Recrystallization of Benzoic Acid

Categories: ChemistryScience


Recrystallization is a widely used method for purifying solid compounds in organic chemistry. It involves dissolving the compound in a suitable solvent at an elevated temperature and then allowing it to slowly crystallize as the solution cools. This process exploits differences in solubility between the desired compound and impurities, resulting in the formation of pure crystals.

Benzoic acid, an aromatic carboxylic acid, was chosen as the compound for recrystallization in this experiment. The technique aims to isolate pure benzoic acid from impurities by utilizing its differential solubility in hot and cold solvent.


  1. Purification of Benzoic Acid via Recrystallization: The primary aim is to employ the recrystallization technique to purify benzoic acid, a common organic compound. By dissolving the impure benzoic acid in a suitable solvent at an elevated temperature and then slowly cooling the solution, the impurities are expected to remain in solution while the pure benzoic acid crystallizes out.
  2. Determination of Melting Point: Following the purification process, the next objective is to determine the melting point of the purified benzoic acid.

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    The melting point serves as a crucial indicator of the compound's purity. A narrow melting point range indicates high purity, while a broader range suggests the presence of impurities. Through careful observation and analysis of the melting behavior, the purity of the benzoic acid can be assessed accurately.

These objectives form the foundation of the experiment, aiming to achieve both purification and characterization of benzoic acid, essential steps in organic chemistry research and practical applications.

Experimental Procedure

The experimental procedure commenced with the dissolution of impure benzoic acid () in hot distilled water (). The solubility of benzoic acid increases with temperature, allowing for the maximum amount of solute to be dissolved. This hot solution was carefully filtered to separate any insoluble impurities from the solution. The use of a filtration apparatus, such as a Buchner funnel, ensured efficient removal of solid impurities while retaining the liquid solution.

Following filtration, the clear filtrate containing dissolved benzoic acid was subjected to controlled cooling. As the solution gradually cooled, the solubility of benzoic acid decreased, leading to the spontaneous crystallization of pure benzoic acid molecules. The slow cooling process facilitated the formation of larger, well-defined crystals, enhancing the purity of the final product.

Once crystallization was complete, the benzoic acid crystals were separated from the remaining solution via suction filtration. Suction filtration, utilizing negative pressure to draw the liquid through a filter medium, effectively isolated the crystalline solid from the liquid phase. The collected crystals were then thoroughly washed with ice-cold water to remove any adhering impurities or residual solvent molecules.

Post-Filtration Treatment

After washing, the wet benzoic acid crystals were transferred to a drying oven. The drying process involved subjecting the crystals to gentle heat (typically around 100°C) to evaporate any remaining water molecules and ensure complete dryness. Proper drying is essential to prevent the formation of clumps or agglomerates and to maintain the integrity of the purified benzoic acid crystals.

Additional Steps for Enhanced Purity

To further enhance the purity of the benzoic acid crystals, additional purification steps may be employed. These can include repeated recrystallization cycles, where the purified crystals are dissolved in a minimal amount of hot solvent and then allowed to recrystallize upon cooling. Each recrystallization cycle serves to concentrate the desired compound while minimizing the presence of impurities.

Furthermore, characterization techniques such as thin-layer chromatography (TLC) or spectroscopic analysis (e.g., infrared spectroscopy) may be utilized to confirm the purity of the recrystallized benzoic acid. These analytical methods provide valuable information regarding the composition and structure of the purified compound, allowing for a comprehensive assessment of its quality and purity.

By meticulously following these steps and incorporating additional purification measures as needed, the recrystallization process can yield highly pure benzoic acid suitable for various research and industrial applications.


The mass of impure benzoic acid was measured to be 1.0132 grams, while the mass of recrystallized benzoic acid was 0.3601 grams. This corresponds to a percentage recovery of 35.54%. The experimental melting point of the recrystallized benzoic acid was found to be 122.1°C, indicating its purity.


The observed low percentage recovery of benzoic acid hints at potential losses encountered during the experimental procedure, indicating that certain factors may have hindered the efficient recovery of the compound. These losses could have arisen from various sources, including incomplete filtration leading to the retention of benzoic acid within the filter medium or the loss of fine particles during transfer between vessels. Additionally, the solubility behavior of benzoic acid in the chosen solvent might not have been ideal, resulting in partial dissolution and subsequent loss of product during the recrystallization process.

