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

 

Place 10 mL 95% Ethanol into a 100 mL beaker, and place it atop

your hot plate, which in turn is placed atop the base of a ringstand. Set the hot plate to 3".

 

Seat a fluted filter paper in a stemless glass funnel, supported on an iron ring, over

the 50 mL beaker - the funnel just barely above the top of the beaker.

 

In addition, place a 100 mL beaker containing 50 mL of 95% Ethanol on the hotplate.

 

Weigh about 1 - 2 gm. of your unknown in a 100 mL beaker, using the top-loading balances

provided in the laboratory.

 

Add small amounts of boiling 95% Ethanol (no more than 30 mL total) to the impure solid in the

beaker.

 

Place the beaker on the hot plate.

 

Stir the mixture with a glass rod as it is being heated. If all the solid does not dissolve

in the boiling solvent after a minute or two, add another 5 mL of solvent. Heat the mixture

for another minute or so (with stirring) and observe whether the solid has

dissolved to any greater degree than previously observed.

 

After the funnel has been warmed by the vapors from the beaker, add your 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 95% Ethanol, to dissolve any crystals

that might be present.

 

Discard the filter paper into 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 95% Ethanol 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 to the water aspirator.

After the mixture has cooled to room temperature, place the beaker 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 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.

 

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

cold ethanol.

 

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.

Return any excess unused ethanol to the plastic ethanol bottle.

 

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 top portion of your buchner funnel, containing the recrystallized product

in your locker. Your product will dry in your locker overnight.

 


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

 

Benzhydrol, 69oC

Biphenyl, 69oC

Stearic Acid, 70oC

 

 Group B

 

Benzamide, 133oC

Cinnamic Acid, 133oC

Urea, 133oC

Phenacetin, 134oC

 

 Group C

Benzanilide, 163oC

Itaconic Acid, 164oC

Sulfanilamide, 166oC


Che 317 Sp2005

 

Lab Report

 

 

 

TITLE: Recrystallization of an impure solid compound and its

Identification

 

Name: (Your Name) Unknown #

 

Date:

 

Weight of crude Unknown: gm.

 

Weight of recrysatallized Unknown: gm.

 

% recovery: %

 

Melting Range of Recrystallized Unknown: oC

 

 

Melting Range of Unknown and Standard(s):

 

Test 1: Standard/MP________________________________________ oC

 

Test 2: Standard/MP________________________________________ oC

 

Test 3: Standard/MP________________________________________ oC

 

Test 4: Standard/MP________________________________________ oC

 

Test 5: Standard/MP________________________________________ oC

 

 

 

 

Identify your unknown by name. Justify your reasoning.