Extraction with Acid and Alkaline

Objective:
1. To recover benzoic acid and p-dichlorobenzene from its mixture using acid-alkaline extraction. 2. To determine the percentage recovery and melting point of benzoic acid and p-diclorobenzene. Apparatus:Separatory funnel(250mL),Buchner funnel,beaker.

Materials:Benzoic acid,p-dichlorobenzene,ether,10% NaOH,conc.HCl,distilled water, anhydrous CaCl2. Introduction:
Organic compounds in an aqueous mixture can be separated by shaking the aqueous solution with a solvent which is immiscible with water. Extraction involves dissolving either a compound or compounds from solid into a solvent or from solution into another solvent. Acid-alkaline extraction is a commonly used method to recover a compound from solid or liquid and also a form of solvent-solvent extraction. Acid-alkaline extraction is a method to separate compounds according to their relative solubility in two different immiscible solutions. Usually, acid-alkaline extraction is being used to recover a compound from solid or liquid. The two solvents used to conduct acid-alkaline extraction must be insoluble to one another. Therefore, the mixture of two solvents will be separated into two layers. The less dense solvent will be at upper layer while the denser solvent will stay at lower layer. In most experiment,one of the solvent used will be water on an aqueous solution and the other will be organic solvent that is insoluble in water.

The organic solvent used have to be volatile because it can be easily removed by evaporation when forming crystals. Phenolic compounds will undergo salt formation with sodium hydroxide solution, NaOH.Therefore,sodium hydroxide solution is allow to use to extract a carboxylic acid or phenolic compound from its solution in an organic solvent.An organic solvent can be used to remove organic impurities from a carboxylic acid or phenol dissolved in aqueous sodium hydroxide.Aqueous solution of Sodium bicarbonate,NaHCO3 are able to convert carboxylic acids to their sodium salts.However,sodium bicarbonate are not sufficiently alkaline to form salts of phenolic compounds.This is the reason why acid-alkaline extraction can be used to perform separation of carboxylic acid and phenolic compound.

Firstly,remove the acid may be removed from solution in an organic solvent using extraction method with sodium bicarbonate solution and then use sodium hydroxide to remove the phenol.Inorganic acid are regularly removed from organic solvents by extraction with sodium hydroxide, sodium carbonate, or sodium carbonate solutions.Dilute hydrochloric acid is used to extract basic substances from mixtures or to remove the basic impurities.The dilute acid converts the base such as ammonia or an organic amine(R3N) into the water-soluble chloride salt(R3NH+,Cl-).Dilute acid solution with organic solvent can be used to extract organic impurities from amine. Sodium salts of carboxylic acid and phenolic compounds are readily reconverted to the parent compounds by treatment with sulphuric or phosphoric acid.The chlorides of amines revert to the original amine upon addition of sodium hydroxide solution. Procedures:

1. 1g of benzoic acid and 1g of p-dichlorobenzene were poured into 40ml of ether solution into a small separatory funnel. 2. 20ml of freshly prepared 10% NaOH was added into the separatory funnel. 3. Stopper the funnel and shaken occasionally inverting and released any surplus pressure through the tap. 4. The funnel is allowed to stand.

5. The stopper is removed and the lower aqueous layer was ran into a conical flask. 6. The residue in the funnel was shook with another 10ml of 10% NaOH and the lower water layer ran off into the same conical flask. 7. Concentrated HCl was added drop wise into the solution until the mixture was acidic to litmus and until no further precipitation occurred. 8. The ether solution was washed with 30ml water.

9. The lower layer was ran off and discarded.
10. Three granules of anhydrous CaCl2 was added to the ether solution and the mixture was shaken occasionally until no turbidity remained. 11. The ether solution was decant into clean dry small conical flask. 12. Two boiling chips was added and warmed gently in the water bath until 15ml of ether remained. 13. Then,the ether solution was decat into a tarred watch glass and placed in the fume hood to allow the rest of the ether to evaporate. 14. The precipitated benzoic acid was filtered using a Buchner funnel and a flask and 5ml of cold distilled water was used to wash. 15. A current of air was drawn through the product for 5 minutes to remove as much water as possible. 16. The crystals was pressed in a folded filter paper to ensure it was completely dry. 17. The weight and melting point of the recovered benzoic acid was recorded. 18. The weight and melting point of p-dichlorobenzene was recorded once all the ether was evaporated. Results:

Weight before separation
Benzoic acid
Mass of benzoic acid = 1.0067g
Mass of watch glass=19.636g
Weight of filter paper = 0.3287g
p-dichlorobenzene
Mass of p-dichlorobenzene = 1.0700g
Weight of evaporating dish = 52.018g

Weight after separation
Benzoic acid
Weight of watch glass,filter paper and benzoic acid recovered = 21.441g Weight of benzoic acid recovered = 1.4763g
p-dichlorobenzene
Weight of evaporating dish and p-dichlorobenzene precipitate= 52.6415g Weight of p-dichlorobenzene recovered = 0.6235g

Melting point of benzoic acid = 120 ̊ C
Theoretical melting point of benzoic acid = 122 ̊C
Melting point of p-dichlorobenzene = 59 ̊C
Theoretical melting point of p-dichlorobenzene = 53 ̊C

Percentage recovery of benzoic acid = weight of compound recoveredweigght of compound used X 100% = 1.4763g1.0067g x 100% =146.65% Relative accuracy of melting point of benzoic acid

= melting point of compound recoveredmelting point of pure compound x 100% = 120 ̊C122 ̊C x 100%
= 98.36%
Percentage recovery of p-dichlorobenzene = weight of compound recoveredweight of compound used X 100% = 0.6235g1.0700g x 100% = 58.27% Relative accuracy of melting point of p-dichlorobenzene

= melting point of compound recoveredmelting point of pure compound x 100% = 59 ̊C53 ̊C x 100%
= 111.32%

Discussion:
In this experiment,benzoic acid and p-dichlorobenzene is used to dissolve in ether,( (C2H5)2O).Benzoic acid is a white and crystalline organic compound belong to the carboxylic acids family.
Benzoic Acid
p-dichlorobenzene is a colourless solid organic compound with strong odour.

p-dichlorobenzene
Benzoic acid and p-dichlorobenzene can dissolve in the ether solution because organic compounds are soluble in organic solvent.Both solvent does not react with the ether solution and only dissolve in it.Sodium hydroxide,NaOH is then added into the ether.Benzoic acid are strong enough to react with a strong aqueous base,sodium hydroxide to yield sodium salt of benzoic acid and water.

