Recrystallization of Different Compounds: Laboratory Report

Abstract

Recrystallization is a purification method commonly used for solid organic substances. In this experiment, we performed the recrystallization of three different compounds: Phthalic Acid, Benzoic Acid, and Naphthalene.

The process involves selecting a suitable solvent, dissolving the solute, decolorizing the solution, filtering suspended solids, recrystallizing the solute, collecting and washing the crystals, and finally drying the product. The goal was to obtain pure crystals by removing impurities through this recrystallization technique.

Introduction

Recrystallization is a crucial technique in organic chemistry for purifying solid compounds.

It involves dissolving a solid compound in a hot solvent, allowing it to recrystallize as the solution cools, and ultimately obtaining pure crystals while leaving impurities behind in the mother liquor. This process is based on the differences in solubility between the desired compound and its impurities at different temperatures.

The seven main steps of recrystallization are as follows:

1. Selecting the appropriate solvent.
2. Dissolving the solute in the hot solvent.
3. Decolorizing the solution if necessary.
4. Filtering suspended solids to remove insoluble impurities.
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5. Recrystallizing the solute by cooling the solution.
6. Collecting and washing the obtained crystals.
7. Drying the purified product.

Activated charcoal is often used during recrystallization to absorb impurities from the solution.

Table of Chemicals

Materials and Methods

We followed the procedures outlined in the laboratory manual for CHM 2210L and CHM 2211L (Weldegirma, 2014) for the recrystallization of Phthalic Acid, Benzoic Acid, and Naphthalene. The chemicals used were obtained from reliable sources, and all equipment was properly cleaned and calibrated before use.

1. Weighed the initial mass of the impure compound for each substance.

2. Dissolved the compound in a suitable hot solvent for each case, ensuring complete dissolution.

3. Added activated charcoal to decolorize the solution if needed.

4. Filtered the solution to remove any suspended solids or insoluble impurities.

5. Allowed the solution to cool slowly to induce crystallization.

6. Collected the crystals by filtration, followed by washing with a small amount of cold solvent.

7. Dried the obtained crystals to a constant mass.

Data Analysis

We recorded the initial mass of the impure compounds, the mass of the collected crystals, and the melting points of the purified compounds. The percent recovery was calculated as follows:

Percent Recovery = (Mass of Collected Crystals / Initial Mass of Impure Compound) × 100%

Results

The results of the experiment are summarized in the following table:

Discussion

The results of the experiment indicate some interesting findings. The percent recovery of Benzoic Acid is notably above 100%, suggesting that an error occurred during the experiment. This could be due to the presence of excess solute or improper filtration.

Phthalic Acid and Naphthalene both exhibited lower than expected percent recoveries, indicating that not all of the solid was collected during the filtration process. This could have resulted from incomplete filtration or loss of crystals during the transfer process.

It's important to note that the observed melting points for Phthalic Acid, Benzoic Acid, and Naphthalene were slightly lower than their respective standard melting points. This deviation could be attributed to the presence of impurities in the crystals obtained or variations in the experimental conditions.

Conclusion

In conclusion, the experiment successfully demonstrated the recrystallization technique for purifying solid organic compounds. While we were able to obtain purified crystals, the main limitation of the experiment was the failure to achieve the expected percent recovery, particularly in the case of Benzoic Acid. This suggests the presence of experimental errors such as excess solute or inadequate filtration. Nevertheless, the recrystallization method remains a valuable tool for obtaining purified substances by removing impurities.

Recommendations

For future experiments involving recrystallization, it is essential to carefully monitor the filtration process to ensure that all crystals are collected. Additionally, proper measurements of the solute and solvent quantities should be made to avoid excess solute, which can lead to results exceeding 100% recovery. Conducting multiple recrystallization cycles may further improve the purity of the obtained crystals.

References

1. Weldegirma, Solomon. Laboratory Manual: CHM 2210L and CHM 2211L. Tampa: Chemistry Department, 2014.
2. "Phthalic Acid Msds." Science Lab. N.P., n.d. Web. 30 Sept. 2014.
3. "Recrystallization Technique." - [www.rhodium.ws]. N.p., n.d. Web. 29 Sept. 2014.