Steam Distillation

Steam distillation is used for separating substances which are immiscible with water, volatile in steam & having high vapour pressure at the boiling temperature of water

It is a method of distillation carried with the aid of steam and used for the separation of high boiling substance from non-volatile impurities

It is the most common example of differential distillation

Principle

• A mixture of immiscible liquids begins to boil when the sum of their vapor pressure is equal to the atmospheric pressure.

• In case of a mixture of turpentine oil and water, mixture boils below the BP of water, though the turpentine boils at a much higher temperature than that of water.

• For example, the BP of turpentine is about 160⁰C. on mixing with water and heated the mixture boils at about 95.6⁰C

• At this temperature, the VP of water is 86.24 kPa (647 mm Hg) and that of turpentine is 15.05 kPa (113 mm Hg)

• The sum of VPs is 101.31 kPa (760 mm Hg) which is normal AP.

• Thus high boiling substances can be distilled at a temperature much below its BP when steam is used

Construction

• It consists of a metallic ‘steam can’ fitted with a cork having two holes.

• Through one of the holes, a long tube is passed so as to reach almost the bottom of steam generator.

• This tube acts as a safety tube, so that in case the pressure inside the steam generator becomes too much, water will be forced out of it and the pressure will be relieved. Moreover, when steam start coming out from the safety tube; it indicates that the steam can is almost empty.

• Through another hole a bent tube is passed.

• Another end of the bent tube is connected to the flask containing non-aqueous liquid (for example, crude containing volatile oil) through a rubber bung. The tube should reach almost the bottom of the flask.

• Through the other hole of the rubber bung, a delivery tube is inserted which connects the flask and the condenser.

• The condenser is connected to a receiver flask using an adaptor.

Provisions are made to heat the steam can and flask.

Working of steam Distillation

The non- aqueous liquid is placed in the flask. A small quantity of water is added to it. Steam can is filled with water.

The steam generator and the flask are heated simultaneously, so that a uniform flow of steam passes through the boiling mixture. The mixture gets heated.

The steam carries the volatile oil and passes into the condenser, which is cooled by cold water.

The condensed immiscible liquid is collected into receiver. Distillation is continued until all the non- aqueous liquid has been distilled. In the receiver, water and organic liquid form two separate layers, which can be easily separated using a separating flask.