Experiment #1: Purification of Benzoic Acid by Recrystallization
John Edward Edward O.Tanchuco & Carolyn Marie D. Legaspi Section AB2 April 29, 2009
Experiment Objectives To purify and separate benzoic acid crystals from an impure sample using crystallization. To perform the proper laboratory techniques in recrystallization. To understand the concepts and computations involving purification and recrystallization.
Experiment Objectives To purify and separate benzoic acid crystals from an impure sample using crystallization. To perform the proper laboratory techniques in recrystallization. To understand the concepts and computations involving purification and recrystallization.
Introduction
Purification involves
the physical separation of contam contamina inant ntss from from a sample sample to produce the desired pure compound.
aims
to remove by-products and impurities impurities from an impure impure sample. sample. for liquids, we usually employ distillatio distillationn but for for solids, solids, we use recrystallization.
Introduction
Recrystallization a
common purification method for organic compounds esp. solids
the
disso dissolut lution ion of the solid solid with an an appropriate solvent at a high temperature temperature & its its recryst recrystalliz allization ation at a low temperature. two types of impurities: (1) more soluble than main component (2) less soluble than main component
Recrystallizat ecrystallization ion
Process
An impure solid compound is dissolved in a solvent and crystallizes as the solution cools. Compounds, which are less soluble, will crystallize first. As crystals form, it selects the correct molecules, that fit in the crystal lattice & ignore the wrong molecules, resulting in a pure solid.
Recrystallization
Process
The crystallization process heavily depends on the differences in the solubilities of the desired solutes and the impurities dissolved in the solvent.
Recrystallization
Process
Limitations the
process cannot separate pure substances and impurities with the same solubilities in a given solvent. the process is not perfect ; it cannot assure a perfectly pure solute, but it does increase its purity. the process is limited to solid solutes.
Recrystallization
Solvent
In the ideal setting« the
solvent would completely dissolve the compound to be purified at high temperature, usually the boiling point of the solvent, and the compound would be completely insoluble in the solvent at room temperature or at 0oC.
and
vice versa for the impurity
Recrystallization
Solvent
In the real world« In
the real world, this will never happen and recrystallization is a technique that has to be practiced and perfected.
Regardless
of crystallization method, the purity of the solid can be verified by taking the melting point.
Recrystallization
Solvent
Common Recrystallization Solvents solvent
formula
polarity
boiling point ( 0C)
water ethanol
H2O CH3CH2OH
very polar polar
100 78
methanol CH3OH polar dichlorometh CH2Cl 2 slightly polar ane diethyl ether (CH3CH2 )2O slightly polar
65 40 35
* Note: Properties of a good recrystallization solvent will be elaborated later.
Recrystallization vs.
Rate
Precipitation
Recrystallization
Precipitation
slow
fast
Formation of selective crystals
random
Shape of crystals
pure regular crystals
amorphous solid
Amount of impurities
negligible
significant
Experimental Step 1: Mix boiling chip, 100 mg impure benzoic acid, & 2 ml distilled water. Dissolve and heat while constantly swirling. Erlenmeyer flask
benzoic acid solution
hot plate
Discussion
W ater is an ideal solvent for benzoic acid. 10C, 2.1 g of benzoic acid dissolves in 1000 ml of water. but at 95C, 68g benzoic acid is soluble per 1000 ml of water. at
this
implies that at different temperatures, benzoic acid has an huge solubility difference in water.
Discussion
Constant swirling at a high temperature. swirling
speeds up the dissolution of benzoic acid in water
agitation
increases the entropy of the system , thus increasing the interaction between benzoic acid and water molecules. the complete dissolution of benzoic acid results to a clear solution.
Discussion
Adding the boiling chip while at room temperature. adding
the boiling chip at room temperature prevents boiling over.
this
means that the solution will not spill out, since the boiling chip induces boiling of the mixture.
Experimental Step 2: Cool the solution. Add activated charcoal. Add a few drops of water. Heat again until observable change is seen.
