Chemistry, Third edition

SOLVENT EXTRACTION 359

BOX 19.4

Reasoning behind multiple extractions

If a compound is shaken in a separating funnel with two immiscible solvents, such as
ethoxyethane and water, some of the compound will dissolve in the ethoxyethane and some in
the water. The amount of compound that dissolves in each depends upon how soluble it is in
the solvents. Kdis defined as the distribution ratio

Kd=concentration of compound in ethoxyethane

concentration of compound in water

Note that Kdis a constant at a particular temperature. for the ethoxyethane–water system,
Kd= 4 at 20 °C.

Supposing 10.0 g of organic product are in 100 cm^3 of water at 20 °C and 100 cm^3 of
ethoxyethane are available for extraction of the compound. Is it better to use the 100 cm^3 of
ethoxyethane in one go, or to perform two extractions using 50 cm^3 of ethoxyethane each time?

1.All 100 cm^3 of ethoxyethane used in one extraction

Letxg of the organic product be extracted into the ethoxyethane layer. The amount of

organic product remaining in the aqueous layer is now (10 x)g.

Using the equation

Kd=concentration of compound in ethoxyethane

concentration of compound in water

4 = x/100

(10x)/100

Solving this equation gives x= 8 g. So, 8 g of product can be recovered from the

ethoxyethane layer.

2.Two extractions are performed, each using 50 cm^3 ethoxyethane

For the first extraction let yg of the organic product be extracted into the ethoxyethane layer.

The amount of organic product left in the water layer will be (10 y).

As before,

4  y/50

(10y)/100

Therefore,y= 6.7 g.

Before the second extraction, 10 6.73.3 g of product remains in the aqueous layer. Let

zg of this be extracted into the ether layer. Then, the amount of organic product left in the

water layer will be (3.3 z).

As before,

4  z/50

(3.3z)/100

Therefore,z= 2.2 g.

The total amount of organic product obtained from the two extractions is

(yz) OR 6.7 2.28.9 g.

This is nearly one more gram of product than that obtained from one extraction. More product
would be obtained with more extractions, i.e. with smaller amounts of ethoxyethane, but the
increase in amount of product obtained becomes smaller with each successive extraction.

Multiple extractions

(i)The distribution ratio of an organic compound X between toluene (methylbenzene) and
water is 7.0. An aqueous mixture (100 cm^3 ) contains 1.0 g of X. The mixture is extracted
once with 200 cm^3 of toluene. What mass of X is extracted into the organic layer?

(ii)Repeat the calculation but for a two-stage extraction, using 100 cm^3 of toluene each time.
Do you extract more X using 200 cm^3 of toluene in one extraction, or 2 100 cm^3 of
toluene in two successive extractions?

Exercise 19C