Recrystallization of Benzoic Acid

Experiment 2: Recrystallization of Benzoic Acid February 21, 2021 Abstract Companies creating chemicals, medications, or food products require high purity materials to ensure a safe, high quality product. The simplest method for purifying many organic compounds is crystallization. After dissolving benzoic acid in heated water, it was cooled, filtered and subsequently dried for one week before determining final yield and measuring melting point to determine purity. From a 1.00 g initial sample of benzoic acid, 0.93 g of recrystallized product was obtained, resulting in a 93 % yield of purified benzoic acid with a melting point range of 121 ˚C to 124 ˚C. The literature provides a melting point of 122 ˚C for benzoic acid, which falls in this experimental range. Introduction Benzoic acid is an aromatic carboxylic acid (Fig.1) which forms a colorless, crystalline solid and can be found naturally in both plants and animals and some microorganisms (del Olmo, 2017; Sandeepa, 2018). It has many uses including being used to create a variety of chemicals, antiseptics, plasticizers, and food preservatives (Thati, 2010). According to Sandeepa (2018) crystallization yields a high purity solid product while consuming less energy than other techniques making it one of the most important techniques for separation and purification. This process is used across many sectors of industry including production of pharmaceuticals and food as well as in the manufacture of electronics and chemicals. In order to be used for these purposes it is necessary to be able to separate benzoic acid from other reactants and obtain a high level of purity. Figure 1. Benzoic Acid structure, consisting of an aromatic ring joined to a carboxylic acid. This experiment demonstrates the process of recrystallization of benzoic acid from water for the purpose of purifying an organic compound. The yield of the recrystallization process can be greatly impacted by the choice of solvent(s) and the temperature used with the solvent(s) as they directly affect supersaturation of the solution (Sandeepa, 2018). Water was chosen in this experiment as benzoic acid has a low solubility of 3.44 g/L at 25 ˚C and much higher solubility of 56.31 g/L at 100 ˚C. Subsequently, melting point will be measured in order to determine the end product’s purity. A very narrow range for melting point is a good indicator of a pure compound. And a lower than expected, or broader than expected, melting point range can indicate the presence of impurities (Setzer, 2008). Initially, solid benzoic acid was dissolved in water in a flask on a hot plate. Water was added in one milliliter increments until the benzoic acid fully dissolved and was then allowed to cool to room temperature. Afterwards, the solution was placed in an ice bath to facilitate crystal formation until fully cooled. The product was then transferred to a vacuum filter apparatus consisting of a side-arm flask and Buchner funnel with filter paper. Three washings of the product in the filter setup (with very cold water to prevent dissolution) were performed before allowing the final product to dry for one week. Once the final mass was obtained, a sample was used with a Mel-Temp melting point apparatus to determine the melting point range of the purified benzoic acid. Discussion Dissolving the initial benzoic acid in heated water should also allow most impurities to be dissolved into the solution as well and to be vacuumed away during filtration. After cooling the solution and allowing the crystalline solid to form, the crystals are washed to remove additional impurities and allow those to be vacuumed away as well. After waiting one week for the yielded product to dry, any remaining water should be evaporated, and a final mass of recrystallized benzoic acid can be measured. In this experiment, the final mass was 0.93 g. With an initial mass of 1.00 g of benzoic acid, this equates to a 93% yield of purified product. Since the benzoic acid is still somewhat soluble in water at 25 ˚C and even a small amount at 0 ˚C, there will be some loss of product in the filtration step due to using water as the solvent. After determining the final yield of the recrystallization process a sample can be taken to be used with the Mel-Temp melting point apparatus. The melting point range is found to be 121- 124 ˚C for the final product in this experiment. A range of 3 ˚C is somewhat broader than would be expected for a pure compound. However, the literature states the melting point of benzoic acid is 122 ˚C and this is in the middle of the range obtained from the sample in the experiment. The widened range can likely be attributed to a small amount of remaining impurity in the sample. The results are generally in line with expected outcomes. There was some loss of mass through the procedure, but the goal was to remove impurities which would explain some of the loss. Without testing the melting point range of the initial benzoic acid, we do not know for sure if we have improved the purity. However, the range of the product is quite close to the actual melting point of benzoic acid indicating a mostly pure product. Using water as the solvent also explains some of the loss of mass that could not be accounted for by impurity removal. Future testing could be done with a different solvent or group of solvents having a better solubility profile for benzoic acid to potentially prevent losses during the crystallization and filtration processes. Table 1. Experiment Results and known values. Initial Mass Benzoic Acid 1.00 g Final Mass (Yield) Benzoic Acid 0.93 g Percentage Yield (relative to starting mass) 93 % Melting Point Range of product 121-124 ˚C Literature Melting Point of Benzoic Acid 122 ˚C