Crystal Clear: An Experiment involving Purification by Recrystallization
By Natalie Trachsel
Objective
In this experiment, which involved purification of impure naphthalene through recrystallization, there were a few goals: first, to learn to choose an effective solvent for crystallization; second, to purify naphthalene using recrystallization; third, to observe the different aspects of the technique of recrystallization in order to determine how certain factors impact others in results (high percent recovery versus high purity of the final naphthalene sample); and finally, to determine crystal purity based on melting point.
Introduction
Recrystallization is the process by which impurities are removed from a compound when an impure solid is dissolved and allowed to recrystallize apart from the dissolved solution, leaving a pure, lattice solid and a separate, impure solution. Recrystallization experiments are an important part of organic chemistry, as they allow for the separation of desired compounds or impurities without breaking any chemical bonds (Fletcher). In this process, a solvent is chosen to dissolve the compound and its impurities based on four solubility characteristics: the solubility of the compound in the solvent, the solubility of the impurities in the solvent, the reactivity and differences between the solvent and the compound, and volatility of the solvent. At the same time, it is important for the compound to be soluble at a high temperature, but only slightly soluble at room temperature, as complete insolubility will prevent the compound from dissolving, and being too soluble at room temperature increases the difficulty of making the compound recrystallize in its purest form. The impurities, then, should be very soluble at room temperature or insoluble in the hot solvent, so as to be able to filter undissolved impurities and allow the soluble impurities to remain dissolved after the compound recrystallizes. The solvent should also have a boiling point lower than the melting point of the compound, because if not, the solid would melt, rather than dissolving, resulting in an improper crystallization. Options for possible solvents for this experiment are displayed in Table 1. Table 2 gives the molecular structure, molar mass, and melting and boiling points of naphthalene. According to this information, the chosen solvent would need a boiling point lower than the melting point of naphthalene, 80.26˚C. Then, as displayed in Table 3, each solvent has a different molecular structure, molar mass, melting point, and boiling point, and thus a different predicted solubility for naphthalene based on polarity. According to Table 3, naphthalene would likely be insoluble at cold temperatures in water, methanol, and acetone, making these likely predictions for possible solvents to be used later in the experiment. Even with this information, however, the results of the solubility tests were based on trial and error, as explained later in the report.
Table 1. Common solvents sorted by decreasing polarity
and their boiling points.
Table 2. Napthalene and its Molecular Characteristics
Table 3. Possible Solvents and their Molecular Characteristics
The equation used in this experiment was Equation 1 (“How To”), which describes percent recovery of naphthalene, where the mass of the purified sample divided by the impure sample mass and multiplied by 100% gives the percent recovery of naphthalene after crystallization.
% Recovery=purified massimpure sample mass100% (1)
Finally, the instrument used to collect data in this experiment was the Mel-Temp apparatus, which works by melting substances in capillary tubes to be observed during the process. Using a small amount, the substance can be observed closest to its true melting point, and with the thermometer, a range of melting points can be recorded for both impure and pure naphthalene.
