Technique of Recrystallization in Crystallography @ Jewel Info 4 U
By:Ritika
Summary: Recrystallization is the
technique of purifying solids based
on their different solubility. That
is, recrystallization deals with the
cleaning method or the
transformation of a crystal
structure, as a result of the change
in external factors, for example
temperature and pressure.
By the process of recrystallization
in crystallography a satisfactory
solution of a substance is
manufactured and the substance is
crystallized by cooling the
solution. The developing crystals
are definitely in a purer form in
all respects, especially if the
supplementary impurities display
some other solution behavior. The
entire process may be done over and
over again until the preferred
degree of purity is attained. The
crystallization process by itself
assists the purification process
since, as the crystals get formed,
they choose the correct molecules
that match the crystal lattice and
disregard the wrong molecules. This
is not a flawless procedure, but it
definitely increases the purity
level of the final creation.
Technique Used for
Recrystallization in Crystallography
* Selection and Addition of
Solvent:
First, careful selection of
appropriate solvent is made and a
little quantity of this solvent is
added to the container containing
impure solid. Choosing the solvent
is very vital and essential care
must be taken to select a solvent
such that the impure compound has a
low solubility at lower
temperatures, however, it may be
totally soluble at high
temperatures. That is, the solvent
must be selected in such a way such
that it fully dissolves the impure
substance when heated and also move
out of the solution on cooling.
Ensure you add little quality of
solvent rather than in larger
amounts such that it fully dissolves
the sample. This is because it is
possible to add more solvent as
needed when heating as described in
the next step. The solubility of the
compound in the solvent used for the
recrystallization process is
essential. In an ideal situation,
the solvent will dissolve the
compound fully to be purified at
high temperatures, which is mostly
the boiling point of the solvent,
and the compound will be totally
insoluble in that solvent at room
temperature or at 0oC. Moreover, the
impurity will either be totally
insoluble in that solvent at high
temperatures, or will be very
soluble in the solvent at low
temperature. So, the process of
recrystallization relies on the
property that for most compounds, as
the temperature of a solvent
increases, the solubility of the
compound in that solvent also
increases.
* Process of Heating:
To allow the solid to dissolve, the
process of heating is carried out
until the solid gets dissolved in
the solvent. For carrying out the
heating process, one can use either
a hot water bath or steam bath.
These would help in carrying out the
heating process in a gentle,
controlled, efficient way. Some
people prefer to use even a hot
plate or gas burner for carrying out
the heating process. The next
process is the process of cooling
which is described below.
* Process of Cooling:
The above heated solution is cooled.
This enables crystallization of the
product. This also allows it to get
more pure solid precipitation
thereby leaving the impurities to be
dissolved in the solvent. The secret
in achieving efficient higher purity
product is slower cooling. So,
ensure that you always allow the
solution to cool to room temperature
before setting the flask in an ice
bath or refrigerator.
The process of forming crystals
normally starts at the bottom of the
container. One can also aid
crystallization by scratching the
container with a glass rod
particularly at the air-solvent
junction. Though this is not an
essential step, by doing this
scratching process, the glass
surface area gets increased giving a
roughened surface on which the solid
can crystallize. Once can also
follow the seeding technique for
aiding the crystallizing processes.
This is done by adding a small
crystal of the desired pure solid to
the cooled solution. But while doing
this, care must be taken to add the
crystal only after the solution is
cooled which otherwise would cause
the crystal to dissolve. Sometimes,
one might encounter that no crystals
has resulted from the solution. Then
the possible reasons for this could
be usage of too much solvent. So,
for correcting this, allow the
solvent to evaporate. Then follow
the process of reheating and cool
the solution as explained above for
pure crystals to be formed.
* Vacuum Filtration Process:
After the formation of crystals now
we have to separate the crystal from
the solution for which vacuum
filtration process is carried out.
The process of vacuum filtration is
applied which isolates the crystals.
For isolating the crystals of
purified solid, one can also try
washing the purified solid with
chilled solvent. But while washing,
ensure that you use only cold
solvent, because otherwise it would
result in dissolving the crystals.
* Process of Drying:
Allow the solid to dry which results
in getting a purified solid. The
waste solution could be discarded.
The previous process of vacuum
filtration itself would have removed
almost all the solvents. This step
namely the open-air drying would
enable removing more solvent, if
anything further is present.
One can repeat the process of
recrystallization if they wish to
further purify the sample.
Crystallization of a solid is not
the same as precipitation of a
solid. In crystallization, there is
a slow, selective formation of the
crystal framework resulting in a
pure compound. In precipitation,
there is a rapid formation of a
solid from a solution that usually
produces an amorphous solid
containing many trapped impurities
within the solid's crystal
framework. Solid-state
recrystallization is what takes
place under circumstances of extreme
pressure and temperature where
grains, atoms or even molecules of a
rock / mineral are crammed even
closer to each other, to create a
whole new crystal structure. It is
possible to check the level of
purity by taking a melting point
range of the solid. It is then
compared with an accepted melting
point range. In general, the thumb
rule is solids show higher melting
points if they are pure. That is,
for pure solids, the melting points
get raised. It is also possible to
use two or more solvents in the
technique of recrystallization in
which case, the technique is termed
as multi-solvent recrystallization.
Related Links :
|
| |
Belly Button Piercing
The Egyptians, the Indians and many other cultures pierced this humble button to show the significance of the person sporting a pierced navel - Royalty, Warriors, exceptional Beauty. |
|
|
|
|
|
|
|
Welcome, you are logged in as 
