Methods for the purification of liquids Distillation

Methods for the purification of liquids Distillation

Distillation is used for separating the constituents of a liquid mixture which differ in their boiling points. Depending upon the difference in the boiling points of the constituents, different types of distillation are employed. These are described below.

i) Simple Distillation

Liquids with boiling points widely apart (about 40K and above) can be purified by simple distillation if they do not decompose under ordinary pressure. Simple distillation involves conversion of a liquid into its vapour by heating in a distilling flask and then condensation of the vapour into a liquid in the receiver.

Mixtures like the following can be separated by this simple distillation.

a  Nitro benzene (b.p 484K) and benzene (b.p.354K)

b. Diethyl ether (b.p.308K) and ethyl alcohol (b.p.351K)

Note : Simple distillation is also helpful in separating non-volatile impurities from liquids.

Fractional Distillation

This method is applicable for the separation and purification of a mixture of two or more miscible liquids whose boiling points lie very close to each other.

This is similar to the ordinary distillation method with the only exception that a fractionating column is introduced in-between the distillation flask and the condenser.

The process of separation of the components in a liquid mixture at their respective boiling points in the form of vapours and the subsequent condensation of those vapours is called fractional distillation.

The fractionating columns used for the purpose are of different shapes.

Steam Distillation

In steam distillation impure compounds are distilled in a current of steam. This method is applicable to solids as well as liquids. For purification by steam distillation, an impure compound must satisfy the following conditions:

1.     It should not decompose at the steam temperature.

2.     It should have a fairly high vapour pressure at 373 K.

3.     It should be insoluble in water.

4.     The impurities present should be non-volatile

1.      Water   2.   Protective tube      3.   Flask4.   Inlet for water

5.      Outlet for water    6.       Pure liquid

The apparatus used for steam distillation is shown in the figure (16.5). The impure compound is taken in the round bottomed flask and a small quantity of water is added. The flask is then heated gently. Now steam is bubbled through the contents in the flask. The vapours of the compound mix up with steam and escape into the condenser. The condensate thus obtained is a mixture of water and the organic compound which can be separated.

Theory of steam distillation

Let p1 represent the vapour pressure of water and p2 the vapour pressure of the organic liquid. In steam distillation the liquid boils at a temperature at which

p1 + p2  = Atmospheric Pressure

This temperature must be lower than the normal boiling point of water or the organic liquid. The reason that p1 + p2 becomes equal to the atmospheric pressure must earlier than p1 or p2 alone. Thus in steam distillation, the impure liquid boils at a temperature which is lower than its normal boiling point. Hence, steam distillation serves the same purpose as distillation under reduced pressure. However, the former is cheaper but less useful than the latter.

Distillation under reduced pressure

This technique is used for purifying or separating thermally unstable liquid compounds which decompose at their normal boiling points.

Principle : Lowering of pressure on the surface of a liquid lowers its boiling point. As a result, a liquid can be boiled and distilled at a temperature much below the normal boiling point without any decomposition.

Procedure : Distillation under reduced pressure or vacuum is carried out in a specially designed glass apparatus as shown in (Fig.).

The receiver is attached to a vacuum pump to reduce pressure. The pressure is measured with the help of a manometer.

Advantages of distillation under reduced pressure

Distillation under reduced pressure has the following advantages:

i)      The compounds which decompose on heating to their boiling points under normal pressure can be purified by distillation under reduced pressure. This is because at a reduced pressure, a liquid would boil at temperature much below its normal boiling point.

In distillation under reduced pressure, a liquid boils at temperature well below the normal boiling point. So, the distillation under reduced pressure is more fuel-economical.

Extraction with solvents

This method is based on the fact that organic substances are more soluble in organic solvents than in water.

The organic substance is extracted from its aqueous solution adopting the following procedure.

1.     The aqueous solution containing organic substance is shaken with a suitable organic solvent which dissolves the substance but is immiscible with water. Two layers are formed; the organic layer and aqueous layer.

2.     The solvent layer containing the organic substance (organic layer) is separated using a `separating funnel'. The impurities remain in the aqueous layer removed by distillation to obtain the organic substance.

The organic solvent is removed by distillation to obtain the organic substance.