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Objective: The purpose of this lab is to gain a better understanding of crystalization techniques. This lab will help us gain a better understanding of how to identify an appropriate solvent and how to remove impurities from a compound using filtration.
Compounds of Study:
Phthalic Acid
Polar
Image from Wikipedia
Naphthalene
Non-Polar
Image from ChemSpider
Water
Polar
Image from www.chem.fsu.edu
Methanol
Polar
Image from Biology Dictionary
Pre-Lab:
1. In Activity #1 and activity #2, you are given the exact conditions for how you will conduct recrystallization. The solvents for perfect recrystallization have already been discovered through research. In 4-6 sentences, summarize the information that you read in the textbook on how you would select a recrystallization solvent if it has not already been researched in advance. In the 4th edition of the textbook, this is section 15.4, “How to Select a Recrystallization Solvent”.
To determine if a solvent would serve as a good recrystallization solvent if not given within the lab procedure the first step should be to look at a table of possible solvents that would be appropriate for recrystallization. To test the solvent, place a small sample of a compound intended to be recrystallized in a test tube and add 5–10 drops of a trial solvent. Once the solvent has been added, the tube should be shaken to thoroughly mix the contents of the tube. If after shaking, the compound dissolves immediately, the compound is probably too soluble in the solvent for recrystallization to be effective. If no solubility is observed after shaking, heat the solvent to its boiling point. Cool the solution to induce crystallization if complete solubility is observed after shaking. A good recrystallization solvent is indicated if, after adding a trial solvent and cooling the solution, the compound forms crystals in 10–20 minutes.
2. If the rule for solubility is “like dissolves like” then how alike will a solute and a recrystallization solvent be in character? Will they be alike? Unlike? Somewhere in-between? Describe in good detail.
"Like dissolve like" refers to the principle that compounds with a given polarity will dissolve other compounds that have similar polarity. However, when it comes to recrystallization if a recrystallization solvent is too similar to the compound crystallization won't occur because the entire compound will be dissolved by the solvent. However, if the compound and solvent are too different, the compound will never dissolve to be able to recrystallize.
3. If we were going to be doing solubility testing, it requires the use of a sand bath. How do you make a sand bath for this testing?
Sand baths are created by heating sand on a heating plate so the sand can be used as a constant source of heat for boiling. While the sand is heated, a boiling stick is used to help promote even/smooth boiling of the desired product.
4. What is the purpose of a boiling stone/boiling stick? What would happen if a stone or stick were not added to a solvent while heating?
The purpose of the boiling stick is to ensure that the product boils smoothly. The stick helps regulate the temperature of the solution, preventing it from becoming superheated and causing unnecessary waste of a solution/product.
5. Charcoal is used to help purify a colored contaminant but it adds an extra step in the recrystallization process. At some point, the charcoal must be filtered from the solution. At the time of filtering, the hot solution should be slightly undersaturated meaning that you should have excess solvent and your solution should not be right at the point of saturation. Why is this? Answer in good detail keeping in mind that recrystallization occurs when a solution is supersaturated.
By filtering the charcoal when it is slightly undersaturated, you are able to prevent accidental crystallization during the filtering process. Charcoal absorbs part of any solution that it is in. Having additional solvent present in the solution without being at the point of supersaturation (where recrystallization occurs) prevents the solution from recrystallizing early, allowing us to have the purest possible final product after recrystallization.
6. When filtering hot solutions, such as when filtering charcoal from methanol in activity #2, why is the fluted filter paper placed into a stemless funnel instead of being placed within a long-stem funnel?
By using a stemless funnel we prevent crystallization from occurring at the exit area of the stem. However, when you use a long-stem funnel, as the solution travels along the filter paper and through the funnel crystallization can occur at the exit area of the stem and cause a blockage of the long-stem funnel.
7. It is very efficient to use vacuum filtration because the pressure quickly pulls the liquor away from the solid. Why don’t you use vacuum filtration to remove charcoal but only use gravity filtration instead? Bear in mind Boyle’s law.
