E x p e r r i i m e n t t : : R e c r y s s t t a l l l l i i z a t t i i o n – P a r t t I I I I : : P u r i i f f i i c a t t i i o n o f f S o l l i i d s
In Part I of the recrystallization experiment, you learned about the factors which make a good recrystallization solvent, and you learned how to determine experimentally what solvents might be good ones. In this part of the recrystallization experiment, you will actually use the recrystallization solvent to purify impure solids.
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Recrystallization with a Single Solvent
Often a single solvent is used to recrystallize an impure solid. Once the solvent has been selected, the recrystallization process proceeds by the following steps. Depending on the impurities present, some of these steps may be omitted, and the possibility for such omissions will be the subject of some of the post-lab questions.
Dissolving the Solid to be Purified
The solid to be purified is placed in an Erlenmeyer flask. This type of flask is used because it allows for vigorous mixing without spilling the sample. Solvent is added to the flask, and the mixture is heated. How much solvent to add varies from recrystallization to recrystallization and depends on the solid and the solvent involved. In general, the quantity of solvent used is determined experimentally. A small amount of solvent is added to the solid and with vigorous swirling is heated to boiling. During this heating, a “boiling stick” is used. This stick serves the function of a boiling stone, but it can more easily be removed once the solid is dissolved. If the solid does not dissolve at the solvent’s boiling point, more hot solvent is added with swirling until the solid has just dissolved at the solvent’s boiling point.
At this point, you may have to exercise some judgment. Keeping in mind that there may be insoluble impurities present, impurities that will not dissolve at all in the solvent, even at its boiling point, you must decide whether the material that has not yet dissolved is the substance to be purified or an insoluble impurity. If it is the substance being purified, you need to add more solvent to dissolve it. If it is an impurity, you do not want to add more solvent. Doing so will dilute the solution of desired solid, thus reducing the amount of solid that will recrystallize from the solution once it has been cooled. So how are you supposed to know whether to add more solvent or not? One way to tell is to see if the solid remaining undissolved looks different from the solid that you started with. Is it a different color or a different consistency? Another way is to see if the amount of undissolved solid stays the same as more solvent is added. If it does stay the same, then it is probably insoluble impurity. If it dissolves with further addition of solvent, it is probably the solid to be purified.
In any event, the bottom line in dissolving the solid is to use as little solvent as is needed to completely dissolve the solid to be purified. Even with this minimum amount of

solvent, some of the solid will remain in solution when it is cooled. “The more solvent you use, the more solid you will lose.”
Adding Decolorizing Carbon
If the impure solid is colored when it should be white, or if the solution of impure solid is colored and shouldn’t be, decolorizing carbon is used. Sometimes called decolorizing charcoal, or by its trade name Norit, decolorizing carbon will adsorb to its surface those impurities that are causing the color. Because the carbon is very powdered and porous it should never be added to a solution that is near its boiling point. This is because the tiny grains of carbon will act like thousands of little boiling stones and cause the solution to boil over. Never add decolorizing carbon to a hot solution.
After the hot mixture has been cooled the carbon is added and the mixture is reheated to the boiling point and filtered.
Filtering the Hot Recrystallization Mixture
The hot recrystallization mixture is filtered by gravity using a stemless or short stem conical glass or plastic funnel. [
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Note: Never filter the hot recrystallization solution with a Buchner or Hirsch funnel. Only use these funnels to filter at room temperature or cooler.] This filtration removes any insoluble impurities, and if used, decolorizing carbon. The filter paper used is fluted filter paper. The mixture must be filtered hot, as near the solvent’s boiling point as possible. [
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Note: It will save you time and yield if you filter the solution into a beaker. When crystallization does occur, it will be easier to scrape the crystals from a beaker than from a flask.] Since the goal of this step is to separate insoluble impurities from the solution containing the desired solid plus soluble impurities, you do not want any crystals to form until the solution has passed through the funnel. To this end, it is helpful if the funnel is heated before the hot mixture is poured onto it. If all goes well, the hot liquid will pass through the funnel, leaving only insoluble impurities and, if used, decolorizing carbon, on the filter paper. At this point you have removed the insoluble impurities.
This hot filtration is the trickiest part of the recrystallization procedure, because crystallization may occur in the funnel and in the pores of the filter paper, preventing the hot liquid from passing through. If crystals do form in the funnel, they can be removed by pouring small amounts of boiling solvent through the funnel to dissolve the crystals.
If the quantity of crystals in the funnel is too great for this, scrape the solid from the filter paper, put it back into the hot recrystallization mixture, add more boiling solvent, and refilter the solution.
Cooling the filtrate
After the insoluble impurities have been filtered off, what remains is a filtrate containing the product to be crystallized plus any soluble impurities. At this point, allow the solution to cool and crystals to form. If crystals do not form, try scratching vigorously the side of the beaker with a glass stirring rod to induce crystallization. If they still do

