11. Run off the lower dichloromethane layer into a conical flask.
12. Transfer the aqueous layer (top layer) into the same 250ml beaker from the first extraction.
13. Separating funnel is cleaned by the tap water before rinse by the distilled water.
14. Transfer the dichloromethane into the separating funnel again.
15. The dichloromethane layer is washed with 30ml distilled water.
16. Run off the lower dichloromethane layer into a dry 100ml conical flask.
17. Anhydrous CaCl2 is added into the dichloromethane layer by using a spatula and shake the mixture occasionally until no turbidity is remained.
18. The dichloromethane is then filtered by using a small piece of cotton wool on a dry glass funnel.
19. Dry dichloromethane is collected into a dry and preweigh 100ml beaker.
20. Add the boiling chips (one or two pieces) into the beaker on a hot water bath (70°C) inside the fume hood.
21. Weigh the beaker again to obtain the mass, melting point and percentage recovery of p-dichlorobenzene.
22. Concentrated HCl is added to solution in 250ml beaker until the mixture is acidic to litmus paper and no further precipitated is formed. 23. The precipitate benzoic acid is filtered by using the Buncher funnel and filter flask.
24. The benzoic acid is washed with 5ml-10ml of cold distilled water on the Buncher funnel.
25. The benzoic acid is transferred to a piece of filter paper.
26. The benzoic acid is pressed by another piece of filter paper until the crystal is completely dry.
27. Determine the mass, melting point and percentage recovery of benzoic acid.
28. The percentage recovery of d-dichlorobenzene and benzoic acid are determined by using the following formula: Percentage recovery =
Part 1: mass of extraction
Initial mass of (a) Benzoic acid : 1.00g
(b) p-dichlorobenzene : 1.05g
Mass of beaker(g)
Mass of beaker + p-dichlorobenzene(g)
Mass of p-dichlorobenzene recovered(g)
Percentage recovery of p-dichlorobenzene =
= = 84.76%
Mass of filter paper(g)
Mass of filter paper + benzoic acid(g)
Mass of crystal benzoic acid recovered(g)
Percentage recovery of benzoic acid =
= = 79.00% Part2 : Melting point
Measured melting point range (°C )
Choosing solvent is important to extract out two immiscible solvents. Some common extraction solvent is dichloromethane, ether and hexane. In this experiment, the benzoic and p-dichlorobenzene are used to dissolve in the dichloromethane, CH2Cl2. Water is needed during the extraction of benzoic acid and p-dichlorobenzene because it is highly polar and immiscible in all organic solvents. Mostly, the compound want to be extracted is soluble in organic solvent but not soluble in water. This is due to the volatility of solvent is too high as most solvents have low boiling point which can use to dry the isolated material very fast. For example, ether is a solvent which is very volatile and has low boiling point. Both the benzoic acid and p-dichloromethane are soluble in dichloromethane as organic compound can dissolve in organic solvent. However, both organic compound is not react with dichloromethane, but just dissolve in it. NaoH is then added into the mixture. NaoH will react with benzoic acid to form sodium salt of benzoic acid and water.
After the addition of NaoH, two layer of solution will be formed. Upper layer formed is organic layer where lower layer is aqueous layer which run off into a conical flask. After this, NaoH is added again into the mixture to ensure that the benzoic react completely with NaoH. This reaction form the similar two layers and the lower layer in run off into same conical flask. The reaction between the sodium hydroxide and benzoic acid produce a lot vapor after shaking the separating funnel for a few times. Surplus pressure will be produced due to the vapor from the reaction. Thus, the vapor should be released through the tap of the separating funnel several times to reduce the pressure inside it.
Then, water is added into the separating funnel to wash the solvent and the lower layer is run off into a conical flask. Granular calcium chloride is added into the organic solvent after the lower layer run off. The purpose to add the granular calcium chloride is used to remove the residual water in the organic solvent. FDA Compliance Articles US Food and Drug Administration March 1995 said that calcium is drying agent and an inorganic salt which readily takes up water to become hydrated. In other word, calcium chloride will react with water if water is available. CaCl2 + H2O → CaO + 2HCl
Then, the heavier hydrated solid which is CaO is sink to the bottom of the beaker and easier to remove. An excess CaCl2 should be used to ensure all the water has been removed. If the water is remained in the organic solvent, it could interfere the compound obtained.
After the water is removed completely, the solution which containing p-dichlorobenzene transfer to a beaker. A few pieces of boiling chips is added and being heated on a hot plate to evaporate the dichloromethane. The boiling chip is small, insoluble and porous stone made by calcium carbonate and silicon carbide. There are a lot of pores inside the boiling chips which provide cavities to trap the air and provide spaces to allow the bubbles of solvent can be formed. Thus when the boiling chips are heated, it will release tiny bubbles which can prevent boiling over. Boiling over a solution cause the loss of solution and lead to inaccurate result. However, boiling chips cannot add to the hot solution directly because it will boil over the solution immediately. The solution is heated continuously until left 20ml of solution inside the beaker. This is because the crystal of p-dicholorobenzene will melt and hence the crystal cannot be recovered. The weight of p-dichlorobenzene is 0.89g with melting point of 52-55°C. The percent recovery of d-dichlorobenzene is 84.76%.
Next, the aqueous layer that collected in the conical flask is added with HCl. The white precipitated will form immediately. Blue litmus paper turns from blue to red colur when excess HCl is added. The HCl is added until no precipitated is formed. This is the neutralization process where sodium benzoate and HCl react each other to form white precipitated of benzoic acid.
The white precipitated of benzoic acid is washed with cold water during filtration to minimize the solubility of benzoic acid in water. Then the benzoic acid formed is dry up by pressing with filter paper. The weight of benzoic acid is 0.79g with melting point of 121-124°C. The percent recovery benzoic acid is 79.00%. Conclusion
Acid and alkali extraction can use to separate compounds according to their chemical properties. Benzoic acid and d-dichlorobenzene can be extracted from the mixture of the dichloromethane. The recovery percentage of the benzoic acid is only 79.00% while the recovery percentage of d-dichlorobenzene shows 84.76%.The melting point of benzoic acid is 121-124 oC but the melting point of d-dichlorobenzene is 52-55 oC.
1. Universiti Tunku Abdul Rahman, Department of Chemical Science, Perak campus, Chemistry Laboratory experiment manual (Page 43,44) 2. http://1chemistry.blogspot.com/2011/09/extraction-with-acid-and-alkaline.html 3. http://en.wikipedia.org/wiki/Acid-base_extraction
4. http://www.vernier.com/experiments/chemo/5/separation_of_organic_compounds_by_acid-base_extraction_techniques/ 5. http://www.chemicalbook.com/ChemicalProductProperty_EN_CB9329690.htm 6. http://en.wikipedia.org/wiki/Calcium_chloride#cite_note-4