Procedure: Materials
Chemicals
Impure naphthalene (mothballs: white or off-white solids); water; methanol (clear, colorless liquid that smells like nail polish remover); hexane (clear, colorless liquid with petroleum-like odor); toluene (clear, colorless liquid that smells like paint thinner); and decolorizing carbon (black solid)
Special Equipment
Gravity filtration apparatus: Erlenmeyer flasks, funnel, coffee filters made into cone filters or fluted filters
Vacuum filtration apparatus: 250 mL side-arm suction flask, vacuum stopper with tubes and rubber hoses, Büchner funnel, fast filter paper
Melting point determination apparatus: Mel-Temp machine, melting point capillary tubes, thermometer, long glass tube
Other Materials
Beakers; small test tubes; test tube rack; small erlenmeyer flasks; boiling chip; hot plate; glass stir rod; rubber policeman; paper towels; spatula; ring stand; ring;p wire mesh; large crystallization dish or watch glass; drying oven
Procedure: Solubility Tests
The experiment was performed with a partner. To prepare, large beaker with cold water was obtained. Then, a series of tests were performed to determine an appropriate solvent system to be used for the recrystallization of naphthalene: First, five small test tubes were labeled 1-5 and placed in a test tube rack. 3 mL of water, methanol, acetone, hexane, and toluene were added to each of tubes 1-5. For organization purposes, each of the tubes was labeled with its respective chemical. Observations about each solvent (color, clarity, odor, boiling point, etc.) were recorded for each solvent. The tubes were placed in the previously-prepared cold water bath for at least two minutes. To each tube, 0.10 g of impure naphthalene was added, and each was mixed well. Observations about the solubility of naphthalene in each solvent were recorded in accordance with the key shown in Table 4 (below). Any cold solvent in which the naphthalene was soluble was considered unsuitable for recrystallization.
Table 4. Abbreviations for Observations on Solubility
Abbreviation
Observation
VS
Very soluble (dissolves completely with little to no mixture)
SO
Soluble (dissolves eventually if shaken enough)
SS
Somewhat soluble (only part dissolves after shaking)
IN
Insoluble (none of the solid appears to dissolve)
For any mixture in which the naphthalene seemed insoluble at cool temperatures, respective tubes were heated in a hot water bath made in a small beaker to see if the naphthalene would dissolve in a hot solvent. When necessary, the solvent was heated to a gentle boil to see if the naphthalene was soluble. Observations about the solubility of naphthalene in these hot solvents was recorded, once again in accordance with the Table 4 key. Any solvent in which the naphthalene was insoluble when hot was also considered unsuitable for recrystallization, and therefore was abandoned for the remaining steps. The test tubes containing the hot solvent with solvated naphthalene were placed into an ice water bath to cool. If the crystals reformed, the respective solvent was chosen for recrystallization steps. If the crystals did not reform, the test tubes were shaken, or their inner walls were scratched with a glass stir rod. Observations about the recrystallization of naphthalene from hot solvents were recorded. Any solvent in which naphthalene failed to recrystallize at room temperature was considered unsuitable for recrystallization, and was therefore abandoned for the remaining steps. If multiple solvents remained, the boiling points of each were compared to the melting point of naphthalene. A good solvent for this recrystallization should have had a boiling point lower than the melting point of naphthalene. The larger the difference in these values, the easier it would be to avoid turning naphthalene into a liquid while attempting to turn it into a pure solid. The boiling point of the chosen solvent for recrystallization, clearly stated, and the melting point of naphthalene (217.9˚C) (“Naphthalene”) were recorded.
Procedure: Recrystallization
Two small erlenmeyer flasks were labeled as “solvent” and “solution.” About 2.0 g of the impure naphthalene were added to the “solution” flask. Then, about 30 mL of the chosen solvent from previous tests, along with a boiling stone, were added to the “solvent” flask. The “solvent” flask was placed on the hot plate, with the opening covered by a watch glass, and heated to boiling. It was important to consider a series of points in this step: first, a boiling chip should always be added before heating any solvent or solution; and second, a solid should never be added to a solution that is at or near its boiling point, as suddenly creating nucleation sites is likely to cause anything from a simple boil-over to a volcano-like reaction. Once the solvent was boiling, the heat was turned down so that the solvent was barely simmering, and a pasteur pipette was used to transfer small portions of the solvent to the “solution” flask containing the naphthalene. At this point, both flasks were placed on the hot plate. Hot solvent was continually added to the naphthalene sample until all of the naphthalene dissolved. To encourage dissolution of the naphthalene, the flask was swirled periodically. The exact amount of naphthalene at the start was recorded. Additionally, an estimate of the amount of solvent added to dissolve the naphthalene was made. Keeping in mind that there may have been insoluble impurities remaining after the naphthalene had dissolved, the naphthalene was allowed to dissolve fully. If subsequent additions of solvent to the solution did not seem to dissolve any of the remaining solid, addition of solvent to the solution was halted. Any observations about impurities remaining were recorded. Naphthalene crystals are white and give colorless solutions (“Naphthalene”). Based on this information, observations regarding the color