not form, try cooling the solution in an ice-water bath. If crystals still don’t form, there is too much solvent in the system. At this point you will have to evaporate some of the solvent to concentrate the solution so that crystals will form. Crystals should be medium in size, the kind that grow only at low temperatures. Crystals that grow very quickly, as soon as they hit the beaker after filtration tend to be very small crystals, ones which can adsorb impurities on their surface. Crystals that grow very slowly and large tend to trap impurity-containing solvent inside them. The best crystals are medium in size and grow over a period of minutes, rather than seconds or hours.
Filtering the Crystals
The crystals are filtered with suction from the solvent containing the soluble impurities using either a Buchner or Hirsch funnel. It is important that filter paper be used that just covers the holes of the funnel.
Washing the Crystals
By this time, the crystals have been separated from a solution containing the soluble impurities, and the crystals are still wet by this solution. If the procedure were stopped at this point, the solvent would begin to evaporate, leaving behind some of the dissolved soluble impurity. Therefore, the crystals must be washed with a small amount of ice cold solvent. This washing is done directly on the funnel with suction still applied. The clean cold solvent displaces the impurity-containing solution that had been wetting the crystals.
Now, when the solvent is evaporated, there will be no soluble impurity left behind.
Drying the Crystals
After the washed crystals have been removed from the funnel, the solvent must be allowed to evaporate. This should be done for at least 24 hours to be certain that the crystals are dry. It is useful to cover the crystals with a piece of filter paper that will allow solvent to evaporate without allowing dust and other impurities to fall on the purified crystals.

The flow diagram below summarizes this purification process. The symbols used in the diagram are:
Product, Soluble Impurities,
Insoluble Impurities hot solvent, filter solid
Insoluble
Impurities filtrate
Product, Soluble Impurities, Solvent cool, filter solid filtrate
Solvent, Soluble
Impurities, trace of
Product
Product, trace of Solvent,
trace of Soluble Impurities cold solvent wash solid
Product, trace of Solvent evaporate solvent
Purified
Product liquid wash
Solvent, Soluble
Impurities, trace of Product
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Recrystallization with Solvent Pairs
There are some solids which cannot be recrystallized from a single solvent. In other words, no single solvent can be found which will not dissolve the solid at room temperature but will dissolve it at the boiling point. When this occurs, chemists employ the technique of solvent pair recrystallization. The solvent pair consists of two solvents that are miscible (soluble in each other in all proportions). The solid to be purified dissolves readily in one of the solvents at room temperature. It does not dissolve readily in the other solvent at any temperature. When the solvents are mixed, the solid dissolves in the solvent pair poorly at room temperature, thanks to the non-dissolving solvent, but well at the boiling point, thanks to the dissolving solvent.
Together, the solvent pair dissolves the solid to be purified just like a single recrystallization solvent. Examples of pairs of solvents that are often used in recrystallization can be found online.
If there is a problem associated with solvent pair recrystallization, it is finding the best ratio of the two solvents to use. It is not a big problem. For ease of discussion, let’s call the solvent in

