Universiti Tunku Abdul Rahman (kampar Campus) Faculty Of Science, Engineering,

Universiti Tunku Abdul Rahman (Kampar Campus) Faculty of Science, Engineering, and Technology Bachelor of Science (Hons) Biotechnology Year 1 Semester 2 Laboratory 1B (UESB 1212) (II) The Properties of Matter Lecturer: Ms. Chew Yin Hoon Student’s Name: Cheah Hong Leong Student’s ID: 08AIB03788 Partner’s Name: Chong Shi Fern Partner’s ID: 08AIB02580 Experiment No. 7 Title: Recrystallization Date: 20 March 2009

Title: Recrystallization

Objective: –

To purify the benzoic acid by filtration of benzoic acid solution and recrystallization of it from its solution.

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To determine the percentage of recovery of the benzoic acid crystal obtained in experiment.

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To determine the purities of the benzoic acid crystal by measuring the melting point and compare it with the theoretical melting point.

Introduction: The most crucial factor in organic chemistry is the purity of the substances used in experiment. Therefore, purification of the compound before used in experiment is very important in organic chemistry. Among the most popular method in purification is the recrystallization. In recrystallization, an impure solute is dissolved in a correct solvent. The insoluble impurities would be removed by filtration while the soluble solute will be crystallized from the solution. A suitable solvent is chosen to dissolve the impure compound or solute. Different solutes have different solubility in different solvent. Thus, the first and foremost step in recrystallization is choosing the suitable solvent. A suitable solvent is dependent on the solutes, but in most cases, the solvent must be readily volatile and unreactive towards the solute. The solute should be more soluble in hot solvent while the impurities should be more insoluble in hot solvent. The solvent must also has very high solute solubility at high temperature; and very low solute solubility at low temperature.

For benzoic acid, the most suitable solvent used in recrystallization is water. Benzoic acid has a carboxylic polar group (-COOH), and it is more soluble in hydroxylic solvent, such as water and ethanol. Water is very good solvent especially for polar compound with melting point larger than 100 oC. For benzoic acid that having melting point of 122 oC, water is a very suitable solvent for it. Recrystallization is based on the solubility of solute in solvent at different temperature. The higher the temperature, the higher the solubility of solute in solvent. Usually the solute will soluble in hot solvent but insoluble in cold solvent. In some cases, such as in benzoic acid case, the impurities are insoluble in hot solvent, therefore hot filtration is used in recrystallization of benzoic acid. In hot filtration, filtration is carried out in high temperature so that the benzoic acid is soluble in water while the impurities are insoluble and thus can be filtered by using filter paper.

Hot filtration must be carried out at high temperature to avoid earlier crystallization in filter paper and filter funnel. Therefore, short-stemmed filter funnel is used to avoid crystallization to occur on stem of funnel. Besides, hot water must also pour into the Erlenmeyer flask that will be used in filtration and the funnel and filter paper must also place on top of the opening of the Erlenmeyer flask so that the apparatus is at hot temperature when the filtration is carried out. To further avoid the crystallization in filter funnel, hot water is used to rinse the funnel. In cold solvent, the solubility of benzoic acid in water is much lower and this solute has no longer soluble in water and precipitated out from the solution as crystal solid. After the crystallization, we need to check for the percentage of recovery of benzoic acid obtained and also the melting point of the obtained benzoic acid crystal. Percentage of Recovery= Weight of benzoic acid recoveredWeight of starting benzoic acid x 100%

The melting point of is measured to determine the purity of the purified benzoic acid. If there are still have some significant impurities in the crystal, the melting point obtained should be lower than that of the theoretical melting point. Experimental Procedures: Materials and Apparatus –

Crude benzoic acid

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Anti-bumping agent

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Water (hot water and ice bath)

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Erlenmeyer flasks

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Beakers

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Short-stemmed funnel

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Filter papers

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Stirrer

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Buchner funnel

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Suction flask

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Suction pump

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Hot plate

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Incubator

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Melting point capillary tube

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Melting point meter

Procedures 1. About 2.0 g of crude benzoic acid was weight with analytical balance and the weight of the crude benzoic acid was recorded. 2. The crude benzoic acid was then put into an Erlenmeyer flask. 3. Hot water was taken into a beaker. Little amount of it as transferred to another

