Chemistry lab report

Experiment #2: RecrystallizationIntroduction
Recrystallization is the most common laboratory method for purifying solid substances.
This technique is based on the fact that solubilities of most solids increase with increasing
solvent temperature. In this process, the impure solid is dissolved in a minimum amount
of hot (boiling) solvent. Impurities that remain undissolved can be removed by hot gravity
filtration. Subsequently, the hot saturated (filtered) solution is cooled down slowly,
allowing for crystals growth at a moderate pace. The recovered material can be
calculated using the following equation:
The process of recrystallization can be broken up into several steps:
Step 1: Selecting a solvent. Selecting the proper solvent is a crucial step in the
recrystallization process. The ideal solvent should (1) be nontoxic and inexpensive; (2) be
nonreactive with the compound; (3) dissolve the solute completely and not dissolve any
impurities at its boiling point or at room temperature, and (4) have a boiling point below
the melting point of the compound.
Simplified it can be stated that the best solvent will dissolve all the solute when the
solution is hot, and none of the solute when the solution is “cold” (at ambient
temperature). The differences in solubilities in hot vs. cold are critical for the
recrystallization process. Table 1 compiles solubilities of benzoic acid in water, ethanol,
and diethyl ether in the “heat” and in the “cold”.
Table 1. Solubility of organic acids in water at varying temperatures
Organic Acid
Benzoic Acid
trans-Cinnamic Acid
Salicylic Acid
Solubility (g/100 mL)
6.8 g/100 mL at 95oC
0.05 g/100 mL at 25oC
0.20 g/100 mL at 25oC
Solubility (g/100 mL)
0.29 g/100 mL at 10oC
The general rule of thumb “like dissolves like” means that polar compounds will
dissolve well in polar solvents, and vice versa, the non-polar compounds will dissolve well
in non-polar solvents. Common recrystallization solvents and their boiling points listed in
Table 2 are arranged in order of increasing solvent polarities.
Occasionally, if a proper single solvent cannot be established, a solvent mixture must
sometimes be employed, whose exact composition is typically found in a monotonous
“trial-and-error” practice (e.g. 50% water + 50% hexane).
1
Table 2. Boiling points and conventional recrystallization suitability of common solvents
Solvent
Boiling Point [oC]
Ligroin (sold as “Hexanes”)
60-90
Hexane
69
Petroleum Ether
60-90
Cyclohexane
81
Carbon Tetrachloride
77
Toluene
111
Benzene
80
Diethyl Ether
35
Chloroform
61
Methylene Chloride
40
Acetone
56
Ethyl Acetate
77
Ethanol
79
Methanol
65
Water
100
a. Hydrocarbons (nonpolar substance)
Suitable for
a
a
a
Similar to hexane
Acid chlorides, anhydrides
Aromatic compounds
Aromatic compounds
Low-melting compounds
Polar compounds
Low-melting compounds
Nitro compounds
Esters, carbonyl compounds
Polar compounds
Polar compounds
Polar compounds
Step 2: Dissolving the solute. Erlenmeyer flasks (not beakers!) should be used for this
procedure because the conical shape of Erlenmeyer flasks reduces the amount of solvent
that evaporates during the heating process and prevents the formation of crystals along
the walls of the flask. The hot solution can also be more easily swirled while dissolving the
solid and without splashing.
The entire impure solid is placed in an Erlenmeyer flask on a hot plate or in the water
bath, and small portions of volumes of the hot solvent are added (the solvent is heated
separately; for the identity of the solvent see step 1). The flask is covered with a watch
glass.
It is absolutely imperative that the solid is dissolved in a minimum amount of hot
solvent. Sometimes a wooden applicator stick is placed in the solution to break up
occasional lumps, prevent superheating and by the same time to provide an excellent
nucleation surface during cooling.
The continuing process of solvent addition and swirling is maintained until the entire
solid just dissolves. Insoluble impurities do not dissolve on heating and should be
removed by hot gravity filtration. Soluble impurities remain in the hot solution. If there is
still undissolved solid, remove the flask from the heat and carry out the following steps.
Step 3: Decolorizing the solution (if necessary). If a crude solid contains colored
impurities, they can be removed by adding a small amount of activated carbon (charcoal)
to the hot solution from step 2. The colored impurities are absorbed in the surface of the
charcoal. However, if too much of the activated carbon is used, it can absorb some of the
compound from the solution along with the impurities. The solution is stirred and heated
for a few minutes and then filtered while hot to remove the decolorizing carbon along with
the impurities.
2
Step 4: Hot gravity filtration. Any solid particles (charcoal) or undissolved impurities
must be removed by hot gravity filtration (a stemless funnel should be used and your
instructor will demonstrate how to fold the filter paper to maximize the surface area), or by
decantation. It is very important that this process is done swiftly to prevent solid
precipitation in the funnel. Often a warm (or even hot) glass funnel is used for in this step.
