Purification Of A Solid By Recrystallization

AND

Identification By Melting Point Determination

Refer back to your recrystallization and melting point experiments. In this experiment you must purify your solid substance from a mixture of solids by using their difference in solubilities in ethanol. Your material will be soluble in hot ethanol and insoluble in cold ethanol.

PROCEDURE

1. Place 30 mL of 95% Ethanol into a 100 mL beaker, and place it atop your hot plate. Set the hot plate to “3".

2. Seat a fluted filter paper in a stemless glass funnel over a 50 mL beaker on the countertop in your hood.

3. Weigh about 0.8 to 1.0 gm. of your unknown into a 100 mL beaker, using the top-loading balances provided in the laboratory.

4. Add small amounts of the now boiling 95% Ethanol (no more than 20 mL total) to the impure solid in the 100 mL beaker.

5. Place the beaker on the hot plate to boil. Stir the mixture with a glass rod as it is being heated until all of the solid has dissolved. Note: there may be insoluble material such as rocks, boiling chips etc that will, of course, never dissolve…..

6. Pour a small amount of Ethanol (~5 mL) into the fluted filter paper to “wet” it.

7. Now pour your boiling solution as quickly as possible directly into the center of the fluted filter paper in the funnel.

8. At this point DO NOT MOVE THE 50 mL BEAKER UNDER THE FUNNEL UNTIL CRYSTALLIZATION HAS BEGUN.

9. Put the fluted filter paper, which may contain a lot of crystals, into the Recovered Organic Solids jar at the front of the lab (white plastic bottle).

10. Put the 50 mL beaker into an ice-water bath. During this cooling period, place ~10 mL 95% Ethanol in another clean beaker and place that beaker in the ice-water bath.

11. 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 to the water aspirator. Place a disk of 4.25 cm filter paper into your Buchner funnel and wet it with a small portion of ice cold ethanol. Remove the beaker from the ice bath, turn on the vacuum, 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 beaker and you have a solid mass, add

another 5 - 10 mL ethanol and stir the mixture, while cooling it in an ice bath.

Then, filter the solid.

12. After all the solution has drained from the funnel, wash the crystals with about 5 mL ice cold ethanol.

13. When all the liquid has again drained from the funnel, turn the vacuum off slowly, and remove the funnel from the flask. Pour the filtrate into the "Recovered Ethanol" bottle.

14. Leave the top part of the Buchner funnel flat in your drawer for the crystals to dry until Monday.

15. Return any excess unused ethanol to the plastic ethanol bottle.

Determination of Melting Point Ranges

PART I

Melting Ranges of Impure and Recrystallized Unknown Solids

NOTE: The Purified Product MUST be completely dry before its melting point can be determined.

You are to determine the melting range of your recrystallization product using the following procedure:

Capillary Tubes and Sample Preparation

Commercially available capillary tubes have one end already sealed and are open at the other end to permit introduction of the sample. The sample is put into the tube as follows:

Place a small amount of the solid whose melting point you wish to determine (your recrystallized product) on a clean watch glass and tap the open end of the capillary tube into the solid on the glass so that a small amount is forced about 2mm - 3mm into the tube. You may have to grind your crystals first into a fine powder in order to get your material into the capillary tube , then to get the solid to the closed end of the tube, take a piece of glass tubing 1 meter long and place this tube vertically on the benchtop, then drop the capillary tube (sealed end down) through the tube several times.

Melting Point Determination

The determination of the melting point involves taking the capillary tube containing the purified unknown and placing them in the slots that are seen through the magnifying glass of your melting point apparatus. The melting point apparatus is then allowed to heat until the contents of both tubes dissolve. Best results are obtained by heating the sample at a rate of about two degrees per minute. Many organic compounds undergo a change in crystalline structure just before melting, usually as a consequence of the release of water of crystallization. The solid takes on a softer, perhaps "wet" appearance, which may also be accompanied by a shrinkage of the sample in the capillary tube. These changes in the sample should NOT be interpreted as the beginning of the melting process. Wait for the tiny drop of liquid to appear. Melting invariably occurs over a temperature range, and the melting point range is defined as the temperature at which the first tiny drop of liquid appears up to and including the temperature at which the solid has melted completely.

PART II

Mixture-Melting Point Determination

Prepare a capillary tube containing only your recrystallized product.

On a piece of glassine weighing paper (DO NOT use filter paper) place a small sample of your purified product and a small amount of known compound you believe to be your product. Make an intimate mixture of the two substances by mixing the solids together thoroughly with your spatula.

Prepare a capillary tube containing the mixture.

On another piece of glassine weighing paper place a small sample of your purified sample and a small amount of another known compound you believe your product could be.

Make an intimate mixture of these two substances by mixing them thoroughly with your spatula.

Prepare a capillary tube containing the mixture from step 6.

Measure the melting ranges of the 3 samples. The apparatus can accommodate three capillary tubes, so melting range determinations can be done simultaneously.

Package the remainder of your recrystallized product in an appropriately labeled sample vial.

NOTES:

a) The actual melting ranges may differ somewhat from the literature values that are given in the list. Do NOT determine the melting range of any of the standards.

b) The melting range of your purified benzoic acid may not be exactly the same as the literature value because: a) the solid is not completely pure b) the sample was heated too rapidly in the melting point apparatus c) or thermometer error, etc.

Solid Compounds and their Melting Points

Group A	Group B	Group C
Benzhydrol, 69⁰C	Benzamide, 133⁰C	Benzanilide, 163⁰C
Biphenyl, 69⁰C	Cinnamic Acid, 133⁰C	Itaconic Acid, 164⁰C
Stearic Acid, 70⁰C	Urea, 133⁰C	Sulfanilamide, 166⁰C
	Phenacetin, 134⁰C