which the solid is soluble, solvent “S” and the solvent in which the solid is insoluble solvent “I”.
First, we dissolve the solid to be purified in some amount of solvent “S”. Then we heat the solution to the boiling point. Then, while keeping the solution boiling, we slowly add solvent
“I”. As “I” is added, the solubility of the solid in the mixed solvent system decreases until the solution becomes saturated. If a little more of “I” is added, the solvent mixture gets cloudy because the solid begins to precipitate. At this point we add just enough “S” to redissolve the solid and proceed as with a single solvent recrystallization.
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1.
Before coming to lab, carefully go over the “Background” section for this experiment, your discussion notes on recrystallization, and watch the podcast demonstrating the procedure for recrystallization. Watching the podcast will make the lab faster for you and will help you answer quiz questions your instructor may ask from this podcast. So watch the entire podcast carefully. To find the podcast, go to the Montgomery College homepage and scroll to the very bottom. Under “News and Events”, select “Podcasts” (tiny font at the very bottom of the page). Under “Browse by Topic”, select “Science and
Technology”. View the Podcast labeled “Solvent Pair Recrystallization”.
2.
Carefully go over the procedures to be followed in this laboratory. This will require you to look at the results for the Recrystallization: Part I lab for benzoic acid and determine from your solubility data which organic solvent to use for recrystallizing benzoic acid. Write the solvent that you have selected here.
3.
In your notebook, write in tabular form the relevant physical constants for benzoic acid, 1,4-dibromobenzene, and ethanol.
4.
Go online and locate and MSDS (Material Safety Data Sheet) for 1,4dibromobenzene. Review and print this document and attach it to your lab report.
! Safety Considerations
! 1,4-dibromobenzene is carcinogenic. Minimize contact with skin and avoid breathing vapors.
! During the solvent pairs experiment, you will be adding decolorizing carbon to the recrystallization mixture and solvent. Be sure that the solvent is not near its boiling point

before you add the carbon. If the solution is hot, the solution may violently splash out of the flask. Never add boiling stones to a hot solution.
! Also during the solvent pairs experiment, you will be adding hot water (b.p. 100ºC) to hot ethanol (b.p. 65ºC). Add the hot water dropwise because if too much hot water is added to the hot ethanol it might cause the ethanol to boil violently and splash out of the flask.
Part 1: Recrystallization of Benzoic Acid from a Single Solvent
1. For this part of the lab, you will compare two recrystallization solvents: water and an organic solvent of your choice (see pre-lab). Weigh two separate 2.0 gram samples of impure benzoic acid. Take enough of this impure solid to take a melting point and put the rest into two separate 125-mL Erlenmeyer flasks. Add boiling sticks to the flasks to prevent bumping when the recrystallization mixture is boiling.
2.
Add 10 mL of water to one of the flasks and 10 mL of your selected organic solvent to the other flask and heat the mixtures to boiling on a hot plate. You should also heat flasks with pure solvent so that you can add additional hot solvent as needed in Step 3.
3.
Keep adding the respective, hot solvents, while boiling, to the mixture until the solid all dissolves. (No insoluble impurities should be present in this sample.) (You may see some crystals forming on the sides of the flasks, but these can be ignored.)
4.
Remove from the heat and allow the solution to cool to room temperature. Crystals should form at this point. After the crystal-solvent mixture has been allowed to sit for 10 minutes, cool the mixture in an ice-water mixture for no more than 5 minutes. Cooling for too long in the ice-water bath may result in crystallization of some of the soluble impurities.
5.
Filter the cold mixture with suction and wash the crystals in the funnel with small amounts of your ice cold solvent.
6.
7.
Only aqueous waste can be poured down the sink.
Remove the crystals from the funnel and place them between two pieces of dry filter paper to allow the crystals to dry.
8.
After at least 24 hours, weigh the crystals and determine their melting point and the melting point of the impure solid. Remember to report a melting point range for all samples.