Furthermore, the slight deviation observed between the experimental melting point and the theoretical value suggests the likelihood of impurities present within the crystalline structure of the benzoic acid. These impurities could have originated from residual contaminants in the starting material or introduced during the experimental procedure. The presence of impurities alters the physical properties of the crystals, including their melting point, leading to discrepancies between the observed and expected values.

Moreover, variations in experimental conditions such as heating rates, cooling rates, or ambient temperature fluctuations could have contributed to the observed discrepancies in recovery and melting point. Small deviations in these parameters can influence the kinetics of the recrystallization process and affect the purity of the final product. Therefore, meticulous attention to experimental conditions and rigorous adherence to procedural protocols are crucial to minimize such deviations and ensure the accuracy of the experimental results.


In conclusion, the recrystallization process has proven to be effective in purifying benzoic acid, as demonstrated by the attainment of a relatively close melting point to the theoretical value. This alignment between the observed and expected melting points signifies the removal of a significant portion of impurities from the crude sample, resulting in a higher degree of purity in the final product. However, while the purification process achieved notable success, as indicated by the satisfactory melting point, the relatively low percentage recovery suggests the presence of potential areas for enhancement in the experimental technique.

The observed low percentage recovery hints at the possibility of losses occurring throughout the various stages of the experiment, from dissolution to crystallization and filtration. These losses could stem from factors such as incomplete dissolution of the impure benzoic acid, inefficient filtration leading to the retention of product in the filter medium, or losses incurred during transfer and handling of the crystalline material. Additionally, variations in experimental conditions, such as heating and cooling rates, solvent choice, or agitation methods, may have contributed to the suboptimal recovery rate.

To address these challenges and improve the efficiency of the recrystallization process, several strategies can be implemented. Firstly, optimizing the choice of solvent and its concentration to maximize the solubility of benzoic acid at elevated temperatures while minimizing its solubility at lower temperatures can enhance the yield of purified product. Secondly, meticulous attention to procedural details, such as proper filtration techniques, careful handling of equipment, and precise temperature control, can help mitigate losses and improve overall recovery rates.

Furthermore, conducting a thorough analysis of potential sources of error and implementing corrective measures, such as adjusting experimental parameters or refining procedural steps, can contribute to enhanced reproducibility and accuracy in future experiments. Additionally, exploring alternative purification methods or complementary techniques, such as chromatography or recrystallization with co-solvents, may offer insights into optimizing the purification process further.


For future experiments, it is recommended to optimize filtration techniques to prevent losses of product and ensure proper crystallization. Additionally, careful attention should be paid to the choice of solvent and cooling rate to achieve maximum yield and purity.


  1. Purpose of Recrystallization: Recrystallization is a process used to purify solid compounds by dissolving them in a solvent at high temperature and allowing them to crystallize as the solution cools.
  2. Anti-bumping Agents: Anti-bumping agents are added to heated solutions to prevent vigorous boiling and bumping, which can lead to loss of material.
  3. Suction Filtration vs. Gravitational Filtration: Suction filtration is preferred over gravitational filtration for separating pure crystals from solution because it is more efficient and faster.
  4. Water as a Solvent for Recrystallization: Water is not ideal for recrystallization of all compounds because it can dissolve many substances, potentially keeping them in solution rather than allowing them to crystallize.
  5. Reasons for Deviation in Melting Point: Deviations in melting point from theoretical values may occur due to impurities present in the crystal lattice or errors in experimental technique.
  6. Usages of Benzoic Acid: Benzoic acid is commonly used as a food preservative, in medicinal applications, as a feedstock, and in various biological and health-related studies.
  7. Class/Family of Benzoic Acid: Benzoic acid belongs to the class of aromatic carboxylic acids.
  8. Synthesis of Benzoic Acid: Benzoic acid can be synthesized from benzyl chloride through oxidation using potassium permanganate and sulphur dioxide as catalysts.


  • Lab Manual
  • Yeap Tok Kheng, Chemistry SPM Highlights, Pearson Malaysia Sdn. Bhd, 2014, p. 341-390


Updated: Feb 26, 2024
Cite this page

Recrystallization of Benzoic Acid. (2024, Feb 26). Retrieved from

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