of the current solution were recorded and compared. If the solution was colored, it needed to be treated with decolorizing carbon. Important information to consider for this step was that decolorizing carbon should not be added to a boiling solution. Rather, the solution would need to be cooled slightly. Then, a spatula-tip (pea-sized amount) of decolorizing carbon could be added, and the solution could be reheated to boiling for a few minutes. Adding too much decolorizing carbon would result in absorption of naphthalene, so extra care had to be taken in this step in consideration of this factor. Whether or not decolorizing carbon was added and the resulting changes in color of the solution were observed. Next, any insoluble impurities and any decolorizing carbon were removed from the solution by gravity filtration through fluted filter paper. Since the glass funnel was much cooler than the hot solution, cooling the solution occurred, resulting in some crystallization of the naphthalene in the stem of the funnel and in the filter paper. To reduce the loss of naphthalene due to this crystallization, the funnel was preheated by running a small amount of hot solvent through the funnel into the clean erlenmeyer flask. The flask was then emptied into a beaker labeled “waste.” The hot solution was added to the funnel in small portions to further reduce the amount of loss of material. The filter paper was rinsed with a little hot solvent (1-2 mL) to dissolve any crystals that formed in the paper or the funnel. After filtering, the hot saturated solution of naphthalene was placed on wire mesh set on a ring clamped to a ring stand to allow the solution to cool to room temperature. Keeping the ring low prevented the flask from tipping. To further cool the solution, the room temperature solution was placed into an ice bath for five minutes. During this cooling process, the opening of the flask was covered with a watch glass in order to ensure that the remaining solvent, also known as the mother liquor (“Mother Liquor”), did not evaporate as the crystals of naphthalene formed. If crystallization did not start, the inside of the erlenmeyer flask was scratched with a glass rod, or a seed crystal of naphthalene was added. After crystallization was complete, the mixture was vacuum filtered using a Büchner funnel to separate the naphthalene crystals from the mother liquor. The crystals could be washed with cold solvent to further isolate them from the solvent. Vacuum filtration was continued for a few minutes to allow the collected naphthalene crystals to dry. Then, the filter paper containing the filtered crystals was transferred onto a watch glass and placed in a 70˚C oven for five minutes to dry. A final mass of the recrystallized naphthalene was recorded, and from this information, the percent recovery of the naphthalene was calculated using Equation 1.
Procedure: Melting Point Determination
To begin with melting point determination of the purified naphthalene, a spatula tip (pea-sized amount) of the recrystallized naphthalene was ground into a powder using a glass rod and a watch glass, making sure that the crystals were not contaminated during this process. The crystals were then pushed into a small pile, into which the open end of a melting point capillary tube was pushed so that some solid was forced into the opening of the tube. To shake the sample to the bottom of the tube, the closed end of the tube was gently tapped on the benchtop. Then, the capillary tube was dropped down the long glass tube onto a hard surface to pack the sample. If the column of the packed sample was over 3 mm, the melting point tube had to be discarded and a new tube had to be packed. In a second melting point capillary tube, the same process took place with un-recrystallized naphthalene. A small amount of the impure naphthalene was loaded into the tube, and both the purified and unpurified samples were marked on their respective tubes so they could be differentiated. Once a “Mel-Temp” melting point machine was available, the power was turned on, but the heating rate control dial was turned completely counter-clockwise to zero. A thermometer was inserted into the slot to determine the initial temperature of the aluminum block. If the temperature of the aluminum block was determined to be higher than 60˚C, it needed to cool. To expedite this process, a moist, but not sopping wet, paper towel was held directly on the block. It was important to note that the aluminum blocks were often hot to the touch during this stage if they had been used previously, so they needed to be powered off and care had to be taken so as to not burn oneself. Once the temperature fell below 60˚C, the packed melting point tubes could be inserted into the holder. The heating rate control dial was turned up until the temperature on the thermometer rose. Because the process could happen quickly, one partner monitored the melting point tubes to watch for any signs of melting, and the other lab partner watched the thermometer to monitor the rate at which the temperature rose. The heating rate control dial was adjusted so that the temperature did not rise too rapidly. As the samples reached their melting points, it was important to ensure that they were heated slowly (at a preferred rate of a 1-2˚C increase per minute) (Hart). The samples were watched closely, and when a sign of melting was noted, the temperature at which this took place was recorded. When the solid in a particular tube completely melted, this temperature was also recorded so as to dictate a range of melting points for each sample. An important piece of information to consider was that every melting point should be reported as a range (Hart). This range was recorded for both the impure and recrystallized naphthalene. To finish, the heating rate control dial was turned to zero and the machine power was turned off.
