Methods of purification Flashcards
The common techniques used for
purification are as follows :
(i) Sublimation
(ii) Crystallisation
(iii) Distillation
(iv) Differential extraction and
(v) Chromatography
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.
Sublimation
- is used
to separate sublimable compounds from non-sublimable impurities. - on heating, some
solid substances change from solid to vapour
state without passing through liquid state.
Crystallisation
- 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. - 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. - . Repeated crystallisation
becomes necessary for the purification
of compounds containing impurities of
comparable solubilities.
Distillation
This important method is used to separate
(i) volatile liquids from nonvolatile impurities
and
(ii) the liquids having sufficient difference
in their boiling points.
Eg: -Chloroform (b.p 334 K) and aniline (b.p. 457
K) are easily separated by the technique of distillation
how distillation is carried out
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
Fractional Distillation:
- 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. - . 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.
fractionating coloumn
- A fractionating column provides many surfaces for heat exchange between the ascending vapours
and the descending condensed liquid. - 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. - Fractionating columns are available in various sizes and designs
- 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.
theoretical plate.
Each
successive condensation and vaporisation unit in the fractionating column is called a
theoretical plate.
Distillation under reduced
pressure:
- 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 - Glycerol can be separated from
spent-lye in soap industry by
using this technique.
