By : James W Zubrick
Email: j.zubrick@hvcc.edu
The essence of a re-crystallization is a purification. Messy, dirty, compounds are cleaned up, purified, and can then hold their heads up in public again. The sequence of events you use will depend a lot on how messy your crude product is, and just how soluble it will be in various solvents. In any case, you’ll have to remember a few things.
1. Find a solvent that will dissolve the solid while hot.
2. The same solvent should not dissolve it while cold.
3. The cold solvent must keep impurities dissolved in it forever or longer.
This is the major problem. And it requires some experimentation. That’s right! Once again, art over science. Usually, you’ll know what you should have prepared, so the task is easier. It requires a trip to your notebook, and possible, a handbook (see Chapter 2, “Keeping a Notebook” and Chapter 3, “Interpreting a Handbook”). You have the data on the solubility of the compound in your notebook. What’s that you say? You don’t have the data in your notebook? Congratulations, you get the highest F in the course.
Information in the notebook (which came from a handbook) for your compound might say, for alcohol (meaning ethyl alcohol), s.h. Since this means soluble in hot alcohol, it implies insoluble in cold alcohol (and you wondered what the i meant). Then alcohol is probably a good solvent for recrystallization of that compound. Also, check on the color or crystalline form. This is important since
1. A color in a supposedly white product is an impurity.
2. A color in a colored product is not an impurity.
3. The wrong color in a product is an impurity.
You can usually assume impurities are present in small amounts. Then you don’t have to guess what possible impurities might be present or what they might be soluble or insoluble in. If your sample is really dirty, the assumption can be fatal. This doesn’t usually happen in an undergraduate lab, but you should be aware of it.
FINDING A GOOD SOLVENT
If the solubility data for your compound are not in handbooks, then
1. Place 0.1 g of your solid (weighed to O.Olg) in a test tube.
2. Add 3 ml of a solvent, stopper the tube, and shake the bejesus out of it. If all of the solid dissolves at room temperature, then your solid is soluble. Do not use this solvent as a recrystallization solvent. (You must make note of this in your notebook, though).
3. If none (or very little) of the solid dissolved at room temperature, un-stopper the tube and heat it (Careful—no flames!) and shake it and heat it and shake it. You may have to heat the solvent to a gentle boil (Careful! Solvents with low boiling points often boil away). If it does not dissolve at all, then do not use this as a recrystallization solvent.
4. If the sample dissolved when HOT, and did NOT dissolve at room temperature, you’re on the trail of a good recrystallization solvent. One last test.
5. Place this last test tube in an ice-water bath, and cool it to about 5°C or so. If lots of crystals come out, this is good, and this is your recrystallization solvent.
6. Suppose your crystals don’t come back when you cool the solution. Get a glass rod into the test tube, stir the solution, rub the inside of the tube with the glass rod, agitate that solution. If crystals still don’t come back, perhaps you’d better find another solvent.
7. Suppose, after all this, you still haven’t found a solvent. Look again. Perhaps your compound completely dissolved in ethanol at room temperature, and would not dissolve in water. AHA! Ethanol and water are miscible (i.e., they mix in all proportions) as well. You will have to perform a mixed-solvent recrystallization (see “Working with a Mixed-Solvent System”).
GENERAL GUIDELINES FOR A RECRYSTALLIZATION
Here are some general rules to follow for purifying any solid compound.
1. Put the solid in an Erlenmeyer flask, not a beaker. If you recrystallize compounds in beakers, you may find the solid climbing the walls of the beaker to get at you as a reminder. A 125-ml Erlenmeyer usually works. Your solid should look comfortable in it, neither cramped, nor with too much space. You probably shouldn’t fill the flask more than one fifth to one fourth full.
2. Heat a large quantity of a proven solvent (see preceding) to the boiling point, and slowly add the hot solvent. Slowly! A word about solvents: Fire! Solvents burn! No flames! A hot plate here would be better. You can even heat solvents in a steam or water bath. But—No flames!
3. Carefully add the hot solvent to the solid to just dissolve it. This can be tricky, since hot solvents evaporate, cool down, and so on. Ask your instructor.
4. Add a slight excess of the hot solvent (5-10 ml) to keep the solid dissolved.
5. If the solution is only slightly colored, the impurities will stay in solution. Otherwise, the big gun, activated charcoal, may be needed (see “Activated Charcoal”). Remember, if you were working with a colored compound, it would be silly to try to get rid of all the color, since you would get rid of all the compound and probably all your grade.
