Precipitation
Chemical precipitation is the formation of a separable solid substance from a solution, either by converting the substance into an insoluble form or by changing the composition of the solvent to diminish the solubility of the substance in it.
Precipitation is the creation of a solid from a
solution. When the reaction occurs in a liquid solution, the solid formed is
called the 'precipitate'. The chemical that causes the solid to form is called
the 'precipitant'. Without sufficient force of gravity to bring the solid
particles together, the precipitate remains in suspension.
After sedimentation, especially when using a centrifuge to press
it into a compact mass, the precipitate may be referred to as a 'pellet'.
Precipitation can be used as a medium. The precipitate-free liquid remaining
above the solid is called the 'supernate' or 'supernatant'. Powders derived
from precipitation have also historically been known as 'flowers'.
When the solid appears in the form of cellulose fibers which have
been through chemical processing, the process is often referred to
as regeneration.
PRINCIPLES Of Precipitation
A number of
parameters can be used to separate a sample of interest from impurities by
reducing its solubility, and removing it from a solution as a solid.
First, changing the ionic strength of the solution can change a substance’s
solubility. This often involves the addition of extra salt (also called salting
out), or the addition of a counter-ion, which forms a less soluble species with
the compound of interest.
Figure: Solubility equilibria are affected
by ionic strength, pH, and temperature.
A
compound of interest (yellow) is separated from impurities (red) by changing
It’s
solubility in a given solvent.
Changing the
pH of a solution may change the net charge of the compound. At a certain pH,
the net charge becomes zero (also called isoelectric point) and the compound
becomes less soluble in water, eventually forming a solid. Temperature also
affects solubility, as higher temperature increases solubility of solids.
The rate of
solid formation determines relative purity. In general, the term precipitation
refers to the formation of a solid at a rapid pace, thereby producing an
amorphous sample with some impurities trapped within. This is common in salting
out and pH change-induced processes. When this process is slowed down, the impurities
are not trapped within the compound and a relatively pure solid is produced.
This technique is employed in re-crystallization. In this process, a compound
is dissolved in enough solvent to be just at the saturation point at an
elevated temperature. This saturated solution is then allowed to cool down
slowly. As the solution cools, the solubility of the component decreases and the compound in excess of the solubility forms a
well-ordered solid (otherwise known as crystals) instead of an amorphous solid.
Impurities in the solution do not get trapped as the slow process allows the
removal of these impurities at the surface of the solid before they are
trapped.
Figure: Difference between precipitation and
recrystallization
Once the solid
has formed (whether as a crystal or as a precipitate), it should be separated
from the rest of the mixture. Filtration is one way to separate them. This
employs a porous material which selectively inhibits the passage of the solid
material but not the solution.
Centrifugation
is another way to separate the precipitate from the rest of the mixture.
Centrifugation uses centripetal acceleration to separate mixtures based on
their densities. Since solid is denser than the aqueous solution, the solid
sediments at the bottom of the container. The solid is also called the pellet
and the aqueous solution, the supernatant. The supernatant can then be decanted
or extracted using a pipet or syringe. Crystals are fragile and centrifugation
is often not employed to separate them from the solution.
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