Discussion
Decolorizing the solution with activated charcoal. activated
charcoal are carbon atoms that are finely separated. these can adsorb impurities (stick to the surface of the substance) from the solution but are quite large to pass through the filter paper. this results to minimization of impurities , and increased purity. ARNING: too much activated carbon W could cause the loss of the pure substance.
Experimental Step 3: Pour the hot solution in the filter syringe and force the liquid through the syringe. benzoic acid Buchner funnel
v
acuum(suction)
filtrate
Discussion
F irst filtration of the solution activated
charcoal used, as well as other impurities, would be separated from the solution and left in the cotton plug.
this
lessens the impurities in the crystallization process, and increases the purity of the yielded substance.
Discussion
F iltering the solution rapidly. as
filtration is taking place so is the crystallization process. the decrease in temperature causes a decrease in the solubility of the benzoic acid crystals. some of the pure crystals would be separated from the filtrate and would be left as residue. a lesser yield would result if the solution was not poured rapidly.
Experimental Step 4: Let the mixture cool in the ice bath.
Discussion
Slow cooling in ice bath. slow
cooling makes the crystals arrange finely, thus ensuring correct molecular arrangements/geometry.
this
helps the crystals form in an undistorted manner and exclude the impurities in crystal formation.
Experimental Step 5: Collect the crystals on a filter paper. Rinse vial with ice ²cold water to collect the remaining crystals in it. Use a seed crystal if necessary.
Discussion
Using a seed crystal. in
cases, that crystallization while cooling does not take place, a seed crystal is employed. the seed crystal has the same structure as the pure crystal to be recovered. the seed crystal serves as a ´source codeµ where the desired solid in the solution begins crystallization.
Discussion
Using a seed crystal. since
the lattice is a perfect fit, the other dissolved crystals would crystallize out as well. impurities would remain dissolved in solution since its structure differs from the seed crystal and cannot fit in the lattice.
Experimental Step 6: Squeeze excess water from the filter paper. Dry it completely & weigh the filter paper.
Discussion
The filter paper and crystals must be completely dried. the
added mass of water while weighing produces an inaccuracy in the desired data (% recovery) due to the solvent molecules.
Results
% recovery of benzoic acid crystals
W eight of impure sample = 100 mg W eight of filter paper = 400 mg W eight of filter paper and benzoic acid = 450 mg W eight of pure crystals = 50mg
% recovery: 50mg/100mg x 100% = 50%
Discussion
Not all groups had the same yield even if everyone used 100 mg of the same impure benzoic acid sample. Even if the same bottle was the source of the benzoic acid, the amount of impurities from each group may have varied. In conclusion, the quantity of impurities and crystals recovered in each group varied.
Discussion Possible error source
Resulting yield
Incomplete dissolution of impure sample
Less yield
Excessive activated chracoal
Less yield
Slow filtration of first filtrate
Less yield
Reduced forcing of liquid from the syringe
Less yield
Rapid cooling
Greater yield
Spilling of filtrate
Less yield
Incompletely dried filter paper
Greater yield
W eighing filter paper while hot
Less yield
Comparative Group Results Group Percent Recovery # 1
Error Source
50%
Spilling
38%
Spilling
80%
Minimal spilling
2 3 4 5 6 7 8 9
Conclusion
Recrystallization is a laboratory technique used in purification of solids. It is not a perfect process and does not produce an accurate yield. A large amount of impurities with the same solubility as the pure substance is also present, making effective separation difficult.
Guide Questions Question#1:
List the properties that an ideal solvent should have to perform the purification of organic compound by recrystallization technique.
Answer
In the ideal setting« the
solvent would completely dissolve the compound to be purified at high temperature, usually the boiling point of the solvent, and the compound would be completely insoluble in the solvent at room temperature or at 0oC.
and
vice versa for the impurity
Answer
A suitable solvent should be:
1. Soluble at the boiling point of the solvent and slightly soluble at room temperature. (if soluble, hard to separate pure crystals because the molecules interact) 2. The impurities should either dissolve at room temperature or insoluble in the hot solvent so that impurities can be removed by filtration.