6. Keep the solvent hot (not boiling) and look carefully to see if there is any trash in the sample. This could be old boiling stones, sand, floor sweepings, and so on. Nothing you’d want to bring home to meet the folks. Don’t confuse real trash with un dissolved good product! If you add more hot solvent, good product will disolve, and trash will not. If you have trash in the sample, do a gravity filtration (see following).
7. Let the Erlenmeyer flask and the hot solution cool. Slow cooling gives better crystals. Garbage doesn’t get trapped in them. But this can take what seems to be an interminable length of time. (I know, the entire lab seems to take an interminable length of time.) So, after the flask cools and it’s just warm to the touch, then put the flask in an ice-water bath to cool. Watch it! The flasks have a habit of turning over in the water baths and letting all sorts of water destroy all your hard work! Also, a really hot flask will shatter if plunged into the ice bath, so again, watch it.
8. When you’re through cooling, filter the crystals on a Buchner funnel.
9. Dry them and take a melting point, as described in Chapter 9.
GRAVITY FILTRATION
If you find yourself with a flask full of hot solvent, your product dissolved in it, along with all sorts of trash, this is for you. You’ll need more hot solvent, a ringstand with a ring attached, possibly a clay triangle, some filter paper, a clean, dry flask, and a stemless funnel. Here’s how gravity filtration works.
1. Fold up a filter cone out of a piece of filter paper (Fig. 42). It should fit nicely, within a single centimeter or so of the top of the funnel. For those who wish to filter with more panache, try using fluted filter paper (see “world famous fan-folded fluted filter paper,” Fig. 52).
2. Get yourself a stemless funnel, or, at least, a short-stemmed funnel. Why? Go ahead and use a stem funnel and watch the crystals come out in the stem as the solution cools, blocking up the funnel (Fig. 43).
3. Put the filter paper cone in the stemless funnel.
4. Support this in a ring attached to a ringstand (Fig. 44). If the funnel is too small and you think it could fall through the ring, you may be able to get a wire or clay triangle to support the funnel in the ring (Fig. 45).
Fig. 42 Folding filter paper for gravity filtration.
Fig. 43 The too long a funnel stem—Oops!
Fig. 44 The gravity filtration setup with a funnel that fits the iron ring.
Fig. 45 A wire triangle holding a small funnel in a large iron ring for gravity filtration.
5. Put the new, clean, dry flask under the funnel to catch the hot solution as it comes through. All set?
6. Get that flask with the solvent, product and trash hot again. (No flames!) You should get some fresh, clear solvent hot as well. (No flames!)
1. Carefully pour the hot solution into the funnel. As it is, some solvents evaporate so quickly that product will probably come out on the filter paper. It is often hard to tell the product from the insoluble trash. Then —
8. Wash the filter paper down with a little hot solvent. The product will redissolve. The trash won’t.
9. You now let the trash-free solution cool and clean crystals should come out. Since you have probably added solvent to the solution, don’t be surprised if no crystals come out of solution. Don’t panic either! Just boil away some of the solvent, let your solution cool, and wait for the crystals again. If they still don’t come back, just repeat the boiling.
Do not boil to dryness!
Somehow, lots of folk think re-crystallization means dissolving the solid, then boiling away all the solvent to dryness. NO! There must be a way to convince these lost souls that the impurities will deposit on the crystals. After the solution has cooled, crystals come out, sit on the bottom of the flask, and must be covered by solvent! Enough solvent to keep those nasty impurities dissolved and off the crystals.
THE BUCHNER FUNNEL AND FILTER FLASK
The Buchner funnel (Fig. 46) is used primarily for separating crystals of product from the liquid above them. If you have been boiling your recrystallization solvents dry, you should be horsewhipped and forced to reread these sections on recrystallization!
1. Get a piece of filter paper large enough to cover all the holes in the bottom plate, yet not curl up the sides of the funnel. It is placed flat on the plate (Fig. 46).
2. Clamp a filter flask to a ringstand. This filter flask, often called a suction flask, is a very heavy-walled flask with a sidearm on the neck. A piece of heavy-walled tubing connects this flask to the water trap (see Fig. 48).
Fig. 46 The Buchner funnel at home and at work.
3. Now use a rubber stopper or filter adapter to stick the Buchner funnel into the top of the filter flask. The Buchner funnel makes the setup top-heavy and prone to be prone—and broken. Clamp the flask first, or go get a new Buchner funnel to replace the one you’ll otherwise break.