Step 5: Crystallizing the solute. The Erlenmeyer flask containing the hot (warm) clear
solution (if it is not perfectly clear this step cannot be performed!!!) is covered with a
watch glass and slowly cool the solution to room temperature undisturbed. Since the
solubility of the vast majority of solids decreases with decreasing temperature, slow
cooling the solution will bring gradual growth of solute crystals. If the hot solution is cooled
too quickly, impurities can be trapped in the crystals. When it has reached room
temperature (some crystals will precipitate) place it in an ice bath to complete
crystallization.
If the dissolved compound fails to crystallize from the solution upon cooling,
crystallization can be induced by a variety of methods. Keeping a wooden applicator stick
in the solution has already been discussed. Alternatively, crystallization can be achieved
by vigorous scratching (it has to be vigorous enough to release minute glass particles into
the solution, which, as it is believed, can initiate and promote crystal growth) the inner
glass wall of the flask with a glass rod or spatula. Crystallization can also be brought by
introducing a small crystal of the pure compound (the so-called seed crystal) into the
solution. The seed crystal functions (just like the glass particles) as a template, to which
other molecules become attached. If neither of these techniques proved successful, it is
highly probable that the compound was dissolved in too much of hot solvent. In that case,
some of the solvent must be boiled off or removed by distillation, before the crystallization
process can again be initiated. Another common problem is “oiling out.” This problem
happens when the solute becomes “contaminated” with solvent. Low-melting point
compounds are more likely to oil out than are high-melting point compounds. This
problem may be solved by redissolving the oil in hot solvent and cool it more slowly.
Step 6: Collecting and washing the crystals. Once the crystallization process is
completed, the obtained crystals have to be separated from the solution (mother liquor)
containing dissolved impurities. Typically this is accomplished by vacuum or suction
filtration using a Büchner funnel. The Erlenmeyer flask is rinsed with a small volume of
cold (warm solvent would re-dissolve the crystals!) recrystallization solvent to transfer all
remaining crystals quantitatively into the funnel. The crystals collected in the funnel are
washed with several portions (few milliliters each) of cold recrystallization solvent to wash
out any remaining mother liquor and dissolved impurities.
Step 7: Drying the product. Before the melting point can be measured or the yield
determined the recrystallized product must be thoroughly dried to remove all traces of
solvent. The suction during vacuum filtration is maintained for a few minutes to allow for
the isolated crystals in the Büchner funnel to be dried by the passing air. The crystals are
then carefully removed from the filter and placed on the watch glass to complete the
drying process in the air, and if necessary, in the oven (The oven temperature must be
considerably lower than the melting point of the product!).
3
Safety Alert and Waste Disposal
All organic acid waste must be disposed into solid organic waste located in the fume
hoods. All water that came in contact with organic acid must not be discarded into the
sink, but must be collected separately in an appropriate container.
Assignment
A. Decolorizing Brown Sugar
1. In a 50-mL Erlenmeyer flask place 2.5 g of brown sugar, 50 mg of charcoal and 5 mL
of water. On a hot plate, bring the solution to a boil.
2. After the sugar has dissolved, remove the flask from the heat and filter the hot solution
by using gravity filtration.
3. In a 50-mL Erlenmeyer flask place 2.5 g of brown sugar and 5 mL of water. On a hot
plate, bring the solution to a boil.
4. After the sugar has dissolved, remove the flask from the heat and set aside.
5. Compare the colors of the filtered (charcoal) and the unfiltered solution (no charcoal).
B. Recrystallizing Organic Acids
In this experiment, approximately 0.1 g of varios organic acids will be purified by
recrystallization from water following the prepared flow chart.
1) Weigh out approximately 0.1 g of benzoic acid, 0.1 g of trans-cinnamic acid, and 0.1 g
of salicylic acid
2) Place benzoic acid and salicylic acid in two separate 25 mL test tubes
3) Place trans-cinnamic acid in a 50 mL test tube
4) Add 1 mL of hot water to each of the test tubes
5) Place all three test tubes in a 250 mL beaker filled to approximately 200 mL with tap
water
6) Place the beaker on the hot plate, bring the water to a boil and then let it simmer
7) Observe if any of the solids in the test tube dissolved
8) If not, add hot water in 0.5 mL increments until each of them does (remember for best
results, dissolve a solid in the least possible amount of hot solvent)
9) Once all the solids completely (!!!!!!!!!!) dissolve (the solution is supposed to be totally
transparent and clear) take the test tubes out of the hot water bath and let them slowly
cool down to ambient
10) At this point crystallization should set in
11) To complete crystallization, place the test tubes in an ice bath for 15 minutes
12) Separate the crystallized material using vacuum filtration
13) Dry the crystals thoroughly (the crystals should be “bone-dry” otherwise they still
contain water). This could give you over 100% recovery!!!
14) Weigh out the crystals and determine the % recovery for each organic acid
4