Chemistry, Third edition

19 · SEPARATING MIXTURES

Soxhlet extraction

A compound can be isolated from a crude, solid product by repeated extraction

using a hot solvent. The apparatus usually used for this purpose is a Soxhlet extrac-

tion apparatus (Fig. 19.16).

The crude material is put into a porous thimble at A, and the extracting solvent is

placed in a flask at the bottom of the apparatus. The solvent in the flask is gently

boiled and its vapour passes into the condenser via the side arm of the apparatus. Sol-

vent vapour condenses and drips into the thimble, where the hot solvent dissolves

some of the desired product present. When the level of solvent reaches the level of the

top of the fine glass tube, the whole volume of solvent, containing dissolved product,

siphons into the solvent flask at the bottom of the apparatus. The process continues

automatically, until all the desired product has been extracted and is present, in solu-

tion, in the solvent flask. If the substances to be extracted are coloured, the process is

stopped when the liquid being siphoned into the flask becomes pale or colourless.

360

Fine

glass

tube

Crude

material

containing

desired

product

Side
arm

Condenser

Heater

Solvent

AA

Fig. 19.16Soxhlet extraction
apparatus.

BOX 19.5

Extraction of caffeine using supercritical fluid extraction

Caffeine has the molecular formula

C 8 H 10 N 4 O 2 and is present in tea, coffee and

cola. It can be extracted from tea or coffee by

Soxhlet extraction, using trichloromethane

(chloroform) as a solvent. A disadvantage of

Soxhlet extraction is that it takes time, often

many hours. Supercritical fluid extraction

(SFE) is a recently developed technique that

can shorten extraction times considerably.

The solvent used is a supercritical fluid,a

substance that is above its critical

temperature and pressure (see page 168).

Carbon dioxide, for example, becomes

supercritical at a temperature of 31 °C and a

pressure of about 73 atm. Supercritical CO 2 is

so dense that it has excellent solvating

powers. It is also inert and will not react with

the substances it is used to extract. For

example, using this solvent, caffeine can be

extracted from coffee beans in minutes.

Decaffeinated coffee beans are preferentially

prepared by removing the caffeine using non-

toxic CO 2 as a solvent, rather than potentially

toxic organic solvents such as hexane,

dichloromethane or chloroform.

Chromatography

Chromatography has become one of the most important methods of separating sub-

stances from a solution. In chromatography, the solution is passed over (or through)

a carefully chosen solid, such as paper or aluminium oxide. Chromatography

depends upon the ability of the solid to slow down the movements of the substances

in the solution. Different substances are separated because they are ‘held back’ by

differing amounts. The ‘holding back’ is caused by the differing degrees of adsorp-

tion between the surface of the solid and the different substances in the mixture, or

by the differing solubilities of the substances in a liquid around the surface of the

solid, or a mixture of both effects. Chromatography is like a race, with the runners

being the substances in the mixture. At the beginning of the race, or the start of the

separation, the competitors are together, whereas by the end of the race the field is

split and the individual runners are well separated.

The word chromatography literally means ‘colour writing’. Chemists and biologists

both use chromatography. For example, chromatographic separations are used exten-

19.7