Lab #2 Objectice Statement: The objective of this lab is to teach the practice of crystallization. Which is done by showing the effects of mixing unpure solutes in a high-temperature solvent solution and then decreasing the temperature after complete solubility. This will cause the pure samples of the solute to recrystallize into a solid form within the unwanted liquid solution, separating the pure solute.
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Compounds of Study:
Phthalic Acid
Slightly polar
Charcoal
Nonpolar
Naphthalene
Very nonpolar
Methanol
Polar
Pre-Lab Questions:
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”.
You would place a small sample of the desired compound to be recrystallized in a test tube and add five to ten drops of the trial solvent. Mix well and observe the status solubility of the compound. If the compound dissolves immediately after mixing, the compound is too soluble for the solvent. If there is no solubility, the addition of heat is needed to reach solubility. If there is complete solubility in your solution, cool the substance for recrystallization to occur, and if there is recrystallization within ten to twenty minutes, your trial solvent is the correct recrystallization solvent. If none of these techniques produce a good recrystallization solvent, mixed-solvent testing can happen. This is when a small portion of solute gets mixed with a viable solvent, and when the solution is increased in temperature, the addition of the less soluble solvent in the mixed pair is slowly added to the solution. Causing the solution to become cloudy, and repetitive addition of the second solvent in the pair, after cloudiness disappears, should cause solvents to form within the solution.
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.
The "like dissolve like" rule relates to the polarity of the solute and recrystallization solvent. This means that polar compounds dissolve polar compounds and nonpolar compounds dissolve nonpolar compounds. Where a polar compound can not dissolve a nonpolar compound. Indicating that the polarity of a solute and a recrystallization solvent need to be alike in polarity for the process of recrystallization to occur.
3. If we were going to be doing solubility testing, it would require the use of a sand bath. How do you make a sand bath for this testing?
A sand bath is made by filling a container with sand on a heated surface. Then submerge another container filled with the product that you wish to heat up within that container of sand. This will evenly heat up the desired product within what is now a 'sand bath'.
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 a boiling stone/boiling stick is to prevent liquids from boiling too violently. Without the stick or stone, the solvent is prone to over-boiling, the potential for your solvent to spill or splash outside of your container, and uneven boiling.
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.
We want to add charcoal to our hot solution before the point of saturation to remove impurities within our liquid solution. Without causing the solution to recyrtyallize in any form prematurally. This way we are insuring the highet percentage yield for out pure solute to be extracted by keeping like a 'window' open for impuritites to be removed from the solution, and not our solute.
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?
In this lab, temperature is an extremely key variable. If we used a long-stem funnel we run the risk of the solution to cool during the filtration process. Possibly causing premature/unwanted crystallization to occur which can either create an impure solute to form or even clog our filtration system with the impure crystals. A stemless funnel allows for a more effective filtration rate.
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.
Using a vacuum filtration system will cause the solution to evaporate instead of sucking up the solution liquid. Gravity filtration will remove any suspended impurities and the charcoal from the increased temperature solution.
8. Why should you wait until the recrystallization solvent has come to room temperature before you filter?
So that the soluble impurities will stay in the solution and not be solid enough to be filtered out, due to the fact that they will not be concentrated enough.
9. What is the appropriate size of a piece of filter paper to be used in vacuum filtration of your crystals?
The filter paper should only be big enough to cover the holes of the Buchner funnel. The paper should not go up the sides of 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?
The purpose of the filtration process is to filter out the most pure samples of the crystals. By not just letting the remaining solvent to simply evaporate away you are not allowing for any pure substance still within the solution to possibly be evaporated out along the process
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?
A. Do you first use the most “like” or “unlike” solvent?
You first add the most "like" solvent as you expect it to be more soluble.
B. When, in the process of recrystallization, is the second solvent added?
The second solvent is added after the solution of the solute and the first solvent has nearly reached its boiling point. Which is when the solid is completely dissolved and the second solvent can be added drop by drop to see if cloudiness appears as an indication of the potential for the solute to recrystallize.
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 so that the compound does not come out of the solution at so high a temperature. It may also help to switch to a solvent with a lower boiling point. Some oils can be crystallized by dissolving them in a small amount of diethyl ether or hexane and allowing the solvent to evaporate slowly in a hood.
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 are to be cleaned and put back in your respective drawers after use. Where sticks are to be cleaned and put back into their containers at the back of the hood.
Experimental
Experiment #1:
Measured .3g of phthalic acid
Placed phthalic acid in a 50 ml Erlenmeyer flask along with a wooden boiling stick, and 10 ml of DI water
Put both the recrystallization flask and an excess solvent flask (containing an extra 25 ml of DI recrystallization solvent) onto a hot plate at heat level 6 a significant amount of phthalic acid dissolved with this amount of DI water.
