Page 190 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

6 SOLVENT EXTRACTION

and pre-concentration of a single chemical species prior to its determination, it
may also be applied to the extraction of groups of metals or classes of
organic compounds, prior to their determination by techniques such as atomic
absorption or chromatography.
To understand the fundamental principles of extraction, the various terms
used for expressing the effectiveness of a separation must first be considered.
For a solute A distributed between two immiscible phases a and b, the Nernst
Distribution (or Partition) Law States that, provided its molecular state is the
same in both liquids and that the temperature is constant:
Concentration of solute in solvent a [A],
-- = KD
-
Concentration of solute in solvent b [A],
where KD is a constant known as the distribution (or partition) coefficient. The
law, as stated, is not thermodynamically rigorous (e.g. it takes no account of
the activities of the various species, and for this reason would be expected to
apply only in very dilute solutions, where the ratio of the activities approaches
unity), but is a useful approximation. The law in its simple form does not apply
when the distributing species undergoes dissociation or association in either
phase. In the practical applications of solvent extraction we are interested
primarily in the fraction of the total solute in one or other phase, quite regardless
of its mode of dissociation, association, or interaction with other dissolved
species. It is convenient to introduce the term distribution ratio D (or extraction
coefficient E):

where the symbol CA denotes the concentration of Ain al1 its forms as determined
analytically.
A problem often encountered in practice is to determine what is the most
efficient method for removing a substance quantitatively from solution. It can
be shown that if V mL of, Say, an aqueous solution containing x, g of a solute
be extracted n times with o-mL portions of a given solvent, then the weight of
solute x, remaining in the water layer is given by the expression:

where D is the distribution ratio between water and the given solvent. It follows,
therefore, that the best method of extraction with a given volume of extracting
liquid is to employ the liquid in several portions rather than to utilise the whole
quantity in a single extraction.
This may be illustrated by the following example. Suppose that 50mL of
water containing 0.1 g of iodine are shaken with 25 mL of carbon tetrachloride.
The distribution coefficient of iodine between water and carbon tetrachloride
at the ordinary laboratory temperature is 1/85, i.e. at equilibrium the iodine
concentration in the aqueous layer is 1185th of that in the carbon tetrachloride
layer. The weight of iodine remaining in the aqueous layer after one extraction
with 25 mL, and also after three extractions with 8.33 mL of the solvent, can
be calculated by application of the above formula. In the first case, if x, g of
iodine remains in the 50 mL of water, its concentration is x1/50g mL-'; the
concentration in the carbon tetrachloride layer will be (0.1 - x,)/25 g mL.-'.