Procedure: Data
In the data collection, the observations about each solvent and the solubility of naphthalene in each were recorded. The solvent used for recrystallization was reported with a statement as to why it was chosen. Then, the crystals were described with an additional discussion of any difficulties encountered during the recrystallization process. The final mass of the recrystallized naphthalene was recorded and used to calculate the percent recovery of naphthalene with Equation 1. After determining the melting points, the ranges for both the impure and recrystallized samples were recorded. Using this data, observations were made in accordance with how said data correlated to the purity of naphthalene.
Procedure: Cleanup
After finishing the procedure, the lab workspace had to be cleaned. First, liquids from the solubility tests were decanted into the appropriate liquid waste containers: water into the aqueous waste container; and methanol, acetone, hexane, and toluene into the organic waste container. Insoluble solids from the solubility tests were placed into the solid waste container. The leftover impure naphthalene, leftover decolorizing carbon, and the remaining recrystallized naphthalene were also placed into the solid waste container. The final filtrate was placed into either the aqueous or organic waste container, according to the solvent chosen for use. Used capillary tubes and pasteur pipettes were placed in the sharps bucket, and used coffee filters, filter papers, paper towels, and gloves were disposed of in the non hazardous waste containers (trash). Next, all glassware from the community drawers was washed with soap and water and returned to the appropriate drawers before leaving. All other equipment was also returned to appropriate spaces. Finally, the bench tops (including those in the hoods) and the balance room surfaces were cleaned and wiped down.
Results: Solvent Observations
Before conducting naphthalene solubility tests, five small test tubes were prepared with 3 mL of water, methanol, acetone, hexane, and toluene, respectively. The initial observations of each solvent was recorded in Table 5.
Table 5. Solvent Observations
Solvent
Observations
Water
Clear liquid
Methanol
Clear liquid
Acetone
Clear liquid, strong scent like nail polish remover
Hexane
Clear liquid, strong, slightly sweet scent
Toluene
Clear liquid, paint thinner-like odor
Results: Solubility Observations
After preparing each of the test tubes with the five solvents, 0.1 g of impure naphthalene was added to each and mixed if necessary. The solubility of naphthalene in each of these solvents, which had been cooled in a previously-prepared cold water bath for at least two minutes, was recorded using abbreviations for observations on solubility, as depicted in Table 4. The solubility observations for naphthalene in the cold solvents were recorded in Table 6.
