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Recrystallization of
Salicylic acid
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Recrystallization
Solid organic cpd.s when isolated from organic reaction are
impure; they are contaminated with small amounts of other
cpd.s produced along with the desired product.
The purification of impure crystalline compound is usually done
by Recrystallization from a suitable solvent or a mixture of
solvents.
Purification of solids by recrystallization is based upon
differences in their solubility in a given solvent or a mix. of
solvents.
Solid organic cpd.s when isolated from organic reaction are
impure; they are contaminated with small amounts of other
cpd.s produced along with the desired product.
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Choosing a solvent for Recrystallization:
The ideal solvent should :
1- Chemically inert toward the solute.
2- It should dissolve the solute to be purified readily at or near it's
boiling point, but sparingly at the lab. temp. or below ( 0 – 25
o
C ).
3- It should dissolve the impurities readily or not at all.
4- not be flammable , low cost , and of low toxicity .
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Simple Recrystallization process consist of:
1- Dissolving the impure substance in some suitable solvent at or near
the boiling point.
2- Filtering the hot solution from the particles of insoluble material &
dust, ( Hot filtration ).
3- Allowing the hot solution to cool thus causing the dissolved
substance to crystallize out.
4- Separating the crystals from the supernatant soln.
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How could we choose a good solvent :
Practically,
Take 0.1g of a pure sample of cpd. to be purified & try to
dissolve it in 1ml of solvent,
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Using Charcoal:
Samples to be purified may contain soluble colored impurities
that may cause the soln. & the crystals to be colored.
Up on Recrystallization these impurities dissolves in the boiling
solvent & adsorbs on the crystals produced up on cooling
yielding a colored product.
Activated charcoal composed of fine carbon particles with a
large active surface area on which the colored impurities will
be adsorbed.
Charcoal is added to the hot soln. before boiling & the soln. is
kept hot at or near the b.p. for about 3 – 5 min. with shaking to
wet the charcoal, the solution is then filtered through a fluted
filter paper.
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Q / Charcoal is Not used for recrystallization of phenolic cpd.s
Ans :Because, They contain ferric ions ( Fe ) that upon heating
the solution for some times it can react with the phenolic -OH
group forming red – violet colored complexes thus impairing the
purification process.
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Recrystallization using mixed solvents:
It is applied when our cpd. is readily soluble in a solvent at room
temp. & insoluble in other solvent, The 2 solvents must be miscible
with each other as Alcohol & water, ether & pentane glacial acetic
acid & water .
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Continuously
Procedure :
1- Put 1 g of impure Salicylic acid sample in a beaker.
2- Try to dissolve it in a minimum amount of hot
water with heating.
3- Filter the solution while it is hot .
4- Cool the filtrate, then Salicylic acid will crystallize.
5- Filter again, ( Cold filtration).
6- Collect the crystals of S.A. on the filter paper and dry
them on oven.
Name of Experiment: Purification of the synthesized
Aim of experiment: Purification of the synthesized Salicylic Acid.
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Percent pure substance % =
pure substance wt.
Sample wt .
x 100
Percent impure substance % =
100
-
Percent pure substance
Calculations :