Objective: 1) To perform an acid-alkaline extraction. 2) To recover benzoic acid and p-dichlorobenzene from its mixture using acid-alkaline extraction. 3) To determine the percent recovery of benzoic acid and p-dichlorobenzene.

The formula for calculating the percentages of recovery:

                

The formula for calculating the relative accuracy of melting point:

        

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Chemicals/reagents: Benzoic acid (1g), p-dichlorobenzene (1g), ether (40ml), 10%  (30ml), conc. , distill water, (35ml), anhydrous .

Glass wares: Separatory funnel, Buchner funnel.

Procedure:

1. Weighted 1g of benzoic acid and 1g of p-dichlorobenzene and mixture in a beaker. The weight was recorded.
2. 40ml of ether were added into the same beaker and poured into the saparatory funnel with retort stand.
3. Added 20ml freshly prepared 10% solutions into the separatory funnel.
4. Stopper the funnel, removed from the retort stand and shake well, occasionally inverting and releasing any surplus pressure through the tap with at least 3 times.
5. The separatory funnel was then placed back the retort stand and let the solution settle down. Two layer of solution was seen after a few minutes.
6. Removed the stopper and separate the aqueous layer into a conical flask.
7. Another 10ml of 10%  solution was added again into the funnel and repeat step 4 to step 5.
8. Removed the stopper and separated the aqueous layer to the previous conical flask.
9. 30 ml of distill water were added into the funnel and shake the mixture. The water layer appeal after settle down was removed into a beaker.
10. The ether layer was removed from the separatory funnel to a dry beaker and added some anhydrous  to absorb excess water in the ether.
11. Boiling chips were also added in to the beaker and transferred to hot water bath to evaporate the ether solution in it around 20 minutes.
12. Meanwhile, added excess concentrated hydrochloride acid into conical flask until the precipitated was formed and changed the blue litmus paper turn red.
13. The precipitate solution was transferred to a Buchner funnel with filter paper to be filtered.
14. Dry crystals were remove from the Buchner funnel and dried for 2 minutes then transferred to watch glass to weighted and recorded down the record.
15. The beaker was removed from the hot water bath and transferred to cold water bath to form crystals in the beaker for 5 minutes and weighted and record down the result.
16. The precipitate from the watch glass and the crystals from beaker were then tested their melting point.

Result:

Weight of initial Benzoic acid        =1.1148g

Weight of initial p-dichlorobenzene        =1.0936g

Weight of watch glass        =28.2146g

Weight of watch glass +Benzoic acid        =29.3208g

Weight of Benzoic acid recovery        =1.1062g

Weight of Beaker        =33.5278g

Weight of Beaker + p-dichlorobenzene        =34.5044g

Weight of p-dichlorobenzene recovery        =0.9766g

m.p. of Benzoic acid        =102 ~ 105

m.p. of p-dichlorobenzene        =53

Calculation:

Discussion:

In this experiment we are doing the extraction to purify the pure benzoic acid and p-dichlorobenzene from the mixture of benzoic acid, p-dichlorobenzene, and sodium hydroxide solution in ether solution. We will first mix them together in a beaker and transferred into the seperatory funnel and being mixed by shaking the funnel with the switch open to release surplus pressure from the funnel. The mixture will undergo the following reaction in the funnel:

        In the mixture, sodium hydroxide solution was added for ionize to form sodium ions and hydroxide ions. Thus, the sodium ions will substitute the benzoic cation to formed salt, sodium benzoate. While the hydrogen ions will combine with hydroxide ions to become water which can use to solute the sodium benzoate. In the mixture, the p-dichlorobenzene is insoluble in water and does not involve any reaction and remaining in ether solution. Thus, two layers formed which the top layer is organic layer and lower layer is aqueous layer. After separate the aqueous layer to a conical flask, the organic layer was used distilled water to wash by dissolved remaining benzoic compound in the organic layer and separated the aqueous layer into same conical flask.

        Concentrated hydrochloride acid was added into the conical flask until the mixture acidic to litmus paper and until no further precipitate occurred. This is to ensure the entire sodium benzoate convert into benzoic acid. Washed the organic layer by using distill water is to remove sodium hydroxide solution. After separated the ether layer and water layer, added anhydrous calcium chloride to remove the remaining water inside the ether solution. The pure organic layer was then transfer to filter out the solidified calcium chloride. Added to the boiling clip to ether layer is to prevent the tiny bubble from during heating for caused the loss of sample. Before used Buchner funnel filter the precipitated benzoic acid, should use distilled water to rinse the filter paper to let the filter paper completely cover the hold of the Buchner funnel for prevent the losses of sample during filter.

        The percentage of recovery of both crystals is not 100% due to some loss during the process of extraction. The weight of benzoic acid recovered and its melting point are 1.1062g and around 103.5. The percentage of recovery of benzoic acid is 99.23% and the relative accuracy of melting point of benzoic acid is 84.84%. Most probably some the products are dissolved in the cold distilled water during filtration. The relative accuracy of melting point for benzoic acid is less than 100% due to the existence of impurities in the products.

The weight of recovered p-dichlorobenzene is about 0. 9766g with melting point is around 53.5. In addition, the percent recovery and relative accuracy of melting point for p-dichlorobenzene are 89.3% and 100.94%. The percent recovery of p-dichlorobenzene is a bit low which has 89.30% may be due to the lost of product in the experiment. Furthermore, the relative accuracy of melting point is more than 100% because the product is not a pure p-dichlorobenzene due to some impurities exist in the product.

Precaution:

1. All the process involves in ether solution must be done in the hood.
2. Make sure to release the pressure to a place without anyone.
3.  When weighting the crystals must make sure the crystals are dry enough to get an accurate result.
4. Handle the concentrated hydrochloride acid be careful because concentrated hydrochloride acid are very corrosive.

Conclusion:

        Mixture of different properties of organic compounds can be separate by using extraction method. In the experiment the percentage of Benzoic acid recovery is 99.23% and the percentage of p-dichlorobenzene recovery is 89.30%. While the relatively accuracy of the melting point for benzoic acid is 84.84% and the melting point for p-dichlorobenzene is 100.94% respectively.

Reference:

1. W.J.Moore, (1956) Physical Chemistry, Longman, London (page 471 – 473)
2. Retrieved February 4, 2012, from

faculty.swosu.edu/william.kelly/extract_pl.ppt

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