Experiment 2

Experiment 2
Separation, Purification and Identification of
the Components of a Mixture
General Principles
Most materials found in nature or produced by manufacturing concerns are mixtures. Even the
purest chemicals available from chemical supply houses contain tiny traces of impurities. The
separation, purification and identification of the components of a mixture are among the
everyday problems faced by both industrial and research chemists. One of the most frequently
used methods for purification is crystallisation.
If recrystallisation is to be effective, the solvent must be properly selected. A good
recrystallisation solvent should:
1. Dissolve a moderate quantity of the substance being purified at an elevated temperature, but
only a small quantity at low temperatures.
2. Not react with the substance, being purified.
3. Dissolve impurities readily at a low temperature or not dissolve them at all.
4. Be readily removable from the purified product. This last requirement usually means that the
solvent should have a fairly low boiling point and should evaporate readily.
If a single solvent cannot be found that meets all these requirements, it is possible to use a
mixture of two solvents.
To obtain a good recovery of purified material, it is best to avoid using unnecessarily large
volumes of solvent. Dissolving the substance in the smallest possible amount of hot solvent
minimises the amount of pure material lost by retention in the mother liquors.
Traces of colouring matter or resinous impurities can sometimes be removed with selective
adsorbent, such as finely divided charcoal.
To do this, add a small amount of decolourising charcoal to the warm solution, before filtering it.
Avoid using excess decolourising agent, however, because, it may also adsorb appreciable
amounts of the substance being purified. To remove insoluble impurities and decolourising
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charcoal, it is necessary to filter the solution while it is hot. Otherwise, when the solution cools,
crystals may deposit prematurely. Rapid filtration can be accomplished by using fluted filter
paper (Figure 2.1) or by using a vacuum filtration (Figure 2.2).
Some substances readily form supersaturated solutions, and crystallisation may not occur
spontaneously when the hot solution is cooled" In such situations, it is sometimes possible to
initiate crystallisation by scratching the walls of the vessel beneath the surface of the solution
with a stirring rod. The best way to induce crystallisation is to ''seed'' the cold solution with one
or two crystals of the substance being purified. Although some compounds readily crystallise,
others may separate from solution as oils, and it may take some time before they crystallise.
This experiment illustrates recrystallisation (I) from a single solvent (water) and (II) from a
mixed solvent (ethanol-water).
Separation of a Solid Material from Liquid Phase
Filtration is commonly used to separate a material in the solid phase from one in the liquid phase
(see Figures 2.1 and 2.2). If your filter paper is properly folded and placed into the funnel, a
liquid passes through it without forming bubbles in the stem of the funnel. Pour the mixture to be
filtered into the funnel carefully to prevent plashing.
You should also wash the precipitate down into the apex of the filter paper. Most precipitates
should be rinsed with distilled water. The rinsing and washing down can be combined into a
single operation. Residues from filtration almost always need to be dried. To expedite drying, the
filter paper should be removed from the funnel and unfolded. The time required for drying
depends upon the amount and nature of residue. In most cases, 24 hours of air-drying in your
locker is sufficient. If an experiment calls for oven drying lay the open filter paper on a watch
glass and place the glass in an oven at 100 to 110 0C. The residue should dry completely in 15 to
20 minutes.
If a centrifuge is needed, your instructor will demonstrate its use to separate a precipitate from a
solution. A centrifuge works much faster than a conventional filter. The solution in the tube is
subjected to a force of 500 to 1000 times that of gravity while it is being centrifuged. Not more
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than a minute of centrifuging are required to cause a precipitate to settle out. You should always
counter balance the tube of solution being centrifuged by placing an equal amount of liquid in
the opposite tube. This prevents the centrifuge from vibrating while in operation. If a precipitate
is well compacted, the supernatant liquid (liquid above the precipitate) may be decanted. If the
solid is not well compacted, you may find it necessary to remove the liquid above the residue
carefully with a pipette or medicine dropper.
Figure 2.1. Filtration of a solution using fluted filter paper.
Figure 2.2. Apparatus for vacuum filtration.
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Chemicals and Equipment
Impure mixture of benzoic acid, lead nitrate, potassium iodide, 100 mL beaker, filter paper,
funnel, Bunsen burner, melting point apparatus, melting point tubes, thermometer, rubber bands,
filter paper, scissors (if strips are not provided), aluminium foil, solvents, hot plate, and assorted
mixtures of food dyes.