We use gravity filtration rather than vacuum filtration because vacuum filtration can alter the pressure of the given sample. As we heat the sample, boiling is affected by both temperature and pressure and changing the pressure using vacuum filtration may affect the results of the recrystallization process.
8. Why should you wait until the recrystallization solvent has come to room temperature before you filter?
The recrystallization solvent should come to room temperature before you filter because allowing the solvent to cool lets pure product form minus any contaminants.
9. What is the appropriate size of a piece of filter paper to be used in vacuum filtration of your crystals?
The size of the filter paper required should correspond with the size of the funnel we are using. The paper should cover all holes in the funnel.
10. What is the purpose of filtering the crystals at the end of the procedure? Why is it not good enough to just decant the excess solvent and then allow the remaining solvent to simply evaporate away from the crystals?
Filtering removes all liquid that is present from the crystallization process, leaving behind only the insoluble materials. This process is important because any liquid that is left behind from the recrystallization
process may contain impurities that could contaminate the desired product.
11. When two solvents are used for recrystallization this is referred to as recrystallization with a solvent pair. This technique must be used when any one solvent by itself does not have the correct polarity. Which solvent is the solute brought to boiling in?
The solute that is boiled is believed to be more soluble.
A. Do you first use the most “like” or “unlike” solvent?
The more "like" solvent should be used first.
B. When, in the process of recrystallization, is the second solvent added?
The second solvent can be added after the first solute is fully dissolved.
12. Low melting solids often “oil out” of a recrystallization solution rather than crystallizing. If this were to happen, how would you change the procedure to ensure good crystals? This answer can be found in your techniques book.
More solvent can be added to prevent the "oiling out" of a recrystallization solution.
13. In the lab you use many items that are considered “consumables” meaning that they get consumed and usually this means that after they are used, they are thrown away. What are you to do with the weight boats and sticks that are “consumed” in this lab?
Weight boats and glass sticks should be washed and set out to dry so they can be used by a later lab group.
Experimental:
Experiment 1:
Measured 0.31 grams of phthalic acid (solid to be recrystallized) using a weight boat on a scale. Collected 8 ml of DI water (recrystallizing solvent) from the tap in a graduated cylinder and combined both the 0.31 grams of phthalic acid and 8 ml of DI water in an Erlenmyer flask. Very little solid appeared to dissolve using this method.
Collected an additional 25 ml of DI water from the tap in a graduated cylinder and placed it in an Erlynmyer flask.
Placed a boiling stick in both Erlymyer flasks.
Set up a hot plate and set the heat to six. Placed flask with both the solid to be recrystallized and the recrystallizing solvent in the center, while placing the flask with recrystallizing solvent only off to the edge of the plate. Carefully monitored hot plate and flasks to look for boiling of contents of the flasks. After approximately three minutes, the contents of the flasks began to boil. Swirled contents of the flask with both the solid and the solvent and added approximately 2 ml of the solvent using a glass pipette until all phthalic acid dissolved.
Let cool for 15 minutes on a cork ring. After cooling for about 1.5 minutes, crystals began to form, the crystals appeared like white shards along the bottom of the flask.
Filtered the flask containing the newly dissolved solid using vacuum filtration to isolate the crystals.
Analyzed color and measured the weight of the crystals (see results section) and then placed the crystals in a beaker. Covered the top of the beaker with a chem wipe to allow the crystals to dry.
Measured the mass crystals and calculated the recovery percentage.
Experiment 2:
Measured 1.25 grams of naphthalene contaminated with congo red using a weight boat and a scale. Collected 8.5 ml of methanol (recrystallizing solvent) in a graduated cylinder and combined both the 1.25 grams of naphthalene contaminated with congo red and 8.5 ml of methanol solution in an Erlenmyer flask. A large amount of solid appeared to dissolve using this method.
Placed a boiling stick in the Erlymyer flask.