Table 6. Solubility Observations of Naphthalene in Cold Solvents
Solvent
Observation (Abbreviated)
Water
IN
Methanol
IN
Acetone
SS
Hexane
SO
Toluene
VS
Once the observations of the solubility of naphthalene in each of the cold solvents were recorded, selections for proceeding solvents were made based on the information. As stated previously, any cold solvent in which the naphthalene was soluble was considered unsuitable for recrystallization. Because naphthalene was somewhat soluble in acetone, soluble in hexane, and very soluble in toluene, these three solvents were eliminated from the following trials. For any mixture in which the naphthalene seemed insoluble at cool temperatures, respective tubes were heated in a hot water bath made in a small beaker to see if the naphthalene would dissolve in a hot solvent. Based on the observations, both water and methanol qualified for these tests. Observations about the solubility of naphthalene in these hot solvents was recorded in Table 7, once again with abbreviations in accordance with the Table 4 key.
Table 7. Solubility Observations of Naphthalene in Hot Solvents
Solvent
Observation (Abbreviated)
Water
VS
Methanol
VS
Any solvent in which the naphthalene was insoluble when hot was also considered unsuitable for recrystallization, and therefore was abandoned for the remaining steps. However, the tests indicated that naphthalene was very soluble in both water and methanol when hot. Therefore, to proceed with solvent selection, the test tubes containing the hot solvent with solvated naphthalene were placed into an ice water bath to cool.
Results: Recrystallization Observations
After cooling, if the crystals reformed, the respective solvent was chosen for recrystallization steps. If the crystals did not reform, the test tubes were shaken, or their inner walls were scratched with a glass stir rod. Observations about the recrystallization of naphthalene from the two remaining hot solvents were recorded in Table 8.
Table 8. Recrystallization Observations of Naphthalene in Cooled Solvents
Solvent
Observation
Water
Crystallization observed
Methanol
Crystallization observed
Any solvent in which naphthalene failed to recrystallize at room temperature was considered unsuitable for recrystallization, and was therefore abandoned for the remaining steps.
Results: Boiling Points vs. Melting Point
Because crystallization was observed in both water and methanol when re-cooled (thus leaving more than one remaining solvent), the boiling points of water and methanol were compared to the melting point of naphthalene. The respective boiling/melting points for the substances were listed in Table 9 for comparison (“Boiling Points”).
Table 9. Boiling and Melting Point Comparisons for Substances
Substance
Boiling/Melting Point
Naphthalene
Melting point: 217.9˚C
Water
Boiling point: 100˚C
Methanol
Boiling point: 64.7˚C
A good solvent for this recrystallization should have had a boiling point lower than the melting point of naphthalene. In this case, both water and methanol demonstrated a boiling point which was significantly lower than the melting point of naphthalene. However, the larger the difference in these values, the easier it would be to avoid turning naphthalene into a liquid while attempting to turn it into a pure solid. Therefore, because methanol demonstrated the greatest difference (a 153.2˚C difference as compared to the 117.9˚C difference between naphthalene and water), methanol was chosen as the solvent to be used in recrystallization.
Results: Recrystallization and Percent Recovery
Once methanol was selected as the most effective solvent to be used in the recrystallization of naphthalene, the recrystallization process could begin. This process started with the preparation of a “solvent” flask, which contained 30 mL of methanol and was heated to a boil, and a “solution” flask to which 2.0 g of impure naphthalene was added. From here, the heated methanol was continually added to the naphthalene sample, which was also placed on the hot plate, until all of the naphthalene dissolved. The amount of methanol added to dissolve the naphthalene was determined to be around 21 mL. After consulting with the AI, however, he stated that this amount may have been too large, so the “solution” flask which now contained methanol and dissolved naphthalene was returned to the hot plate to allow some of the methanol to evaporate. In the meantime, observations were made about any impurities which remained in the solution after the naphthalene had completely dissolved. These observations were that small, dark, insoluble particles like stones remained at the bottom of the flask. In addition, observations were made and recorded regarding the solution of dissolved naphthalene