2. on a Hot Plate, Heat Some of the Solvent You Have Selected in an Erlenmeyer Flask. Put

Recrystallization

1. Select the appropriate solvent for recrystallization. Sometimes you can find data on solubility for your compound or related compounds in the CRC Handbook or in the chemical literature. Test one or more possible solvents using small amounts in test tubes. The compound you are trying to recrystallize should be insoluble in the cold solvent, dissolve in the solvent when heated, and recrystallize when the solvent is cooled. A mixed solvent may be required.

2. On a hot plate, heat some of the solvent you have selected in an erlenmeyer flask. Put the impure, solid compound in a beaker. Placing the beaker on the hot plate, quickly add a small amount of the hot solvent. If the compound does not completely dissolve, add additional small amounts of the solvent until complete dissolution is achieved. You want to use a minimum of the hot solvent to dissolve the compound. Do not needlessly boil off the solvent; if the compound is going to dissolve it will do so in a few seconds.

3. Once the impure compound is totally dissolved you must decide whether to decolorize or not. If necessary, add a small amount of decolorizing carbon to the hot solution. Place your stemless glass funnel in the top of a beaker. Fold a fluted filter paper (page 46 of your lab text) and place it in the stemless funnel. Add a few mL of the pure solvent to the beaker. Put the hot filtration assembly on the hot plate and add the hot, decolorized solution of your compound to the filter. The purpose of the stemless funnel, beaker, and hot plate is to keep the compound to be purified in solution while you filter out the carbon.

4. Let the hot solution cool to room temperature on the bench top. Then use an ice-water bath to cool the solution even more. If crystallization does not begin, try scratching the wall of the beaker with your glass rod.

5. Vacuum filter the cooled solution to recover the recrystallized solid. You may want to wash the compound with a small amount of cold solvent if the compound is not too soluble in the cold solvent.

6. After air drying for a couple of days you can weigh and take the melting point of the purified compound before packaging it.

PURIFICATION OF A SOLID BY RECRYSTALLIZATION

A mixture of solids can be separated from each other if they have different solubilities in a particular solvent. In this experiment you will dissolve a solid with hot water, any insoluble substances will be caught in the filter paper when you filter the hot solution. This "filtrate", when cooled, will yield a pure solid that when allowed to dry will melt in a 1-2 degree range. If any impurities remain in the solid the melting range will be wider and the temperature at which your solid begins to melt will be lower than that of the pure solid. PROCEDURE Day 1

Place 10 mL water into a 50 mL beaker, and place it atop your hot plate, which in turn is sitting on the base of a ringstand.

Seat a fluted filter paper in a stemless glass funnel, supported on an iron ring, over the 50 mL beaker with the funnel just barely above the top of the beaker. In addition, place a 100 mL beaker containing 50 mL of water on the hotplate

Weigh about 1 gm. of benzoic acid in a 50 mL beaker, using the top-loading balances provided in the laboratory.

Add small amounts of hot water (not more than 20 mL total) to the impure solid in the beaker until it is completely dissolved. Place the beaker on the hot plate and set the dial to "6".

Stir the mixture with a glass rod until the solution begins to boil, then continue heating for an additional minute.

After the funnel has been warmed by the vapors from the beaker, add your hot solution containing your material to the funnel. Only add enough solution to cover half the filter paper at one time. Once the solution has been filtered, add an additional aliquot of hot water, to dissolve any soluble crystals that might have precipitated onto the filter paper. Remove the beaker from the hot plate.

Discard the filter paper in the Recovered Organic Solids jar at the front of the lab.

Allow the beaker containing the clear filtrate to cool to room temperature on the lab bench . During this cooling period, place 10 - 20 mL water in your 50 mL beaker and place the beaker in an ice-water bath.

Clamp your 125 mL suction flask to a ringstand with a three-prong clamp, and place the smallest Buchner funnel (with rubber stopper attached) into the neck of the flask. Attach a length of black vacuum tubing to the side-arm of the flask and to the house vacuum line (the valve with the Yellow cap), or a water aspirator ( if the house vacuum is not available).

Purification Of A Solid By Recrystallization

After the mixture has cooled to room temperature, place the flask in the ice bath. Place a disk of 4.25 cm filter paper into your Buchner funnel and wet it with a small portion of ice cold water. Remove the flask from the ice bath, turn on the house vacuum (or water) and filter the precipitated crystals with suction.

a) If there are no - or very few - crystals at this point, see the instructor. b) If there appears to be no liquid in the flask and you have a solid mass, add another 5 - 10 mL water and stir the mixture, while cooling it in an ice bath. Then, filter the solid.

After all the solution has drained from the funnel, wash the crystals with about 5 mL ice- cold water. When all the liquid has again drained from the funnel, turn the vacuum or water off slowly, and remove the funnel from the flask. Pour the filtrate down the drain.

NOTE: Pay no attention to any solid that may have precipitated from the solution inside the suction flask. This is quite normal and results from the evaporation of some of the water from the saturated solution under the reduced pressure in the flask. Any such solid is impure and should not be recovered.

Put the funnel containing the recrystallized product back on the filter flask. Cover the funnel with a disk of 9.0 cm filter paper and re-apply suction for 5 minutes.

Put the top portion of your Buchner funnel, containing the recrystallized benzoic acid in your locker. Your product will dry in your locker overnight.

Remove the product from your locker, weigh it, determine the percent of the starting material that you recovered, and its' melting range. (The melting range procedure is described in the next portion of this experiment). The lab reports from this procedure and for the next are combined into one. Turn in the product in a properly labeled vial.