Two layers of solution are formed which the upper layer is organic solvent while the lower layer is aqueous layer.The sodium salt of benzoic acid(sodium benzoate) is dissolve in the aqueous layer which runs off into a conical flask.NaOH is added another time into the solvent to ensure all the benzoic acid is reacted completely with sodium hydroxide.Then,the similar two layers are formed and the aqueous layer is transferred into the same conical flask using the exactly same method.The reaction between benzoic acid and sodium hydroxide produces a lot of vapour after a few times of shaking.Therefore,the surplus pressure produced from the reaction is being released the tap of separatory funnel for a few times to reduce high pressure inside.Distilled water is added into separatory funnel to wash the solvent.After the aqueous layer run off,the granular calcium chloride is added into the organic solvent.This is to remove residual water in the organic solvent.Sodium chloride, CaCl2, is a salt of calcium and chloride.It is solid at room temperature.

Calcium chloride is known as drying agent in the organic solvent which are not dissolves in the solvent but drying the solvent.Calcium chloride reacts with water to form hydrates. The upper layer contained p-dichlorobenzene in ether is run off into a conical flask.Three boiling chips(calcium carbonate) is added into the conical flask and warmed in a water bath until 15ml of ether left in the conical flask.Then,left the solution in the fume hood to get the amount of p-dichlorobenzene being recovered.There are a lot of pores inside the boiling chips which provide cavities both to trap air and to provide spaces to allow bubbles of solvent can be form.The sodium benzoate in aqueous layer collected in the conical flask is added with hydrochloric acid,HCl and formed white precipitate. Then,sodium benzoate in aqueous layer collected in the conical flask is added with hydrochloric acid,HCl and white precipitate formed from the reaction.Neutralization take place when sodium benzoate and HCl is reacted with one another to produce precipitate of benzoic acid.Precipitate of benzoic acid is the main product while the sodium chloride salt and water are the side product.

The solubility of benzoic acid in the water is minimized by washing the white precipitate using cold distilled water during filtration.After the filtration,dry the benzoic acid crystal in the fume hood. According to the results obtained from the experiment,the percentage recovery of benzoic acid is 146.65% while p-dichlorobenzene is 58.27%.The percentage recovery of benzoic acid is very high because it is contaminated with impurities and therefore the weight of benzoic acid recovered is more than the weight of benzoic acid used.Therefore,the percentage error of the recovery of benzoic acid is 46.65% which is not a proper value.On the other hand,the percentage recovery of p-chlorobenzene is consider low which is only 58.27% because the solution might over warmed and caused the p-dichlorobenzene in the solution evaporated with the water vapour and ether solution.Therefore,the percentage error of the recovery of p-dichlorobenzene is 41.73% because the p-dichlorobenzene recovered is much lower than the weight of p-dichlorobenzene used.

The relative accuracy of melting point of benzoic acid and p-dichlorobenzene is 98.36% and 111.32% respectively.The relative accuracy of melting point of p-dichlorobenzene is more than 100% is because the product is not a pure p-dichlorobenzene due to the existence of impurities in the product.The percentage error of melting point of benzoic acid and p-dichlorobenzene is 1.64% and 11.32% respectively.The percentage error of benzoic acid and p-dichlorobenzene is also due to the impurities added throughout the experiment without notice. There are a few precaution steps throughout the experiment.Firstly,make sure all the nearby flames must be extinguished when the separation is being carried out.This is because ether is highly flammable.Secondly,swirl the separatory funnel gently before stopper it.It is important to swirl the separatory funnel if acid and sodium bicarbonate are present because carbon dioxide is given off by their reaction.Swirling will get off some carbon dioxide and minimize pressure buildup during the shaking process.Thirdly,make sure boiling chips are used when heating a solvent but never added to a hot solution.This is because it will cause immediately boiling over of solution and no results can be obtained.Last but not least,must wear gloves when handling concentrated HCl with care.This is because HCl is a very strong acid.

Conclusion:
Acid-alkaline extraction is used to recover benzoic acid and p-dichlorobenzene.The percentage recovery of benzoic acid and p-dichlorobenzene is 146.65% and 58.27% respectively.The melting point of benzoic acid and p-dichlorobenzene is 120 ˚C and 59˚C respectively.The melting point obtained is different with the theoretical value due to the presence of impurities.

References:

1. P-dichlorobenzene.Retrieved at 5/2/2013 from
http://www.chemicalland21.com/industrialchem/organic/P-DICHLOROBENZENE.htm

2. Benzoic acid.Retrieved at 5/2/2013 from http://global.britannica.com/EBchecked/topic/61357/benzoic-acid

3. Sodium benzoate.Retrieved at 4/2/2013 from http://pubchem.ncbi.nlm.nih.gov/summary/summary.cgi?cid=517055

4. Carboxylic acids.Retrieved at 3/2/2013 from http://www.chemguide.co.uk/organicprops/acids/background.html