Chemistry, Third edition

SOLVENT EXTRACTION

replace the ions now bound to the exhausted resin and the column is ready for use

again.

For example, hard wateris caused by the presence of calcium and magnesium ions

in solution. Sodium aluminosilicate is a water softener which replaces the calcium

and magnesium ions in hard water with sodium ions:

2NaAlSi 2 O 6 (s)Ca^2 (aq)\==\Ca(AlSi 2 O 6 ) 2 (s)2Na(aq)

To regenerate the aluminosilicate a concentrated solution of sodium chloride is

poured through the resin. Sodium ions replace the calcium and magnesium ions.

The reaction is now the reverse of the one shown in the above equation.

357

Fig. 19.13An ion exchanger.

Solvent extraction

Frequently in organic preparations a mixture of compounds is produced. Inorganic

compounds may be present and, to dissolve these, it is necessary to wash the reaction

mixture with water. Now the required organic product may be in solution or suspen-

sion in water. If this mixture is shaken with an organic solvent, which is immiscible

with water but in which the organic product is soluble, the organic product can be

extracted from the water. Ethoxyethane (diethyl ether) is a good organic solvent for

extracting organic products from aqueous solutions for the following reasons:

1.It is immiscible with water;

2.It is a good solvent;

3.It has a low boiling point and can therefore be removed easily by evaporation.

Ethoxyethane is shaken with the aqueous mixture in a separating funnel. The

desired organic product moves from the aqueous layer into the ethoxyethane layer.

The ethoxyethane layer can then be separated off (Fig. 19.15(a)). Note that after one

separation, the water layer often still contains some of the desired organic product

and, for this reason, the water layer is usually extracted with ethoxyethane a further

one or two times with fresh solvent in order to collect as much organic product as

possible (Fig. 19.15(b)). The separated layers are combined, then dried by shaking

with, for example, a little anhydrous magnesium sulfate and then filtered (Fig.

19.15(c)). The pure, dry organic product is recovered by evaporating off the ethoxy-

ethane (boiling point 35 °C).

19.6