vogel_practical_ochem_3.pdf

22] THEORY OF GENERAL TECHNIQUE 45
Let w2 grams remain in the aqueous layer after the second extraction,
then :
w2fv _ R
(wl — w2)/s
Kv
or w9 = w 1 Kv +s
/ Kv \ 2
Similarly if wn grams remain in the aqueous layer after the nth extraction:
/ Kv
Wn = W, '"«\Kv-+S, ;) CD-
We desire to make wn as small as possible for a given weight of solvent,
i.e., the product of n and s is constant, hence n should be large and 8
small; in other words, the best results are obtained by dividing the
extraction solvent into several portions rather than by making a single
extraction with the whole quantity. It must be emphasised that the
expression deduced above applies strictly to a solvent which may be
regarded as completely immiscible with water, such as benzene, chloroform
or carbon tetrachloride ; if the solvent is slightly miscible, e.g., ether, the
equation (1) is only approximate, but is nevertheless useful for indicating
the qualitative nature of the results to be expected.
Let us consider a specific example, viz., the extraction of a solution of
4'0 g. of w-butyric acid in 100 ml. of water at 15° with 100 ml. of benzene
at 15°. The partition coefficient of the acid between benzene and water
may be taken as 3 (or J between water and benzene) at 15°. For a
single extraction with benzene, we have :
Af $ x 100 \ . A
For three extractions with 33 • 3 ml. portions of fresh benzene :
i X 100
./ * x 100 \»
4 (iFT"33^3) ==0 - 5g -
Hence one extraction with 100 ml. of benzene removes 3*0 g. (or 75 per
cent.) of the w-butyric acid, whilst three extractions remove 3 • 5 g. (or
87 • 5 per cent.) of the total acid. This clearly shows the greater efficiency
of extraction obtainable with several extractions when the total volume
of solvent is the same. Moreover, the smaller the distribution coefficient
between the organic solvent and the water, the larger the number of
extractions that will be necessary.
The above considerations apply also to the removal of a soluble impurity
by extraction (or washing) with an immiscible solvent. Several
washings with portions of the solvent give better results than a single
washing with the total volume of the solvent.
3*