Set up a hot plate and set the heat to six. Placed flask with both the solid to be recrystallized and the recrystallizing solvent in the center. Carefully monitored hot plate and flask to look for boiling of contents of the flask. After approximately two minutes, the contents of the flasks began to boil. Added 3 large scoops of charcoal and 20 ml of methanol to the solution and let the solution boil for five minutes.
Removed solution from hot plate and allowed to cool on a cork ring for 15 minutes.
Filtered solution using a short-stemmed funnel and fluted filter paper, resulting in a yellow-tinted solution. Washed solution with an additional 5 ml of methanol to allow any original solvent to fully drain.
Evaporated any remaining solvent by placing the flask back onto the hot plate and allowing the flask to return to a boil.
Removed the flask from the hot plate and allowed it to cool on a cork ring for approximately 15 minutes. After cooling for about 2.5 minutes, crystals began to form, the crystals appeared like white shards along the bottom of the flask.
Filtered the flask containing the newly dissolved solid using vacuum filtration to isolate the crystals.
Analyzed color and measured the weight of the crystals (see results section) and then placed the crystals in a beaker. Covered the top of the beaker with a chem wipe to allow the crystals to dry.
Measured the crystals and calculated the recovery percentage.
Calculated the melting point of the naphthalene to assess the purity of the crystals.
Results:
Experiment 1:
Appearance of crystals: White in color, crystals are shiny and shard-like in appearance. They appear to look almost like broken glass.
Initial weight of phthalic acid: 0.31 grams
Final weight of crystals collected: 0.27 grams
Recovery percentage ((Final Weight/Initial) *100%): 0.27/0.31*100%=87%
Experiment 2:
Appearance of crystals: Dull white in color, shard-like in appearance.
Initial weight of naphthalene contaminated with congo red: 1.25 grams
Final weight of crystals collected: 1.02 grams
Recovery percentage ((Final Weight/Initial) *100%): 1.02/1.25*100%=81%
Melting Point: 80.2-82.2 degrees Celcius.
Discussion:
Experiment 1:
The recrystallization of phthalic acid provided us with 0.27 grams of white, shard-like crystals. The experiment began with 0.31 grams of phthalic acid which makes the recovery percentage for this experiment 83%. While the appearance of the crystals is consistent with the literature regarding pure phthalic acid crystals, because the phthalic acid used in this experiment was a pure substance, we would have expected a yield closer to 100%. With improved laboratory techniques, we would expect the yield to be around 95-100% recovery of phthalic acid. It is possible that too much DI water was added to the solid and solvent during the dissolving process.
Experiment 2:
The purification and recrystallization of naphthalene contaminated with congo red provided us with 1.02 grams of white, shard-like crystals. The experiment was conducted with 1.25 grams of naphthalene contaminated with congo red initially which makes the recovery percentage 81%. The white, shard-like crystals indicate that the naphthalene had become purified during the recrystallization process. The melting point obtained for the purified crystals was 80.2-82.2 degrees Celcius which was within the range for the literature value of naphthalene of 80.1 degrees Celsius. However, the recovery percentage is much lower than we would expect, even with having to purify the solid that is being recrystallized. It is highly likely that the inexperience of my lab partner and myself caused errors in the way our experiment was conducted, resulting in unnecessary loss of product. It is possible that too much methanol was added during the dissolving or purification processes which prevented the naphthalene from crystalizing.
Conclusions:
Based on the results of this experiment we can determine that naphthalene is a white crystalline solid. The melting point of the purified naphthalene was determined to be 80.2-82.2 degrees.
Reflections:
I learned about how substances can be purified through the recrystallization process and practiced this process using pure and impure solids. I felt much more confident in performing vacuum filtration for experiment two after having performed the procedure once before in experiment one and feel that I would be able to repeat this process again in a later lab. One thing I would do differently for this lab would be to be more cautious in the amount of solvent that I am adding to my solids when I am dissolving them for recrystallization. I had lower than expected recovery percentages for both procedures and wonder if adding less solvent could have helped me to recover more solid in my final results.
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