4. The water trap is in turn connected to a source of vacuum, most likely, a water aspirator (Fig. 47).
5. The faucet on the water aspirator should be turned on full blast! This should suck down the filter paper, which you now wet with some of the cold recrystallization solvent. This will make the paper stick to the plate. You may have to push down on the Buchner funnel a bit to get a good seal between the rubber adapter and the funnel.
6. Swirl and pour the crystals and solvent slowly, directly into the center of the filter paper, as if to build a small mound of product there. Slowly! Don’t flood the funnel by filling it right to the brim, and waiting for the level to go down. If you do that, the paper may float up, ruining the whole setup.
7. Use a very small amount of the same cold recrystallization solvent and a spatula to remove any crystals left in the flask. Then you can use some of the fresh, cold recrystallization solvent and slowly pour it over the crystals to wash away any old recrystallization solvent and dissolved impurities.
8. Leave the aspirator on and let air pass through the crystals to help them dry. You can put a thin rubber sheet, a rubber dam, over the funnel. The vacuum pulls it in and the crystals are pressed clean and dry. And you won’t have air or moisture blowing through, and possibly decomposing, your product. Rubber dams are neat.
9. When the crystals are dry, and you have a water trap, just turn off the water aspirator. Water won’t back up into your flask. [If you've been foolhardy and filtered without a water trap, just remove the rubber tube connected to the filter flask sidearm (Fig. 47)].
10. At this point, you may have a cake of crystals in your Buchner funnel. The easiest way to handle this is to carefully lift the cake of crystals out of the funnel along with the filter paper, plop the whole thing onto a larger piece of filter paper, and let the whole thing dry overnight. If you are pressed for time, scrape the damp filter cake from the filter paper, but don’t scrape any filter paper fibers into the crystals. Repeatedly press the crystals out between dry sheets of filter paper, changing sheets until the crystals no longer show any solvent spot after pressing. Those of you who use heat lamps may find your white crystalline product turning into instant charred remains. 11. When your cake is completely dried, weigh a vial, put in the product, and weigh the vial again. Subtracting the weight of the vial from the weight of the vial and sample will give the weight of the product. This weighing by difference is easier and less messy than weighing the crystals directly on the balance. This weight should be included in the label on your product vial (see Chapter 8, “On Products”).
Just a Note
I’ve said that a Buchner funnel is used primarily for separating Crystals of product from the liquid above them. And in the section on drying agents, I tell people to use a gravity filtration setup to separate a drying agent from a liquid product. Recently, I’ve had some people get the notion that you can Buchner filter products from drying agents. I don’t advise that. You will probably lose a lot of your product, especially if it has a low boiling point (<100°C). Under this vacuum filtration your product simply evaporates along with your grade.
ACTIVATED CHARCOAL
Activated charcoal is ultra finely divided carbon with lots of places to suck up big, huge, polar, colored impurity molecules. Unfortunately, if you use too much, it’ll suck up your product! And, if your product was white, or yellow, it’ll have a funny gray color from the excess charcoal. Sometimes, the impurities are untouched and only the product gets absorbed. Again, it’s a matter of trial and error. Try not to use too much. Suppose you’ve got a hot solution of some solid, and the solution is highly colored. Well,
1. First, make sure your product should not be colored!
2. Take the flask with your filthy product off the heat and swirl the flask.
This dissipates any superheated areas so that when you add the activated charcoal, the solution doesn’t foam out of the flask and onto your shoes.
3. Add the activated charcoal. Put a small amount, about the size of a pea, ©n your spatula, then throw the charcoal in. Stir. The solution may turn black. Stir and heat.
4. Set up the gravity filtration and filter off the carbon. It is especially important to wash off any product caught on the charcoal, and it is really hard to see anything here. You should take advantage of fluted filter paper. It should give a more efficient filtration.
5. Yes, have some extra fresh solvent heated as well. You’ll need to add a few milliliters of this to the hot solution to help keep the crystals from coming out on the filter paper. And you’ll need more to help wash the crystals off of the paper when they come out on it anyway.
6. This solution should be much cleaner than the original solution. If not, you’ll have to add charcoal and filter again. There is a point of diminishing returns, however, and one or two treatments is usually all you should do. Get some guidance from your instructor.
Your solid products should not be gray. Liquid products (yes, you can do liquids!) will let you know that you didn’t get all the charcoal out. Often, you can’t see charcoal contamination in liquids while you’re working with them. The particles stay suspended for awhile, but after a few days, you can see a layer of charcoal on the bottom of the container. Sneaky, those liquids. By the time the instructor gets to grade all the products — voila—the charcoal has appeared.
