Methods of Purification 1 Flashcards
how do you certain whether a compound is pure or not
Finally, the purity of a compound is
ascertained by determining its melting or
boiling point. Most of the pure compounds
have sharp melting points and boiling points.
New methods of checking the purity of an
organic compound are based on different
types of chromatographic and spectroscopic
techniques.
purificatinon through sublimation
You have learnt earlier that on heating, some
solid substances change from solid to vapour
state without passing through liquid state.
The purification technique based on the above
principle is known as sublimation and is used
to separate sublimable compounds from nonsublimable impurities.
when is crystallisation used for purification
This is one of the most commonly used
techniques for the purification of solid organic
compounds. It is based on the difference in
the solubilities of the compound and the
impurities in a suitable solvent.
how is crystallisation done
The impure
compound is dissolved in a solvent in which
it is sparingly soluble at room temperature
but appreciably soluble at higher
temperature. The solution is concentrated to
get a nearly saturated solution. On cooling
the solution, pure compound crystallises out
and is removed by filtration. The filtrate
(mother liquor) contains impurities and small
quantity of the compound.
If the compound
is highly soluble in one solvent and very little
soluble in another solvent, crystallisation can
be satisfactorily carried out in a mixture of
these solvents.
Impurities, which impart
colour to the solution are removed by
adsorbing over activated charcoal.
Repeated
crystallisation becomes necessary for the
purification of compounds containing
impurities of comparable solubilities.
what compounds if distillation used for
This important method is used to separate (i)
volatile liquids from nonvolatile impurities and
(ii) the liquids having sufficient difference in
their boiling points. Liquids having different
boiling points vaporise at different
temperatures. The vapours are cooled and the
liquids so formed are collected separately.
explain simple distillation
Chloroform (b.p 334 K) and aniline (b.p. 457
K) are easily separated by the technique of
distillation (Fig 12.5). The liquid mixture is taken in a round bottom flask and
heated carefully. On boiling, the
vapours of lower boiling component
are formed first. The vapours are
condensed by using a condenser and
the liquid is collected in a receiver. The
vapours of higher boiling component
form later and the liquid can be
collected separately.
when is fractional distillation used
If the
difference in boiling points of two
liquids is not much, simple distillation
cannot be used to separate them. The
vapours of such liquids are formed
within the same temperature range and
are condensed simultaneously. The
technique of fractional distillation is
used in such cases. In this technique,
vapours of a liquid mixture are passed
through a fractionating column before
condensation. The fractionating
column is fitted over the mouth of the
round bottom flask
explain fractional distillation
Vapours of the liquid with higher boiling
point condense before the vapours of the
liquid with lower boiling point. The vapours
rising up in the fractionating column become
richer in more volatile component. By the time
the vapours reach to the top of the
fractionating column, these are rich in the
more volatile component. Fractionating
columns are available in various sizes and
designs( glass bead fractionating column, bubble plate column)
what is the purpose of a fractionating column
A fractionating
column provides many surfaces for heat
exchange between the ascending vapours
and the descending condensed liquid. Some
of the condensing liquid in the fractionating
column obtains heat from the ascending
vapours and revaporises. The vapours thus
become richer in low boiling component. The
vapours of low boiling component ascend to
the top of the column. On reaching the top,
the vapours become pure in low boiling
component and pass through the condenser
and the pure liquid is collected in a receiver.
After a series of successive distillations, the
remaining liquid in the distillation flask gets
enriched in high boiling component.
what is a theoretical plate
Each
successive condensation and vaporisation
unit in the fractionating column is called a theoretical plate. Commercially, columns
with hundreds of plates are available.
what are some commercial applications of distillation
fractional distillation is to separate different
fractions of crude oil in petroleum industry.
what is distillation under reduced pressure used for
This
method is used to purify liquids having very
high boiling points and those, which
decompose at or below their boiling points.
Such liquids are made to boil at a temperature
lower than their normal boiling points by
reducing the pressure on their surface. A
liquid boils at a temperature at which its
vapour pressure is equal to the external
pressure. The pressure is reduced with the
help of a water pump or vacuum pump
(Fig.12.8). Glycerol can be separated from
spent-lye in soap industry by using this
technique.
how is steam distillation carried out
In steam distillation, steam from a
steam generator is passed through a heated
flask containing the liquid to be distilled.
The mixture of steam and the volatile
organic compound is condensed and
collected. The compound is later separated
from water using a separating funnel. In
steam distillation, the liquid boils when
the sum of vapour pressures due to the
organic liquid (p1) and that due to water
(p2) becomes equal to the atmospheric
pressure (p), i.e. p =p1+ p2. Since p1 is
lower than p, the organic liquid vaporises
at lower temperature than its boiling
point.
what substance is steam distillation
This technique is
applied to separate substances which are
steam volatile and are immiscible with
water.
Thus, if one of the substances in the
mixture is water and the other, a water
insoluble substance, then the mixture will boil
close to but below, 373K. A mixture of water
and the substance is obtained which can be
separated by using a separating funnel.
Aniline is separated by this technique from
aniline – water mixture (Fig.12.9).
when is differential extraction used
When an organic compound is present in an
aqueous medium, it is separated by shaking it with an organic solvent in which it is more
soluble than in water. The organic solvent and
the aqueous solution should be immiscible
with each other so that they form two distinct
layers which can be separated by separatory
funnel. The organic solvent is later removed
by distillation or by evaporation to get back
the compound. Differential extraction is
carried out in a separatory funnel
