### Introduction The process of crystallization involves the precise manipulation of solubility and temperature, offering a powerful tool to separate and purify individual chemicals from their mixtures. The choice of solvent plays a pivotal role in this technique, as solvents exhibit varying solubility behaviors at different temperatures. This method finds particular utility when aiming to purify substances with minor impurities, as the differing solubility of the impurities versus the target compound enables their exclusion during the crystallization process. Once a sample is dry and solvent free, the melting point of the chemical can be taken using a Mel-Temp or Thomas Hoover heating system and compared against literature values to evaluate purity. If a sample undergoes the process of crystallization/purification, it will have a higher melting point and more narrow melting point range than the original sample. ### Materials and Methods ![triangle](crystallization_lab_chemicals_used.png) ![triangle](crystallization_lab_materials_used.png) #### Crystallization of Phthalic Acid from Water 103.4mg of phthalic acid and 0.5mL of deionized (DI) water were put into a test tube and heated. An additional 1.5mL of DI water was added to the test tube in order to facilitate dissolving. The solution was removed from the steam batch and allowed to cool to room temperature. Once cooled to room temperature, the test tube was placed into an ice bath for 10 minutes. The liquid solution was pipetted off using a Pasteur pipette and 10 drops of ice-cold ethanol were added to the test tube. The test tube was swirled around and contents of the test tube were then transferred over to a Hirsch funnel with filter paper attached to a vacuum apparatus. The solution was allowed to evaporate and the remaining crystals were transferred off the filter paper onto a scale where the weight of the crystals was taken. The crystallized phthalic acid was then loaded into a 2mmx4cm capillary tube and analyzed in a Mel-Temp instrument to determine the melting point. The original phthalic acid sample was measured using the same technique. #### Crystallization of Benzoic Acid from Cyclohexane 95.7mg of benzoic acid and 0.5mL of cyclohexane were put into a test tube and heated. An additional 1.0mL of cyclohexane was added to the test tube in order to facilitate dissolving. The solution was removed from the steam batch and allowed to cool to room temperature. Once cooled to room temperature, the test tube was placed into an ice bath for 10 minutes. The liquid solution was pipetted off using a Pasteur pipette and 10 drops of ice-cold cyclohexane were added to the test tube. The test tube was swirled around and contents of the test tube were then transferred over to a Hirsch funnel with filter paper attached to a vacuum apparatus. The solution was allowed to evaporate and the remaining crystals were transferred off the filter paper onto a scale where the weight of the crystals was taken. The crystallized benzoic acid was then loaded into a 2mmx4cm capillary tube and analyzed in a Mel-Temp instrument to determine the melting point. The original benzoic acid sample was measured using the same technique. #### Purification of Unknown Chemical Two samples weighing 50.5mg and 48.6mg of Unknown Chemical #4 (UC4) were obtained to determine which solvent should be used for the recrystallization process. These samples were added to either a test tube containing 0.5mL of water or a test tube containing 0.5mL of cyclohexane. Both test tubes were heated and observed to determine which test tube UC4 dissolved in. The sample dissolved in cyclohexane, so a third sample of UC4 was obtained weighing 202.3mg and added to a test tube containing 3mL of cyclohexane. This test tube was placed into a steam bath until the solids had dissolved. The solution was allowed to cool to room temperature at which point, the inside of the glass was scratched with a microspatula to encourage crystallization. The solution was then placed into an ice bath for 10 minutes and once removed, the remaining solution was drawn off using a Pasteur pipette. 10 drops of ice-cold cyclohexane were added to the test tube and the test tube was swirled around. The contents of the test tube were then transferred over to a Hirsch funnel with filter paper attached to a vacuum apparatus. The solution was filtered out and the crystals were allowed to dry. Once dry, the crystals were weighed and the melting point was determined by loading the crystals into a 2mmx4cm capillary tube and analyzing it in a Mel-Temp instrument to determine the melting point. The original UC4 sample was measured using the same technique. ### Results #### Crystallization of Phthalic Acid from Water 81.3mg of phthalic acid was recovered after performing the crystallization process. With a starting weight of 103.4mg, this results in a relative percent recovery of 78.6%: ![triangle](crystallization_lab_percent_recovery_phthalic_acid.png)

