SIMPLE
DISTILLATION
• Simple distillation
is a process of converting a single constituent from a liquid (or mixture) into
its vapour, transferring the vapour to another place and recovering the liquid
by condensing the vapour, usually by allowing it to come in contact with a cold
surface.
• This process is known differential
distillation, as distillation is based on the differences in volatilities
and vapour pressures of the components in the mixture.
• Principle:
Liquid boils when its vapour pressure is equal to atmospheric pressure. Simple
distillation is conducted at its boiling point. The higher the relative
volatility of a liquid, the better is the separation by simple distillation.
Heat is supplied to the liquid so that it boils. The resulting vapour is
transferred to a different place and condensed.
Assembling
of apparatus
The construction of a simple distillation apparatus is shown
in Figure. It consists of a distillation flask with a side arm sloping
downwards.
Condenser is fitted into the side arm by means of a cork.
The condenser is usually water condenser, i.e., jacketed for
circulation of water. The condenser is connected to a receiver flask using an
adapter with ground glass joints.
On a laboratory scale, the whole apparatus is made of glass.
Simple
Distillation Process
• When a mixture AB of a specific composition is heated, the
total vapor pressure (composed of the contributions of PA and PB) will rise
until it is equal to the external vapor pressure.
• The mixture will begin to boil.
• Assume a two component mixture with a composition of
30%A:70%B (point W). The boiling point of this mixture is found by drawing a
vertical line from W to where it intersects the lower curve (point X).
• A horizontal line drawn from X to where it intersects the vertical
axis (the temperature) gives the boiling point of composition W.
• From the point (X) where this horizontal line intersects
the upper curve (vapor) drop a vertical line to intersect the lower axis (the
composition).
• Point Z gives the composition of the vapor which is in
equilibrium with a liquid of composition W at its boiling point.
Procedure
The liquid to be distilled is filled into the flask to
one-half to two-third of its volume.
Bumping is avoided by adding small pieces of porcelain before
distillation.
A thermometer is inserted into the cork and fixed to the
flask.
The thermometer bulb must be just below the level of the
side arm.
Water is circulated through the jacket of the condenser.
The contents are heated gradually. The liquid begins to boil
after some time.
The vapour begins to rise up and passes down the side arm
into the condenser.
The temperature rises rapidly and reaches a constant value.
The temperature of the distillate is noted down, which is
equal to the boiling point of the liquid.
The vapour is condensed and collected into the receiver.
The flame is adjusted so that the distillate is collected at
the rate of one to two drops per second.
Distillation should be continued until a small volume of
liquid remains in the flask.
Applications:
(I) Simple distillation is used for the preparation of
distilled water and water for injection
(2) Volatile and aromatic waters are prepared
(3) Organic solvents are purified
(4) A few official compounds are prepared by distillation
Examples are spirit of nitrous ether and aromatic spirit of
ammonia
(5) Non-volatile solids are separated from volatile liquids