and methanol: the solution was clear, yellow-orange colored, and by this time the naphthalene had dissolved completely. Naphthalene crystals are white and give colorless solutions, and the solution formed was colored, so based on the procedure, decolorizing carbon could have been added to treat the solution in hopes of changing it to a state more similar to the expected naphthalene properties. However, after another consultation with the AI, he recommended that this step be skipped so as to prevent too much absorption of the naphthalene from the solution, and the trials proceeded. Next, any insoluble impurities and any decolorizing carbon (in this case, none was added) were removed from the solution by gravity filtration through fluted filter paper. Observations from the gravity filtration process were recorded: after filtering, the black insoluble impurities observed in the solution remained on the paper along with some naphthalene crystals, a likely result of the quick cooling of the solution. To dissolve these crystals into the solution, more methanol was boiled to pour through. Crystallization was also noted in the filtered solution, which was then reheated to dissolve said crystals in the next step. After the gravity-filtered solution was reheated, it was cooled, first using a wire stand and following with an ice bath. During this time, the naphthalene was able to recrystallize. The resulting crystals were filtered through a Büchner funnel (vacuum filtration apparatus) to separate the naphthalene crystals from the mother liquor. To further separate the solvent and the crystals, they crystals were rinsed with small amounts of cold solvent. Observations about the remaining crystals on the filter paper were recorded: after funneling, the crystals were a combination of pure white and yellow tint, and after drying completely in the 70˚C oven, they remained a mix of white and slightly yellow crystals. This indicated that the naphthalene had not been completely purified, as the procedure did not result in the predicted pure white crystals, but doing so was recognized as one of the biggest difficulties encountered in the experiment. The mass of the sample of purified crystals was determined to be 0.503 g, whereas the impure sample was 2.0 g to begin. Using Equation 1, the percent recovery of naphthalene was determined to be 25.15%:
% Recovery=0.503 g2.0 g100%=25.15% recovery
Results: Melting Point Determination
A portion of the crystals produced, along with a sample of impure naphthalene, were used in the determination of melting point ranges for both substances. Once the tubes were packed and loaded into the “Mel-Temp” melting point machine, the process of melting point determination began. The observations from these trials were recorded in Table 7. (Note: The beginning temperature of the melting point machine was 53˚C.)
Table 7. Melting Point Observations
Substance
Temperature at which Melting Began
Temperature at which Melting Completed
Purified naphthalene
55˚C
72˚C
Impure naphthalene
67˚C
76˚C
Discussion
Conclusion
After conducting this experiment, it was determined that the percent recovery of naphthalene was 25.15%, and the melting range of the recrystallized naphthalene sample was 55˚C to 72˚C. Although there was error in the conduction of this experiment, it was overall successful, as the objectives for the experiment were met. It showed how to choose an effective solvent for crystallization– in this case, methanol was chosen–, it exhibited the process of purifying naphthalene using recrystallization, it demonstrated how to use the different aspects of the technique of recrystallization in order to determine how certain factors impact others in results (high percent recovery versus high purity of the final naphthalene sample), and it showed how to determine crystal purity based on melting point ranges.
Works Cited
“Boiling Points for Common Liquids and Gases.” Young's Modulus of Elasticity for Metals and Alloys, Engineering Toolbox, 2003, www.engineeringtoolbox.com/boiling-points-fluids-gases-d_155.html.
Fletcher, Beth. “Recrystallization.” Firewalking Myth vs Physics, Pitt.edu, www.pitt.edu/~ceder/add_info/recrystallization.html.
Hart, H., et al. “Melting Point Determination.” Chemistry Sites: Clemson, Clemson University, 2012, chemistry.sites.clemson.edu/organic/Labs/2270Docs/MeltingPoint.pdf.
“How to Calculate Percent Recovery.” ScienceStruck, ScienceStruck, sciencestruck.com/how-to-calculate-percent-recovery.
“Mother Liquor.” Merriam-Webster, Merriam-Webster, www.merriam-webster.com/dictionary/mother%20liquor.
“Naphthalene.” National Center for Biotechnology Information. PubChem Compound Database, U.S. National Library of Medicine, pubchem.ncbi.nlm.nih.gov/compound/naphthalene.