Recrystallization of Benzoic acid Chemistry 204-002 Date: 4/9/2011 Hector Ortega Recrystallization is a method used to purify compounds, and this is a very important procedure because compounds are naturally impure. Polarity and solubility play a big role in this experiment and these characteristics are exploited in order to have a crystallization of the desired compound. In this experimented benzoic acid was crystallize using ethanol and water. I chose these solvents based on part C of the experiment (table 1. 1). The compound is too soluble in ethanol but too insoluble in water.
Benzoic acid is a polar compound in comparison with ethanol which is a less polar compound. However at low temperatures benzoic acid is insoluble in water. Compounds are soluble in solvents with the same polarity, like dissolves like. Table 1. 1 Benzoic Acid solubility Dissolve Heat Ice H2O No No No Ethanol Yes Yes Yes Hexane No No Acetone Yes Yes Toluene No Yes No The benzoic Acid was dissolved in a very small amount of hot ethanol once dissolved water was added drop wise using a disposable pipet until it got cloudy.
Once it reached this point the substance was removed from the steam bed and let cool down at room temperature. It is important to let this cool down slowly so impurities don’t get trapped in between the crystals. After 20 minutes the crystals were filtered out using a vacuum filter method. The crystals were carefully removed from the filter paper and weighted. These crystals were dried out for a period of 2 weeks, to later get a melting point. The MSDS value was compared with the experimental value to verify purity of the crystals Page | 1 Recrystallization of Benzoic acid
Chemistry 204-002 Date: 4/9/2011 Hector Ortega Data Collected: Ethanol and Water used as solvents Benzoic acid Grams Started 0. 154 Recovered 0. 126 Final Recovered 0. 064 Percent Recovery 81. 81% *Before Drying for two weeks Final Recovery 41. 52% Melting Point 118. 9° C - 119. 9° C MSDS MP 122. 4° C Percent difference 3. 684% Crystallizing benzoic acid using a mixture of solvents is the most efficient procedure compared to using one solvent alone. By comparing the experimental and MSDS melting points we can see that our recovered crystals had very little impurities only a 3. 8 % difference. If I was to repeat the experiment I would improve the recovery process by letting the substance cool down even more slowly to avoid impurities to get trapped in the crystals. When the crystals were moved from the filter paper to the weighing boat some of the crystals were lost. This could be improved by letting the sample dry longer so the crystals would not stick to the filter paper or the spatula. Another possible error could have happened when taking the melting point, the rate at which the temperature was rising was a little too high.
In the future slowly and gradually rising the temperature would give better more accurate results. Page | 2 Recrystallization of Benzoic acid Chemistry 204-002 Date: 4/9/2011 Hector Ortega 3D embedded Structures « Che m 3 D E m b e d » Benzoic Acid « Che m 3 D E m b e d » Water Page | 3 Recrystallization of Benzoic acid Chemistry 204-002 Date: 4/9/2011 Hector Ortega « Che m 3 D E m b e d » Ethanol References Zubrick (8th ed. ) Chs. 6, 11 and 13 Chemdraw Ultra Version 12. 0 Page | 4