Answer
A suitable solvent should be:
3. The solvent should not react since their interaction would result to a lesser yield of pure crystals. 4. The solvent should be volatile enough to be easily removed from the solute/s. This allows rapid drying of the solid compound after it has isolated from the solution.
Guide Questions Question#2:
W hat advantages does water have as a crystallization solvent?
Answer
exists
as a liquid and can be used for a wide temperature range (1ÜC -100ÜC) non-flammable and non-volatile polar
which can easily dissolve molecules esp. polar substances high heat capacity and can dissolve different substances safe and non-toxic compared to other organic solvents
Guide Questions Question#3:
Two students crystallized 10g samples of benzoic acid from water, the first dissolving benzoic acid at 80ÜC and filtering at 10ÜC , the second dissolving at 95ÜC and filtering at 18ÜC . Calculate the quantity of water each student was required to use and the maximum recovery of benzoic acid possible in each case.
Answer 1st Student:
@ 80ÜC 10 g = 2.75 g X 1 ml 100 ml X 1 = 363.6364 ml @ 10ÜC
Y 1 g
= 0.21g
2nd Student:
@ 95ÜC 10 g = 6.80 g X 2 ml 100 ml X 2 = 147.0588 ml @ 18ÜC
Y 2 g
= 0.27g
363.63 ml 100ml Y 1 = 0.7636 g
147.06 ml 100ml Y 2 = 0.3971 g
10 g ² 0.7636 g = 9.24 g
10 g ² 0.3971 g = 9.60 g
Guide Questions Question # 4:
A Solid (X) is soluble in water to the extent of 1 g per 100 g of water at room temperature and 10 g per 100 g of water at the boiling point.
Guide Questions a) How would you purify X from a mixture of 10 g of X with 0.1 g impurity Y, which is completely insoluble in water and 1 g impurity Z having the same solubility characteristics in water as X?
Answer
Dissolve the mixture in 100 g water and heat until boiling (100C ). Cool the mixture to room temperature. Then filter to separate impurity Y from the mixture. Add water to the filtrate up to 100ml. Heat the mixture again until boiling until all crystals dissolve. Slowly cool the mixture to room temperature. Filter the mixture.
Answer
The residue left on the filter paper is the purified X crystals. The formation of impurity Z is insignificant since it is minimal in quantity (1g) and has a different lattice structure from the solid X structure thus excluding Z in crystal formation.
Guide Question b) How much pure X could be obtained after one recrystallization from water? ANSW ER: 10g ² 1g x 100% = 90% 10 g Since the solubilities of different solutes are independent of each other, at room temperature, it would be assumed that 1 g of solutes X and Z would dissolve. So, 9 g of pure X solute will crystallize.
Guide Question c) How much pure X could be obtained after one recrystallization from a mixture of 10g of X with 9 g of Z?
Answer
To retrieve pure solid X crystals from the mixture, all X and Z must be dissolved. The solubilities of X and Z are independent of each other and to dissolve all crystals, solid X is used as basis for it is present in a larger amount. 10g = 1g Xg
100g
X = 1000 g water
Answer
To ensure that Z doesn·t crystallize back, the final amount of water must ensure that solute Z crystals remain dissolved in water under room temperature. This can be done by heating the solution until 900 g water is left. 9g = 1g Yg
100g
Y= 900 g water
Answer
The solution is slowly cooled until crystals are formed. Z g = 1g
Z = 9 g solid X
900g 100g In 900 g water, 9 g of solutes X and Z are dissolved which means only 1 g of pure solid X can crystallize from the solution.
d) Based on the results obtained, what is suggested about the use of crystallization as a purification technique? ANSW ER: The crystallization technique is very inaccurate because it heavily relies on the differences in solubility. Therefore, it is difficult to separate substances with the same solubilities especially when present in the same mixture with significant amounts.
