RECRYSTALLIZATION Adolfo, Angela Nicole G., Alcaraz Frances Frances Jillian F., Aliman, Aliman, Stephanie Stephanie T., Astor Astor,, Jashmin D., D., Bauzon, Franchesca D., Cabral, Danica Rose G.
Department of Pharmacy, Faculty of Pharmacy University of Santo Tomas ABSTRACT Recrystallization is a purifying method of organic solids where the amount of solute that can be dissolved by a solvent increases with temperature. In the experiment, each of the three test tubes contains pure pur e acetanilide dissolved in water, wat er, ethanol, and hexane respectively. The test tubes were observed at room temperature, during heating (37-40 C) and upon cooling. Moreover, there was crude acetanilide obtained from mixture of aniline, distilled water and acetic anhydride. Water was used as the recrystallizing solvent to obtain the pure acetanilide. The crude and pure acetanilide was weighed. After which, the percentage yield of acetanilide was computed and the melting point of the recrystallized acetanilide was determined. Therefore, recrystallization is a good purification method compatible with the nature of acetanilide. INTRODUCTION A solid sample can be purified
in a purer form because the impurities won't
through the process of recrystallization, to
crystallize
prepare a highly concentrated solution at a
impurities behind in the solution. A filtration
high temperature. Melting point can be used
process must be used to separate the more
to determine the purity of this sample. The
pure crystals at this point. The procedure can
solution
the
be repeated. Solubility curves cur ves can be used to
temperature causes the solubility of the
predict the outcome of a recrystallization
impurities in the solution and the substance
procedure. The slower the rate of cooling, cooli ng, the
being purified to decrease. The impure
larger the crystals are that form. The
substance
the
disadvantage of recrystallization is that it
impurities- assuming that there was more
takes a long time. Also, it is very important
impure substance than there were impurities.
that the proper solvent is used. This can only
The impure substance will crystallize
be determined by trial and error, based on
is
cooled.
then
Decreasing
crystallizes
before
yet,
therefore
leaving
the
predictions and observations. The solution
of obtaining a pure sample of some product,
must be soluble at high temperatures and
or precipitate.
insoluble
at
low
temperatures.
The
advantage or recrystallization is that, when carried out correctly, it is a very effective way The following are the objectives 1.)
To be able to select an appropriate
3.)
To compare the melting points of
impure and recrystallized acetanilide
recrystallizing solvent 2.)
To separate and purify acetanilide by
recrystallization METHODOLOGY Prepare three (3) different test tubes
In an Erlenmeyer flask, mix 2mL of
and place a corn-grain sized amount of pure
Aniline and 20mL of distilled water, and then
acetanilide, then add distilled water to the
slowly add 3mL of acetic anhydride. Note the
first test tube, hexane to the next one, and
changes then cool the flask in an ice bath to
ethanol to the last. Before placing it in a water
speed up the crystallization of the solution.
bath for 1-5 minutes, make sure that the test
The solution was then filtered to obtain the
tubes were shaken. The test tubes were
fully formed crystals, let it dry by pressing
placed to an ice bath right after, and note the
with filter paper and weigh the crude
solubility of the pure acetanilide in the
Acetanilide.
different solvents at varying temperature.
For the second crystallization, pour
impurities and heat it again. After heating, the
20mL of water of the recrystallizing solvent
solution should be filtered immediately while
when the crude Acetanilide was completely
it is still hot using a fluted filter paper. The
dried. The solution was placed in a water
filtrate was placed on an ice bath to hasten
bath until all the solid dissolves. If the
the crystallization. Collect the crystals and let
solution shows a change in color, add a pinch
it dry by pressing in-between filter papers,
of activated charcoal to remove the colored
and then weigh it.
After heating, the solution was filtered
placed on an ice bath to hasten the
immediately while it is hot and the filtrate was
crystallization of the solution. When the
solution is done crystallizing, the solution
between filter papers. The yielded pure
was filtered and the crystals were collected.
acetanilide was weighed.
The crystals were dried by pressing in-
RESULTS AND DISCUSSION
Table 1 - Solubility of Pure Acetanilide in Different Solvent
A recrystallizing solvent is a solvent that shows the desired solubility
Solvent
behavior for the substance to be crystallized,
Room
During
Upon
temp.
heating
cooling
and the process of recrystallization starts with finding the right crystallizing solvent. In choosing the recrystallizing solvent, the
Water
Insoluble Soluble
Insoluble
Methanol
Soluble
Soluble
Hexane
Insoluble Insoluble
Soluble
compound being purified must be insoluble in solvent at room temperature and must be
Insoluble
soluble in boiling solvent. The solvent’s boiling point must be lower than the melting
In the experiment, three solvents
point of the compound. The unwanted
were used, such as Water, Methanol, and
impurities
Hexane. The solvents were used to identify
should
be
soluble
at
room
temperature or insoluble during heating.
