Experiment 1: Crystallization of Impure Acetanilide Objective

EXPERIMENT 1 : Crystallization of Impure Acetanilide
OBJECTIVE
1. To understand the basis of purifying organic solid compound.
2. To know that recrystallization is one of the techniques to purify the
organic solid compound.
INTRODUCTION
Recrystallization is the process that removes impurities from solid organic
compounds that are at room temperature. This process is built on the principle that
the solubility of a compound increases with temperature and decreases as it cools
down, into its crystal form. When it is at or near to the boiling point, a solution is
created by dissolving at solute in a solvent. There are seven major steps in the
recrystallization process. First, we need to choose specific and proper solvent because
if the solvent is not specified, we need to test variety of solvent that work the best for
the solute that we are trying to crystallize. Second, the impure solute must be dissolve
in minimum volume of hot solvent to ensure that the solute is completely dissolve.
Third, we need to remove any soluble impurities by filtration. Because any impurities
that remain in the solvent will affect the melting point of the pure crystal that we will
obtain at the end of the experiment. Fourth, the solute must be decolorize using
activated carbon. Fifth, the formation of crystal obtains when the solute was poured
on Buchner vacuum to vaporize the solute, forming the crystal. Lastly, we can purify
the crystal by determining its melting point. To ensure that we actually obtain the
pure crystal, we can see that in the last step there is no foreign substance in the crystal
and the melting point will be higher compared to impure crystal.
METHODS
1. 3.0 g of impure acetanilide was weight out accurately and was put in the 250 ml (A)
Erlenmeyer flask. The physical appearance of that crystal was observed. A capillary
tube was filled with crystals about 3 mm high and its melting point was determined.
2. The material was added to about 40 ml distilled water.
3. The flask was placed on a hot plate and slowly the contents was heated to boil. The
contents stirred occasionally.
4. When the solvents started to boil, 5 ml of water was added until any further
disappearance of the solute cannot detected. The solution was keep boiling.
5. The solution was cooled slightly and a small amount of decolorizing charcoal or
activated carbon was added. The solution was reheated to boil.
6. At this period, another beaker containing 20 ml of distilled water was heated to boil.
7. A piece of filter paper was fluted and placed inside of a stem less funnel.
8. The stem less funnel and fluted filter paper was placed directly in the 250 ml (B)
Erlenmeyer flask. The filter paper was wet using a small amount of distilled water so
that the rising vapour from the boiling solvent can heated the funnel and filter paper.
9. The solution was poured from flask A to flask B immediately through the funnel and
filter paper assembly.
10. The flask A was rinsed with 10 ml of distilled water and poured again through the
funnel and filter paper assembly.
11. After collecting the filtrate, a small amount of the hot distilled water was passed
through the filter paper to wash the residue.
12. The flask was set aside to cool slowly to room temperature (about 15 minutes) and
then was chilled in an ice bath (about 15 minutes) to induce crystallization. Then the
crystals collected using a vacuum filter apparatus.
13. A Buchner funnel of appropriate size was selected for the amount of residue collected,
not the amount of filtrate. A piece of filter paper was put in the funnel that fits exactly
and the paper was wet using a small amount of solvent which is distilled water.
14. The solution was filter to isolate the crystals. The flask was swirl to suspend the crystals
and then poured the slurry into the funnel.
15. Some crystals may have been left behind the flask. The crystals washed with a very
small amount of ice cold water as an effective quantitative transfer of all this material.
16. When the crystals have been collected and washed, the aspirator was allowed to run
for several minutes so that the crystals have an opportunity to dry.
17. When the crystals have been dried as much as possible in the Buchner funnel, the
aspirator was switched off and the Buchner funnel was taken off from the vacuum
source.
18. The crystals spread out in a beaker or a crystallizing dish to provide the most efficient
drying as the crystals will have a maximum exposed surface area.
19. After the crystals were dried, the final weight was determined. After that it was put in
the plastic bag and labelled. The melting point of acetanilide was determined.
RESULT
Mass of impure Acetanilide : 3.025 g
Mass of filter paper : 0.361 g
Mass of filter paper + Mass of pure Acetanilide : 1.529 g
Mass of pure Acetanilide : 1.168 g
Melting point of impure Acetanilide : 123.5 OC
Melting point of pure Acetanilide : 130.5 OC
DISCUSSION
From the experiment, the mass of impure Acetanilide is 3.025 g while the mass for
pure Acetanilide that we obtained is 1.168 g. Next, the melting point that we obtained for the
pure Acetanilide is 130.5 OC which is higher that the melting point for the impure Acetanilide.
An impure solid will have a lower melting point than the pure solid. This is because less
attractive forces within the solid, thus less energy to break up those forces, thus lower melting
point of Acetanilide.
There are several steps of precaution that we should be taken during this experiment.
The Erlenmeyer flask will contain very hot liquid. So, it must be handle with care because hot
glass looks like it is cools. Use a test tube clamp, a three prong clam, or a paper towel to
handle the hot Erlenmeyer flask. Next, do not add the decolorizing charcoal or activated
carbon to the hot solution. It may boil over when the carbon is added and will cause error to
the result. Furthermore, we need to use stem less funnel to avoid premature recrystallization
because a funnel with a stem will cool the filtrate as it passes through the stem. Besides, the
filter paper was wet using hot distilled water to prevent the crystal from forming at the filter
paper during filtration. Lastly, when the crystals are dried as much as possible in the Buchner
funnel, a scapula was used to remove them to a beaker or crystallizing dish. This will ensure
the crystals will not contaminate by the filter paper fibre as they dry.
CONCLUSION
After finish this experiment, we enhance the knowledge to understand the basis of purifying
organic solid compound and we know that recrystallization is one of the techniques to purify
the organic compound.
REFERENCES
1) http://www.wiredchemist.com/chemistry/instructional/laboratory-
tutorials/recrystallization
2) http://chemwiki.ucdavis.edu/Physical_Chemistry/Physical_Properties_of_Matter/Sol
utions_and_Mixtures/Case_Studies/RECRYSTALLIZATION
3) http://www.wiredchemist.com/chemistry/instructional/laboratory-
tutorials/recrystallization
4) http://www.xula.edu/chemistry/documents/orgleclab/07Recr.pdf
5) http://www.chem.umass.edu/~samal/269/cryst1.pdf

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