Separation and Purification
In this experiment you will be given a murky mixture composed of lead nitrate (solid), benzoic
acid (solid), and some other impurities. Your objective will be to separate colourless, needle-like
crystals of benzoic acid and lead nitrate (in the form of lead halide precipitate) from the other
components. The separation is based on the difference between (the solubility of benzoic acid
and that of the lead nitrate and the impurities, in hot and cold water. Benzoic acid is quite soluble
in hot water but less soluble in cold water. But lead nitrate, a solid, is relatively soluble in both
hot and cold water. You can verify that the products, which you isolate, are the right materials by
comparing their melting points with that of the pure compounds.
1. Place about a 1 g sample of the mixture (provided by your instructor) in a small beaker and
add approximately 25 mL of water and small quantity of active carbon. Heat the mixture
gently while stirring to near its boiling point.
2. Filter the hot solution quickly by using fluted filter paper into another beaker (Figure 2.1). If
traces of colouring matter present, add a small amount of decolourising charcoal to the warm
solution, before filtering it.
3. Heat the filtrate to boiling, remove from the heat, and allow to be cooled slowly without
disturbance. You may have to use ice bath to complete the crystallisation.
4. After crystals form, filter the solution and wash the residue with cold water several times and
show them to your instructor (you may need to recrystallise the solid). Dry the crystals in
the oven at 100 0C. Allow the sample to dry thoroughly before melting point is taken.
Add carefully and slowly enough potassium iodide solution to the filtrate (the mother
liquors). Cool the mixture in an ice water for several minutes. Filter the mixture and wash
the residue (lead iodide) few times with cold water. Dry as above.
6. Determine the percentage and melting point of pure benzoic acid.
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Recrystallisation of p-Dibromobenzene
This part of the experiment illustrates the technique of using a mixed solvent for
recrystallisation. The compound X is soluble in cold as well as in hot ethanol. Therefore, ethanol
alone is not a good recrystallisation solvent for this substance. On the other hand, the compound
has a very low solubility in hot or cold water. Therefore, water alone is also unsatisfactory as a
recrystallisation solvent for this substance. But because ethanol and water are miscible, it is
possible to find a mixture of the two solvents in which compound X is soluble when hot but
relatively insoluble when cold. This experiment is also designed to acquaint you with the type of
small-scale recrystallisation that is commonly used to purify products you will prepare in later
experiments in this manual.
Weigh out a 0.2g sample of impure p-dibromobenzene or m-dinitrobenzene and use a few
milligrams to determine the melting-point range. Transfer the solid into a small Erlenmeyer flask
or small test tube, add 5 mL of ethanol, and heat the mixture gently on a steam bath (hot water
bath). (Caution: Ethanol is flammable; do not use an open flame as a heat source in this
Swirl or stir the solution until the solid dissolves. If any undissolved solid remains after several
minutes of heating, filter the hot solution using fluted filter paper. If, however, the solid dissolves
completely, add water in small portions to the hot solution until it becomes cloudy or just fails to
clear when stirred. Then add a little ethanol (few drops), until the turbidity just disappears.
Allow the flask to Cool to room temperature you may need to cool the flask in a pan of ice water
for several minute, and then collect the crystals by vacuum filtration (Figure 2.2). Rinse the
crystals on the filter (with the vacuum off) with a few millilitres of an ice-cold 50-50 ethanolwater mixture. Stir the contents on the funnel (carefully) for just a few seconds then turn the
vacuum on. Repeat this operation until a white product is obtained or until most of the orange
impurity is removed. Allow the pure crystals to air dry on the filter (with the vacuum on) for a
few minutes, and then, transfer them to a clean sheet of filter paper for final drying. After the
crystals are dry, weigh the product, determine its melting point and the percentage recovery, and
turn in the labelled sample to your instructor.
All the solid products should be properly labelled and handed out to your instructor.
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Waste Disposal
Discard your used melting point tubes in the waste glass container and pour the aqueous mother
liquor solutions into an aqueous waste bottle, both provided by your instructor.
1. What are the requirements, to be a good recrystallisation solvent?
2. Explain how can we initiate crystallisation process?
3. (a) What are the major differences between a homogeneous mixture and a compound?
(b) Identify one heterogeneous, one homogeneous mixture, one element, and five compounds
that you have encountered in this experiment.
4. Describe physical methods that could be used to separate each of these mixtures into their
component compounds and/or elements.
(a) A solution of sodium chloride and water.
(b) A solid mixture of sodium chloride and iodine. Iodine is relatively soluble in carbon
tetrachloride or chloroform while sodium chloride is not. Also, iodine sublimes (passes directly
from the solid phase to the gaseous phase before it has reached its melting point).
(c) A mixture of sodium nitrate (very soluble in both hot and cold water), lead chloride (much
more soluble in hot than in cold water), iodine (relatively) insoluble in water but soluble in
chloroform or carbon tetrachloride), and sand.
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