245 * Lab #6
Recrystallization
There will be times when it is necessary to purify a solid chemical. These may include products from a reaction, determining the identification of an unknown of questionable purity or just to determine crystal shape. A solid chemical must be pure to obtain an accurate melting point. As with the melting point procedure, recrystallization requires that certain steps be taken to insure good and complete recrystallization. Your goal is to retrieve as much of the chemical as possible and that it be pure.
The first important step is to determine a solvent for use in dissolving the solid. If you know what impurities there may be in the solid, then you should look for a solvent that may select out the chemical that you are recrystallizing. (i.e. the impurity would be less soluble in the solvent than the chemical that you want to recrystallize) The solvent should in no way react with the solid. And you should definitely choose a solvent that is the least hazardous (flammable/toxic) as you will be required to heat the solvent. The reason that you want to heat the solvent before dissolving the solid is that more grams of solid will dissolve in a hot solvent than in a cold solvent. This allows for easier recrystallization of the solid as the solution cools. If you use too much or too cool a solvent, it will be much harder to start the recrystallization, as well as, recover the entire sample from the solution.
The solvent we will be using today is water. The solid that we want to recrystallize is benzoic acid. (Before lab, check for the solubility's of benzoic acid in water at different temperatures) You will want to add only enough hot water to just dissolve the benzoic acid. You need to keep the solution hot but not vigorously boiling. (However, solid impurities will remain undissolved.)
The benzoic acid should now be dissolved in the water. Other insoluble impurities will not dissolve. If the solid is originally white or colorless, the color of the water can be an indication of impurities. The first filtration that you do is to remove any impurities not dissolved in the hot water. It is important to note that there is a good chance of the benzoic acid crystallizing onto the filter paper if the filtration system is not adequately prepared. In case that you do get considerable recrystallization on the filter paper, pipette a small amount of hot water around the filter paper to dissolve the benzoic acid. If this is not successful, please dispose of the filter paper into the benzoic acid WASTE bottle. Do not put contaminated filter paper in the trash.
The filtrate will now contain the dissolved benzoic acid and any impurity soluble in hot water. As the solution cools the benzoic acid crystals will begin to form because benzoic acid is much less soluble in cool water than in hot water. You want the solution to cool slowly so the crystals have more time to grow into medium sized crystals. If you do a nice recrystallization, you may easily see the correct crystal formation for that chemical. If it is too fast, though, smaller, less definitive, crystals will form. When the crystallization looks complete (no more solid precipitating out) you complete the experiment by placing the flask in an ice water bath for 5 minutes then collecting the benzoic acid by vacuum filtration using a preweighed filter paper. Wash the solid with approximately 5 mls of ice cold water. You will need to determine the weight and melting point of the recrystallized benzoic acid. The acid must be dry to get an accurate value so you will need to store it until the next lab. You will also need to use this in the Thin Layer chromatography lab. Remove the preweighed filter paper with the benzoic acid and store it in a preweighed envelopes provided. Store in your glassware kit cabinet.
Important concepts: a) choice of solvent , b) use of a single solvent , c) temperature vs. solubility and d) use of a mixed solvent system
WASTE Spill and Disposal - G:
-Please dispose of ALL Benzoic acid and contaminated debris into its waste
bottle- Store recrystallized benzoic acid in brown envelopes until ready to
dispose. Dispose of in envelopes separate labeled container.
WASTE Spill and Disposal - E:
Any acetone used to clean glassware. *THERE IS AN ACETONE
WASTE JUG IN THE HOOD*
DO NOT PUT USED WEIGH BOATS IN
THE HW CHEMICAL CONTAINER. CONTAMINATED PAPER TOWELS AND WEIGH BOATS ARE PUT
INTO THE CLEAR PLASTIC HW BOX IN THE HOOD. PAPER ENVELOPES W/CHEMICALS SHOULD
BE FOLDED MULTIPLE TIMES AND SEALED BEFORE BEING PUT INTO THE SPECIFIED HW CONTAINER.
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CHEMICALS: |
LOCATION |
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Impure Benzoic Acid |
Org_prep |
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Benzoic Acid WASTE bottle |
Hood #5 |
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SUPPLIES: |
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Brown envelopes |
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fluted filter paper |
White Supply Cabinet TOP |
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25-50 mL EMF |
White Supply Cabinet |
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Buchner Filter Assemblies |
White Supply Cabinet |
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Filter Flasks |
White Supply Cabinet |
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Latex Tubing |
C-24 |
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Stemless funnels |
White Supply Cabinet |