Recrystallization - Purification of Benzoic Acid CHE311L PDF

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The Purification of Benzoic Acid by Recrystallization

CHE311L 9/29/16

Summary: This experiment was carried out to obtain pure benzoic acid from an impure benzoic acid mixture by recrystallization. This was done by first obtaining the melting point of impure benzoic acid using the mel-temp and capillary tube method. Following was the determination of the ideal solvent – a choice between water, 95% ethanol or hexanes. Water proved to be the ideal solvent. Recrystallization of the benzoic acid then took place.

Introduction: Recrystallization is a method used for purifying a compound. It involves manipulating the solubility of that compound in solvent system in order to separate the pure substance from its impurities. When choosing the solvent system for recrystallization, the ideal solvent must be used. In an ideal solvent, the compound to be purified is “infinitely insoluble” at lower temperatures, but “infinite soluble” at high ones, and is dissolved in as little hot solvent possible. As the solvent cools, crystals of the pure substance begin to form. The insoluble, impure components of an ideal solvent can be removed by a process called hot filtration. In this process, the hot solution is passed through an already-hot filter, separating the impurities and the pure product + soluble impurities (filtrate) which will recrystallize. Washing the crystals with cold solvent removes the soluble impurities leaving only the final purified product. Recrystallization is considered a necessary but wasteful procedure because some product will be lost regardless of how efficiently the experiment is carried out. The liquid that remains after recrystallization will contain some small amounts of the solute, causing product loss.

Methods: A. Determining the melting point of the impure solid A capillary tube packed with 2-3 mm of impure benzoic acid and placed in a mel-temp, then heated at a rate of 2 C/minute to obtain the melting point range of that impure benzoic acid. B. Identifying the ideal solvent 1 mL each of 3 solvents were tested to find the ideal solvent. These included water, 95% ethanol and hexanes. Each was placed in a separate test tube along with approx. 2mg of impure benzoic acid. Solutions were stirred to determine the solubility of the benzoic acid in each. Results were described using soluble, insoluble or slightly soluble. The solvent in which the benzoic acid completely dissolved (ethanol) was eliminated as a choice for ideal solvent. The remaining two solutions were heated then stirred to further observe solubility. A data table for this information was created. Observations were recorded after cooling. C. Large scale recrystallization of the impure benzoic acid Recrystallization solvent (water) was heated, while 2g of impure benzoic acid was added to an Erlenmeyer flask. Hot solvent was added to the flask while stirring, until no more solid dissolved. De-colorization with activated charcoal was not necessary. Hot filtration was then performed, the funnel was heated with hot solvent, and then solution was passed through to separate impurities from pure soluble solute. Solution was cooled too room temperature then placed in an ice bath to allow further recrystallization. Vacuum filtration was then used to isolate pure crystals from solvent, which were washed with a small amount of ice-cold solvent to remove further impurities. Crystals were left to dry until next lab period and percent recovery calculated. Melting point of the pure solid was obtained.

Results:

Data Tables Melting Point Range of Impure Benzoic Acid 91  C – 173.5  C Table 1. Solubility of Impure Benzoic Acid Solvent

Solubility Abbreviation Water ic/sh 95% Ethanol sc/ih Hexanes Ssc/ssh Ic: insoluble cold ih: insoluble hot Sc: soluble cold

sh: soluble hot

Ssc: slightly soluble cold

ssh: slightly soluble hot

Calculations: Percentage recovery = (mass pure material/ mass crude material) * 100%

Discussion:

Conclusion: Melting point range for impure benzoic acid determined to be 91  C – 173.5 C. Water proved to be the ideal solvent after testing, and was used to complete the recrystallization process.

References: [ CITATION Org \l 1033 ]...