Recrystallization is a technique used to purify solids based on differences in their solubility, involving dissolving the impure solid in a hot solvent, filtering to remove insoluble materials, and obtaining pure crystals during controlled cooling as the solvent crystallizes out of solution. The process works best when an appropriate solvent is selected that dissolves the compound at high temperatures but causes it to crystallize upon cooling, allowing purification through multiple recrystallization attempts if needed.

1. SARASWATHI VIDYA BHAVANS
COLLEGE OF PHARMACY, DOMBIVLI -E
RECRYSTALLISATION
By - Mrs. Priyanka Kalamkar
Asst Professor

2. Solid organic compounds when isolated from organic
reactions are seldom pure; they are usually contaminated
with small amounts of other compounds (impurities) which
are produced along with desired product.
RECRYSTALLISATION
The purification of impure crystalline compounds is
usually effected by crystallization from a suitable
solvent or mixture of solvents.
The purification of solids by crystallization is based upon
differences in their solubility in a given solvent or mixture
of solvents.

3. Crystallization is a laboratory technique used
to purify solids based on their different
solubilities.
It works only when the proper solvent is
used.
RECRYSTALLISATION
Crystallisation

4. Process
RECRYSTALLISATION
Step 1: Dissolving the impure substance in some suitable
solvent at or near the boiling point
Step 2: Filtering the hot solution from particles of
insoluble material and dust
Step 3: allowing the hot solution to cool thus causing
the dissolved substance to crystallize out
Step 4: separating the crystals from the supernatant
solution (mother liquor)
The resulting solid after drying , is tested for purity (MP /TLC)
and if found impure is again recrystallized from fresh solvent.
The process is repeated until the pure compound is obtained.

6. Step-1
Step-2
Step-3
Step-4
Step-5Dissolve the
impure solid
in a minimum
volume of hot
solvent
Hot filtration
Cooling &
crystallization
Filtration of
crystals

7. Desirable characteristics of a solvent for
recrystallisation are as follows:
1. A high solvent power for the substance to be purified at elevated
temperatures and a comparatively low solvent power at the laboratory
temperature or below
2. It should dissolve the impurities readily or to only a very
small extent
3. It should yield well formed crystals of purified
compound
4. It must be capable of easy removal from the
crystal of the purified compound ie. Possess a
relatively low boiling point

8. Commonsolventsforcrystallisation

9. Precautions:
Use of ETHER as a solvent for
recrystallisation should be avoided
----its great flammability
----tendency to creep up walls of the
containing vessels, thus depositing solid
matter by complete evaporation instead
of crystallisation
Use of CS2 (BP 460C )
-dangerously low flash
point
forms very explosive
mixtures with Air.

10. Solvent selection

11. Choice of solvent for crystallisation
0.1 g of sub in Test tube + 1 ml of solvent = heated
(take precautions if solvent is flammable)
If sample
dissolves easily in
cold sample or
when gently
warmed
Solvent is
unsuitable
If all the solid does not dissolve on
heating , more solvent is added in 0.5
ml portions & again heated to boiling
after each addition. If 3 ml of solvent
is added & substance does not
dissolve on heating. The substance is
called as sparingly soluble
Solvent is unsuitable
If sample is
insoluble at room
temp and upon
heating dissolves
Go to next
step

12. If the substance dissolves in the hot solvent then Tube is cooled to determine whether
crystallisation occurs
Crystals
separates on
cooling
Good
solvent
If crystallization does not take place rapidly , this may be due to absence of
suitable nuclei for crystal growth
Tube should be scratched below the surface of the solution with a glass
rod----the fine scratches on the wall may serve as excellent nuclei for
crystal growth
If the substance is found to be sparingly soluble in one solvent and too
soluble in another solvent -----to allow recrystallisation mixed solvents
can be used.

13. Recrystallisation using mixed solvent
 Requirement : two solvents must be completely miscible
 Recrystallisation from the mixed solvents is carried out near the boiling point of
mixture.
 The compound is dissolved in the solvent in which it is soluble---and the hot solvent
in which the substance is sparingly soluble is added cautiously until a slight
turbidity is produced.
 The turbidity is then cleared by addition of a small quantity of a first solvent and
the mixture is allowed to cool at room temperature ; crystals will separate
 Ex of mixed solvents used commonly used
1. Alcohol + water
2. Alcohols + toluene
3. Toluene + light petroleum
4. Galcial acetic acid + water

14. Filtration of Hot solution
 The hot solution must be rapidly filtered before undue
cooling has occurred.
 This is usually done with a fluted filter paper supported in
a relatively large funnel with short wide stem, separation
of crystals in and clogging of the stem is avoided.
 The funnel should be warmed in an electric oven before
the filtration is started

15. FILTRATIONASSEMBLY

16. The main advantage of this
method is that
 It increases the speed of
filtration by letting air
enter the flask along its
sides to bring about
pressure equalization.
 The fluted filter paper
has a larger surface area
through which the
solvent can seep
Advantage of fluted filter
paper

17. ? Questions
1] What happens if you add too much solvent
during recrystallization?
2]During filtration, why is it important to only
wash your solid with ice-cold solvent?
3] Why should you fold and put creases in your
filter paper prior to filtration?

18. References:
1] Vogel's textbook of quantitative chemical analysis. – 5th
ed.
2] https://slideplayer.com/slide/7629283/