Erlenmeyer flask equipped with short-stemmed funnel and filter paper. 4. Anti-bumping agent was added to the Erlenmeyer flasks containing crude benzoic acid, hot water and beaker containing hot water. 5. Both the beaker and the Erlenmeyer flask containing hot water and anti-bumping agent were placed onto a hot plate. 6. The hot water in the beaker was poured little by little into the Erlenmeyer flask

containing crude benzoic acid and anti-bumping agent until all the benzoic acid was dissolved in hot water completely. While doing so, the solution was stirred. 7. After the benzoic acid was completely dissolved, the Erlenmeyer flask was put onto the hot plate to further heat the solution. 8. Meanwhile, the Erlenmeyer flask equipped with short-stemmed funnel and filter paper was prepared for filtration. Hot water was filtered through the fluted filter paper before the filtration was carried out. 9. The hot benzoic acid solution was filtered through the fluted filter paper into the heated Erlenmeyer flask. The original beaker and filter paper were rinsed with hot water into the heated Erlenmeyer flask. 10. The Erlenmeyer flask containing the filtered benzoic solution was taken away

from the hot plate. The solution was transferred into a beaker and was allowed to cool down to lower temperature for about 15 minutes. 11. An ice bath was prepared. The beaker containing the benzoic acid solution was

put into the ice bath until crystal was seen formed in the solution. 12. Apparatus needed for suction filtration were prepared for used. The filter papers put on Buchner funnel were rinsed with cold water before the filtration was carried out.

13. The suction pump was switched on, and the crystals of benzoic acid were poured slowly onto the filter papers on Buchner funnel. 14. After all the crystals had already poured and filtered. The beaker was rinsed with cold water and then poured onto the filter papers to ensure that no remaining crystals were there anymore. 15. After the filtration, the filter papers containing the crystal of benzoic acid were transferred into an incubator so that the crystal can be allowed to air-dry. 16. Two days after the experiment, when the crystal benzoic acid had been completely dried, the filter papers together with the crystal benzoic acid were taken out from the incubators. 17. The weight of the crystal benzoic acid was determined using analytical balance. 18. The melting point of the crystal benzoic acid was determined using melting point meter.

Results: Weight of filter papers (2 pieces) and starting benzoic acid solid = 2.6663 g Weight of filter papers (2 pieces) = 0.6475 g Weight of starting benzoic acid solid = 2.0188 g Weight of filter papers (2 pieces) and recovered benzoic acid solid = 1.6945 g Weight of recovered benzoic acid solid = 1.0470 g Melting point of recovered benzoic acid = 122 oC

Analysis and Calculations: Percentage of Recovery= Weight of benzoic acid recoveredWeight of starting benzoic acid x 100% Percentage of Recovery= 1.0470 g2.0188 g x 100%

Percentage of Recovery = 51.86 %

Discussion: During the filtration of hot solution of benzoic acid, the filter paper was involuntarily torn by the stirrer used to stir the solution pour into the filter paper on funnel. Fearing that some of the impurities might have escaped into the filtered solution in Erlenmeyer flask, we decided to repeat the filtration. Another filter paper was placed on the funnel under the torn filter paper. The filtered solution was pour again through the filter paper and funnel for second filtration. Although filter paper had already torn during the first filtration, but this problem had been treated by having a second filtration to filter again any escaped impurities. The percentage of recovery obtained was only 51.86 %. Not all the dissolved benzoic acid was crystallizes after the filtration. One of the possible reasons for this low percentage of recovery was the hot filtration. During the hot filtration, some crystal solid was observed to be formed in the filter paper on funnel. The lowering of temperature in filter paper and funnel causing the lowering of solubility of the dissolved benzoic acid, leading to the formation of crystal in the filter paper on funnel during the filtration process. Although the filter paper had been already rinsed with hot water after that, but by minor quantity there was still some crystal remained in the filter paper. This error can be fixed by numerous rinses the filter paper by hot water. Another possible reason was the cooling of the filtered solution. The temperature of the solution might not as low as expected while the solution was thought to be cold enough to be filtered through the Buchner funnel. When the solution in the beaker was immersed into the ice bath, the solution in the middle part of the beaker might not cold enough. Therefore crystallization of the benzoic acid was incomplete when the cold filtration was carried out. This error can be fixed by stirring the solution in the beaker while immersed into the ice bath so that the temperature of the solution was even. The melting point of the recovered benzoic acid obtained was 122 oC. The melting point of the pure benzoic acid given was 122 oC too. This showing that the recovered benzoic

acid obtained was really pure since there was no deviation from the theoretical melting point. This might due to the double filtration when it was found that the filter paper had been torn by the stirrer.

Conclusion: The percentage of recovery = 51.86 % The melting point of recovered benzoic acid = 122 oC

References: 1. Bell C. E., Taber D. F., and Clark A. K. (2001). Organic Chemistry Laboratory with Qualitative Analysis. Standard & Microscale Experiments, Third Edition, Thomson Brooks/Cole. 2. Benzoic

Acid.

(n.d.).

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http://en.wikipedia.org/wiki/Benzoic_acid 3. Solubility.

(n.d.).

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