Approximately 1 ml of hot solvent was added until the phthalic acid fully dissolved
Took the recrystallization flask off the hot plate and let it cool to room temperature did not recrystallize after 9 minutes so adjusting the procedure
Added .09 grams of solid phthalic acid into the recrystallization flask and reboiled to dissolve the solute believed to have added too much solvent at the beginning of the procedure
Took the flask off the hot plate and let it cool to room temperature Crystals formed within 4 minutes when scrapped the bottom of the flask with a wooden boiling stick
Filtered the crystals by vacuum filtration and scraped crystals into a beaker to dry. Crystals were tiny white needles, looking like powder
analyzed by color and mass (see results below)
Experiment #2:
Measured 1.25g of impure-colored naphthalene
Placed naphthalene in a 50ml Erlenmeyer flask along with a wooden boiling stick and 5ml of methanol
Put the recrystallization flask and both recrystallization solvent flasks on a hot plate at a heat level of 4 almost all the naphthalene diluted instantly in the methanol and took very little time to boil.
Added approximately .5ml of hot methanol until the naphthalene completely dissolved
Removed the recrystallization flask off the hot plate and let cool for 2 minutes.
Added around 10ml of excess solvent and a scoop of charcoal and stirred in the recrystallization flask on a hot plate for five minutes Kept solvent level at around 20ml in the flask and swirled
Used fluted paper and a short-stemmed funnel to filter charcoal out of the solution. added too much charcoal as the solution was a mercury grey and slightly red color
Returned solution to the hot plate to boil out 5-10ml of solvent
Removed the recrystallization flask from the hot plate and added a lot of excess room temperature solvent pair water to recrystallize naphthalene in the solution.
Used vacuum filtration and scraped the crystals into a beaker to dry. Crystals were tiny white to pinkish crystals that looked almost spongey.
Analyzed by color and mass (see results below)
Results
Experiment #1:
.26g of crystals recovered
Crystals were bright white. Must have been extremely small structured because it made a powder texture. Resembled baking soda. Record the color of the crystals and the final mass that you weigh once the crystals are dry.
Percent Recovery = .26 g recrystallized phthalic acid/ .3 g of starting phthalic acid is a recovery percentage of 86.67%.
Experiment #2:
.47 g recorded
Crystals clumped together to form an almost porous texture. The crystals were slightly dull, not shiny white, but almost seemed a little gray.
Percent Recovery = .47 g recrystallized naphthalene/ 1.25 g of starting naphthalene is a recovery percentage of 37.6%.
Melting point was 73 degrees Celsius
Conclusion
Experiment #1:
The recrystallization of phthalic acid resulted in the formation of 0. 26 grams of white needle crystals. Because the experiment began with 0.3 grams, the recovery was around 87%. 87% still being relatively low as we want to get 100% recrystallization recovery as we know our starting phthalic acid was 100% pure. There was not much, if any, contaminant in the phthalic acid, and therefore with good technique, I should have been able to recover closer to 95-100% yield. The phthalic acid recovered is still pure, as it matches the white needle-like formation according to the literature, however, we would want closer to 95%-100% to be completely satisfied with our work. The recrystallization was not performed with a high level of skill and there was probably too much DI water used when doing the recrystallization and the other 13% was still dissolved in the cold methanol. It is also highly likely that the other .04 grams of phthalic acid was not recrystallized as the temperature of the solution decreased during the filtration process, not allowing for some of the phthalic acid to recrystallize.
Experiment #2:
The recrystallization of naphthalene resulted in the formation of 0.47 grams of dull white powder-like crystals. Because the experiment began with 1.25 grams, the recovery was 37.6%. 37.6% is extremely low and we want to get as close to 100% recrystallization recovery as we know our starting phthalic acid was a little impure. There was some contaminant in the naphthalene in the form of red dye, and therefore with good technique, I should have been able to recover closer to 90-95% yield. The naphthalene recovered is still pure, as it matches the white powder-like formation according to the literature, however, we would want a much higher percent recovery to be completely satisfied with our work. The recrystallization was not performed with a high level of skill and there was probably too much methanol used when doing the recrystallization which would cause the naphthalene to still be dissolved in solution. A contamination accident that happened during the lab caused the solution to become very contaminated. When working with the charcoal filtration technique, too much charcoal was added to remove the congo red dye and the solution was still visible darker. Too much solvent-pair additive at the end would also affect recrystallization efficiency.
Reflections
During this lab, I learned about the importance of proper laboratory preparation and technique. I felt like I did not prepare and plan out the procedure as much as I would have wished beforehand, and that resulted in a lot of hiccups during the lab that I can see affected my work efficiency. Especially with a practice like recrystallization, where outside variables like temperature control, boiling, and contamination need to be limited to ensure a high percent recovery, preparation, and time efficiency is something I want to work on for my future labs. If I were able to repeat this lab, I would make sure to have better control over the boiling of the solution, along with temperature control during the filtration process. That is where I believe I lost a good amount of my naphthalene during the lab. To work on understanding the techniques that the lab is supposed to each as the solvent-pair additive for the second experiment was not fully understood on my end during the procedure and I believe I added too much DI water.
Post Lab Questions:
NA for Lab #2
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