Equation 1: Phthalic Acid Percent Recovery

The purified phthalic acid was loaded into a capillary tube and the melting point was analyzed. The purified phthalic acid began melting at 202℃ and was completely melted at 208℃. The starting phthalic acid substance began melting at 193℃ and was completely melted at 204℃. #### Crystallization of Benzoic Acid from Cyclohexane 77.8mg of benzoic acid was recovered after performing the crystallization process. With a starting weight of 95.7mg, this results in a relative percent recovery of 81.3%: ![triangle](crystallization_lab_percent_recovery_benzoic_acid.png)

Equation 2: Benzoic Acid Percent Recovery

The purified benzoic acid was loaded into a capillary tube and the melting point was analyzed. The purified benzoic acid began melting at 117℃ and was completely melted at 120℃. The starting benzoic acid substance began melting at 105℃ and was completely melted at 114℃. #### Purification of Unknown Chemical #4 165.7mg of UC4 was recovered after performing the crystallization process. With a starting weight of 202.3mg, this results in a relative percent recovery of 81.9%: ![triangle](crystallization_lab_percent_recovery_unknown.png)

Equation 3: UC4 Percent Recovery

The purified UC4 was loaded into a capillary tube and the melting point was analyzed. The purified UC4 began melting at 64℃ and was completely melted at 67℃. The starting UC4 substance began melting at 56℃ and was completely melted at 63℃. ### Discussion Phthalic acid and benzoic acid were able to be purified, but not without step yield losses during the recrystallization phase. Phthalic acid has a literature melting point of 210℃ (3) and the melting point obtained of the purified phthalic acid in this experiment was 208℃ with a melting range of 202℃-208℃ (6℃). The starting phthalic acid had a melting point of 204℃ with a melting range of 193℃-204℃ (11℃). The more narrow range of the purified phthalic acid (6℃ vs 11℃) along with the melting point value closer to the literature value (starting: 204℃, purified: 208℃, literature: 210℃) indicate that impurities were removed during the recrystallization process. However, along with these impurities, 22.3mg of material was lost during the process, resulting in a 78.6% recovery. Benzoic acid has a literature melting point of 121℃-123℃ (1) and the melting point obtained of the purified benzoic acid in this experiment was 120℃ with a melting range of 117℃-120℃ (3℃). The starting benzoic acid had a melting point of 114℃ with a melting range of 105℃-114℃ (9℃). The more narrow range of the purified benzoic acid (3℃ vs 9℃) along with the melting point value closer to the literature value (starting: 114℃, purified: 120℃, literature: 121℃-123℃) indicate that impurities were removed during the recrystallization process. Similar to the phthalic acid recrystallization process, some material was lost along with these impurities. 17.9mg of material was lost during the process, resulting in a 81.3% recovery. The melting point obtained for UC4 in this experiment was 67℃ (2) with a melting range of 64℃-67℃ (3℃). The starting UC4 material had a melting point of 63℃ with a melting range of 56℃-63℃ (7℃). Based on the melting point, the closest chemical would be biphenyl, which has a literature melting point of 69℃. A total of 36.6mg of material was lost during the process, resulting in a 81.9% recovery. ### Conclusion The experiment demonstrated the benefits and risks of performing a recrystallization process. Purified samples of phthalic acid, benzoic acid, and biphenyl were obtained but not without a loss in total material. Utilizing the Mel-Temp unit, the melting points of the recrystallized materials were able to act as a surrogate for determining the purity of the samples. The increase in melting point and the narrowing of the melting point range (as compared to the starting material) both provided an indication that the sample had become more pure. The hypothesis for this experiment was that purified samples would have increased melting points, and that was proven to be correct. ### References 1. PubChem. (2019). Benzoic Acid. Nih.gov; PubChem. https://pubchem.ncbi.nlm.nih.gov/compound/benzoic-acid#section=Melting-Point 2. PubChem. (2019). Biphenyl. Nih.gov; PubChem. https://pubchem.ncbi.nlm.nih.gov/compound/Biphenyl#section=Melting-Point 3. PubChem. (2019). Phthalic Acid. Nih.gov; PubChem. https://pubchem.ncbi.nlm.nih.gov/compound/phthalic-acid#section=Melting-Point Don't forget to check out our [Molecule Namer study tool](/study-tools#namer) and our [Reaction Solver study tool](/study-tools#solver)!