Part 2: Recrystallization of 1,4-dibromobenzene from a Solvent Pair
1.
Weigh 2.0 grams of impure 1,4-dibromobenzene. Take enough of this impure solid to take a melting point and put the rest into a 125-mL Erlenmeyer flask. Put a boiling stick into the flask to prevent bumping when the recrystallization mixture is boiling.
2.
Add 25 mL of ethanol to the flask and bring it to a boil on a hot plate. Cool the solution to below its boiling point.
3.
Because the solid mixture was colored and because pure 1,4-dibromobenzene is colorless, add about 0.050 g of decolorizing carbon to the ethanol-dibromobenzene mixture. Place the flask back on the hot plate and bring to a boil. While heating, warm a stemless funnel on your hot plane. After a minute of boiling, filter the hot solution into an appropriately sized Erlenmeyer flask through fluted filter paper and a heated stemless funnel.
4. If crystallization occurs in the funnel take the steps outlined in the “Background” section of this experiment to get them out of the funnel.
5. Heat the solution to boiling and dropwise add, with swirling, boiling water to it. Keep adding boiling water to the hot solution until cloudiness appears. At this point, add small amounts of hot ethanol to the boiling mixture until the cloudiness just disappears.
6.
Allow the hot solution to cool to room temperature. Crystals should form at this point.
After the crystal-solvent mixture has been allowed to sit for 10 minutes, cool the mixture in an ice-water mixture for no more than 5 minutes. Cooling for too long in the ice-water bath may result in crystallization of some of the soluble impurities.
7.
Filter the cold mixture with suction and wash the crystals in the funnel with small amounts of ice cold water.
8.
Remove the crystals from the funnel and place them between two pieces of dry filter paper to allow the crystals to dry.
9.
All ethanol waste will be disposed of in a ethanol waste container in the hood. This includes all filtrates (mother liquors) from the experiment.
10.
After at least 24 hours weigh the crystals and determine the melting point of both your purified solid and the melting point of the impure solid. Remember to report a melting point range for both. See Appendix 4 for detailed melting point information. After taking the melting points of the crystals obtained in this experiment, put the remaining crystals of 1,4-dibromobenzene in a test tube and the crystals of benzoic acid in a test tube. Label the test tubes neatly with the name of the compound, your name, your instructor’s name and the experimental melting point of the crystals and turn them in to your instructor with your lab report.

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1.
Describe the characteristics of a good recrystallization solvent. Also give the characteristics of a good solvent pair system. (Do not simply copy the information provided in the introduction of Recrystallization - Part I. Explain in your own words.)
2.
For each of the crystallizations that you carried out, give the grams of dry solid that you recovered, and the melting point ranges of the impure and purified solids.
3.
Based on your melting point data, comment on the how well you were able to purify these samples. Also, compare the two solvents used for benzoic acid. Which worked better? How did you decide?
4. Calculate the percentage recovery of each of your recrystallized solids. Explain why you did not get 100% yield. Where did your loss of product occur? Which were avoidable losses and which unavoidable?
Percent Recovery = mass of recovered crystals
x 100 mass of impure crystals
5. Answer the following questions: a.
Under what circumstances would it be reasonable to omit from the recrystallization procedure the step involving filtration of the hot solution through fluted filter paper? b.
Under what circumstances would it be acceptable to omit from the recrystallization the addition of decolorizing carbon? c.
What would happen to the amount of benzoic acid recovered from your recrystallization if you used too much solvent to dissolve your crystals? Explain your answer. d.
Why is it necessary to wait at least 24 hours after filtering them to take a melting point of the recovered crystals? What would be the effect of not waiting? e.
In the solvent pair recrystallization that you did in this experiment, you were told to dissolve the impure 1,4-dibromobenzene in 25 mL of ethanol. You mistakenly add 35 mL. Will this error cause you to recover less dibromobenzene than if you had followed directions? Explain your answer.