which solution would best suit for the experiment. As tabulated on Table 1, hexane
The solubility of Acetanilide was
was insoluble in all temperatures and the
tested for the three given solvents: Water,
opposite for methanol which is soluble in all
Hexane, and Methanol. A compound usually
temperatures; as for water, it is insoluble at
exhibits one of the three general solubility
room temperature and upon cooling, but
behaviors: (1), the compound has a high
during heating it is soluble. Therefore, two of
solubility in both hot and cold solvent, (2), the
the chosen solvents cannot be used for the
compound has a low solubility in both hot and
recrystallization of acetanilide. Including the
cold solvent, and (3), the compound has a
verity that both hexane and methanol were
high solubility in hot solvent and low solubility
known to be highly volatile and flammable.
in cold solvent. A solvent showing the last
And as distilled water being the only option
characteristic or behavior is the one suitable
left, it is ceased that water is the ideal
as a Recrystallizing Solvent.
recrystallizing solvent for acetanilide. In acetylation of aniline with acetic anhydride, the yielding acetanilide exhibits
an exothermic reaction. Exothermic reaction
Thus, crystals form. Hastening crystallization
is the release of heat from the system. In
is done through ice bath. This method
case of cooling, the heat has to be removed
decreases the solubility of acetanilide.
from the system, the temperature is lowered. the weight of the crude acetanilide is 4.80 g
first in a reaction must be determined first. It
The crude acetanilide was then subjected to
also limits the amount of product to be
the addition of the recrystallizing solvent
formed.
which is water and placed in a hot plate. During the process of heating in a water bath,
Calculation for the determination of the
impurities may cause the solution to be
limiting reagent: (First Calculation)
colored, or have color. To let these color vanish, activated charcoal can be applied to remove the colored impurities of the solution.
2 ml aniline x
1.0217 g 1 ml
= 2.0434 g
2 ml aniline = 2.0434 g aniline
Activated charcoal was used to adsorbs the impurities because it has a large surface area and it can remove impurities more effectively. Only a small amount of
3 ml acetic anhydride x
1.082 g 1 ml
= 3.246 g
activated charcoal should be added. Too much activated charcoal will not only adsorb
3 ml acetic anhydride = 3.246 g acetic anhydride
the impurities but also the desired product. When the acetanilide was dissolved, the solution should be immediately filtered
The
calculated
values
are
the
while hot using a fluted filter paper. The
amounts of the two reactants in terms of
filtrate was subjected to ice bath to speed
mass. These are used to determine if the
up the cooling process. The crystals
amount of each reactant is enough or more
collected were dried by pressing in between
than what is needed for the chemical
filter papers.
reaction.
To calculate the percentage yield of the recrystallized acetanilide, the limiting reagent which is the reactant that is used up
% Recovery =
3.70 g 4.80 g
× 100
% Recovery = 77.08% Figure 1. Calculation for the determination of the limiting reagent.
The percentage recovery was 77%, which means there was only a minimum amount
of
acetanilide
lost
and
the
aniline
experiment was a success. The pure
because it needed 2.96 g to react with 3 ml
acetanilide that was collected was colored
of acetic anhydride but only 2.0434 g is
white.
The
limiting
reagent
is
available to be used. Theoretical yield, the maximum yield of pure crystals that could be obtained by cooling or evaporating a given solution, will be the mass of the limiting reagent. This refers to the quantity of pure crystals deposited from the solution.
CONCLUSION As a conclusion, the process of recrystallization is a quick, simple, and easy way to purify a compound whether it is known or unknown. Recrystallization is a good
% =
ℎ
% =
2.70 2.970
purification method compatible with the
× 100
nature of acetanilide. It is efficient for dissolving the solid to be crystallized in a hot
100
solvent or solvent mixture and by then, cooling the solution slowly. It is also
% = 90.9%
somehow useful for identifying an unknown compound because impurities affect boiling and melting points which can cause or may
The calculated percentage yield was 90.9 %. Percentage recovery is the percent of the pure form of a compound formed during the reaction. This refers to the percent of
pure
acetanilide
formed
after
recrystallization. % Recovery =
mass of pure acetanalide mass of crude acetanalide × 100
lead to the identification of another different compound. Charcoal, or for a concrete example that was used in the experiment is the activated charcoal, is useful in removing some of the impurities of the mixture.
REFERENCES Gocke, M., Pustovoytov, K., & Kuzyakov, Y. (2011). Carbonate recrystallization in rootfree soil and rhizosphere of Triticum aestivum and Lolium perenne estimated by ¹⁴C labeling. Biogeochemistry, 103(1/3), 209-222. Retrieved from http://www.jstor.org/stable/41490444 Sakai, T., Belyakov, A., Kaibyshev, R., Miura, H., & Jonas, J. (2014). Dynamic and Postdynamic Recrystallization under Hot, Cold and Severe Plastic Deformation Conditions,
DOCUMENTATION
130-207. (2014). http://dx.doi.org/10.1016/j.pmatsci.2013.09. 002
Dillamore, I. L. & Katoh, H. (2013). The Mechanisms of Recrystallization in Cubic Metals with Particular Reference to Their Orientation-Dependence, 8(1), 73rd ser., 7383. (2013, July 18). http://dx.doi.org/10.1179/msc.1974.8.1.73
