Last Updated 28 Jan 2021

Extraction of Benzoic Acid

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Introduction: The theory behind the extraction of a solution containing benzoic acid, cellulose, and methyl orange involves many components pertaining to the fundamental ideas of solubility and polarity. Using the concepts of like dissolving like and acid base reactions, a solution of organic acid (benzoic acid), a water soluble compound cellulose, and an organic soluble compound methyl orange, can be separated and benzoic acid can be isolated by a method of extraction.

At the fundamental level, organic solvents separate from aqueous solutions based on their varying densities, typically the organic layer being less dense and therefore on top of the aqueous layer1. Therefore any solutes in those solutions are also separated and can be isolated from one another. This phenomenon is the basis for the liquid-liquid extraction and leads into the concept of the partition coefficient. According to MtCg the ratio of concentrations of a solute in each layer is defined by the partition coefficient K, where K = C2 / C1.

The distribution coefficient can therefore be viewed as the ratio of the concentration of the compound in organic solvent to the concentration of the same compound in aqueous solution. This constant allows for the calculation of dissolved compound in each layer of the solution, so that after several extractions, the yield can be sufficient for the purposes of the experiment. A second type of extraction, acid-base extraction, involves the addition of acids and bases in solutions to change the polarity of organic acids and bases to their corresponding water-soluble salts2.

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This allows us to take advantage of the liquid-liquid extraction to separate organic acids and bases from solution mixtures. Such extraction can be considered chemically active extraction because the polarity of one substance is altered to increase solubility in water. This type of extraction is necessary when both compounds are non-polar and dissolved well in organic solvents. For Experiment #1 Extraction of Benzoic Acid, a solution of benzoic acid, cellulose and methyl orange will first be added to ether, heated and filtered to separate the organic insoluble cellulose leaving benzoic acid nd methyl orange. Then after adding NaOH, the benzoic acid will react to form the water-soluble salt, sodium benzoate, which can be separated via the liquid extraction technique. Finally adding HCl to the aqueous sodium benzoate will cause the acid-base reaction to undergo restoring benzoic acid in the organic layer and NaCl in the water layer. Reagent Table: Experimental: To start, a solution of diethyl ether was mixed with 4. 06g of crude benzoic acid with two impurities.

Boiling stones were added to the mixture and heated over the steam bath until the ether solution began to boil. Once the solution was boiling to most efficiently dissolve the inorganic parts of the solution, the ether insoluble material was filtered out of solution using the Buchner funnel vacuum filtration technique. The filtrate ether remaining was added to a separatory funnel with 30mL of 1M NaOH, mixed and vented properly, and then rested as the solution settled into separate organic and aqueous layers.

The bottom layer (aqueous) was drained from the separatory funnel into a beaker and another 30mL of NaOH was added to the left over ether, swirled and vented, separated, and the aqueous layer drained off into the same beaker as the first aqueous extraction. Next the aqueous solution that was collected was cooled on ice and 50mL of chilled 6M HCl was added to the mixture. At this point, a Bruchner funnel was used to collect the solid precipitate via vacuum filtration and those collected crystals were weighed out and set aside for the recrystalization of benzoic acid.

Results: The most important measurements made during this lab were the initial weight of the crude benzoic acid, which was 4. 06g, and the mass of the crystals after the separation from the two impurities, 4. 88g. The fact that the crystals weighed more than the starting material can be accounted for based on the lack of proper drying time for the crystals, and will be accounted for when calculating the percent composition.

Discussion: After the results of the lab, there were a few points to consider in the final analysis. First off, and most importantly, after the crystals were obtained from the aqueous solution obtained by adding NaOH, the crystals were used immediately for the recrystalization and were not given the proper time to dry out, so the weight of the wet crystals must be used to calculate the percentage composition, which will be inaccurate.

Another important note was after the HCl was added to the aqueous solution, the solution turned bright pink, indicating that some of the methyl orange, which should have remained in the organic layer in the separatory funnel, had made it into our aqueous solution. To counteract this, when the vacuum filtration technique was used, the solid precipitate was washed with distiller water to ensure the crystals did not contain a pink tint to them.

The reason this lab works was because the first filtration got rid of the organic insoluble cellulose leaving methyl orange and benzoic acid in the ether mixture. Then, using the acid-base extraction technique, benzoic acid was altered to sodium benzoate, rendering it soluble in aqueous solution, which allowed for the separation of the sodium benzoate from the methyl orange (still dissolved in ether) and then the reformation of benzoic acid by adding a strong acid to the aqueous dissolved salt. References: MtCg pg. 129 MtCg pg. 131

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