ORGANIC I LABORATORY (MAJOR'S)
KELLY
PURIFICATION OF SOLIDS BY CRYSTALLIZATION AND
IDENTIFICATION BY MELTING POINTS.
I. PURPOSE: To learn how solids are purified by recrystallization and to identify
an unknown compound by its melting point
II. READING ASSIGNMENT:
and pp 46-51 for melting points.
Landgrebe pp109-121 for recrystallization
EQUIPMENT AND SUPPLIES:
A. From Stockroom
Yellow squeeze bulb for Pasteur pipette (1)
B. From front desk
Pasteur pipette (1)
III. GENERAL INSTRUCTIONS:
A. You will work individually on this experiment. Pick up two grams of
unknown from the lab assistant. This unknown is contaminated with a
colored impurity. You will purify the unknown by recrystallization and
identify it by taking its melting point in comparison with authentic samples
provided in the next lab period. You will turn in the unknown when you turn
in your report. The unknown will be graded for purity and yield. .
B. Be sure to wear your goggles during the entire period. A few drops of
methanol in the eye will cause partial blindness. Take no chances. If you get
methanol on your hands, wash it off. It does not soak through the skin very
quickly.
C. Be sure to pour all chemicals into labeled waste containers.
IV.
A.
B.
C.
D.
OVERVIEW OF PROCEDURE
Find the best solvent for your unknown.
Dissolve your unknown in the minimum amount of boiling solvent.
Add decolorizing carbon and boil to remove colored impurities.
Filter the hot solution through a fluted filter paper to remove the carbon and
any insoluble .impurities.
E. Allow the hot, saturated solution to cool slowly to induce crystallization of
the pure unknown.
F. Determine the recovered witght, per-cent recovery, and melting point of the
unknown.
G. Identify the unknown by comparison of melting points and mixed melting
points with samples of authentic compounds provided by your instructor.
Purification of Solids by CrystallizationPage 2
Compounds for Melting Point Unknowns
Compound
p-Dichlorobenzene
Naphthalene
Phenanthrene
m-Anisic acid
m-nitroaniline
acetanilide
p-nitroaniline
Salicylic acid
Melting point (˚C)
53
80
101
102
113
114
150
159
V. PROCEDURE (Recrystallization):
A. Determine the best solvent for recrystallizing your unknown. Use water,
methylene chloride and methyl alcohol as potential solvents. Use the
procedure below:Remember: An ideal solvent will not dissolve the material at
room temperature, will dissolve it when the solvent is near boiling, and will
allow crystals to reform from the saturated solution when cooled down.
To select a suitable solvent, a small sample (0.05-0.1g) of ths solid to be
recrystallized is added to a test tube and 1-2 ml of the solvent to be tested is
added, the tube stoppered and shaken vigorously. If solution occurs easily at
room temperature the solvent is not suitable. If solution does not occur, the
tube is heated to just boiling in a sand bath (remember to remove th cork
first), if the sample does not dissolve even with heating, the solvent is
unsuitable. If the solid does dissolve in the warm solvent, it should be allowed
to cooled to see if crystals will form at room temp (or slightly below).
C. Once the best recrystallization solvent is found, follow the steps below.
1. Add the solid to a 125 mL erlenmeyer flask and add about 10 mL of the
solvent. Add a boiling stick. Support the flask on a ring clamp and heat
over a hot plate. Bring the solvent to a gentle boil and stir for 5 minutes
while keeping a gentle boil. Then add another 10 mL and repeat the
process until all the solid dissolves. This could take from 40 to 70 mL of
solvent. During this time the solid may melt. If it does, it will look like
an oil in the solvent. Just continue to add solvent until the oil dissolves.
If there are solid impurities in the oil, do not try to add enough solvent to
dissolve them. They will never dissolve! They are either sand or carbon
black.
2. While you are weighing out 0.2 g of decolorizing charcoal, let the solution
cool just below boiling. Add the decolorizing charcoal slowly to the
Purification of Solids by CrystallizationPage 3
solution. Bring the solution back to the boiling point and gently boil for
five minutes.
3. Clamp an empty erlenmeyer flask over a hot plate and insert your heated
glass funnel. Load the funnel with a fluted filter paper. While holding the
boiling saturated solution with a clamp, pour the hot solution, a little at a
time, on the filter paper. Keep the remainder of the solution boiling until
you are ready to add more to the filter paper. Avoid dripping the
solution under the filter paper by pouring down a glass rod. The
dampened filter paper is very easily torn. Do not lift or poke it, or a
rupture will occur. Some crystallization of the unknown is normal during
this step.
4. Allow the filtered solution to cool slowly, without stirring or swirling,
until you can comfortably hold it in your hand. Scratch the wall of the
vessel with a glass rod just under the surface of the solution. Crystals
should form. You will have to scratch hard enough that the glass wall is
scratched. After crystals have formed, cool in ice water for five minutes.
If an oil comes out during the cooling, add 5 mL of water and hold the
mixture at the boiling point until the oil is back in solution. Then start the
slow cooling process again.
5. Moisten a filter paper with a small amount of solvent and suction filter the
slurry of crystals in a Buchner funnel. If some crystals are left in the
original flask, you may swirl them with some of the filtered mother liquor
and try to transfer them to the Buchner funnel. Always break the
vacuum by disconnecting the aspirator end of the hose before turning off
the water flow to the aspirator.
6. After the crystals are free of solvent, wash with 5 mL of ice cold solvent.
7. Transfer the crystals to a clean filter paper and leave in your drawer
until next lab period, at which time you will take the melting point
and get a final weight.
VI. Procedure (melting points)
A. Determine melting points of your recrystallized unknown sample as well as
the authentic samples provided.
.
B. Always fill melting point capillaries to a depth of no more than 1/8 inch from
the bottom. Make sure the crystals are finely ground and well packed in the
capillary.
C. Do not use the melting point apparatus until you have had instruction on its
operation.
Purification of Solids by CrystallizationPage 4
D. Before using the melting point apparatus, make sure it has cooled down
below 50°C. Insert the capillary and apply heat that will cause the
temperature to rise about 10 degrees per minute. WATCH THE SAMPLE
CAREFULLY FOR ANY SIGN OF MELTING. Record the temperature
when melting first occurs, then when the sample is all liquid; this is the
melting point range, the melting point range should be no more than 2-3
degrees. Repeat the procedure with a fresh sample and tube this time when
the temperature of the sample reaches a temperature 10 degrees below the
first temperature you recorded as the melting point...SLOW THE HEATING
RATE TO ABOUT 3 DEGREES PER MINUTE. Duplicate results should
agree within 2-3 degrees C.
E. Determine a more accurate melting point of each of the following in a second
melting run with capillaries of the following samples all in place.
1. pure unknown
2. three mixtures of your unknown with standard compounds that have a
similar melting point.
VII.
A.
B.
C.
Report
Title Page
Experimental Procedure
Observation Sheets
1. Weight paper
2. Weight product + paper
3. Weight of product only (difference)
D. Calculate percent recovery of final crystals
E. Melting point and range of unknown and authentic samples.
Identify your unknown, justify your reasoning.
F. Discussion of likely purity of your crystals taking in consideration the
following.
1. Color
2. Melting point
3. Other observations that might relate to purity (You must think of some
valid ones here).
G. Answer questions:
pages 121-122: 2, 6, and 7
pages 61: 1, 2 and 3