Instant Notes: Analytical Chemistry

two solutes with distribution ratios D 1 and D 2 , a separationor selectivity factor,

b, is defined as

b= D 1 /D 2 where D 1 >D 2 (6)

Selectivity factors exceeding 10^4 or 10^5 (logbvalues exceeding four or five) are

necessary to achieve a quantitative separation of two solutes, as for most prac-

tical purposes, a separation would be considered complete if one solute could be

extracted with greater than 99% efficiency whilst extracting less than 1% of

another. The extraction of many solutes can be enhanced or supressed by

adjusting solution conditions, e.g. pH or the addition of complexing agents (vide

infra).

Solvent extraction Solvent extraction(SE) is used as a means of sample pre-treatment or clean-up
to separate analytes from matrix components that would interfere with their
detection or quantitation (Topic A4). It is also used to pre-concentrate analytes
present in samples at very low levels and which might otherwise be difficult or
impossible to detect or quantify. Most extractions are carried out batchwise in a
few minutes using separating funnels. However, the efficient extraction of
solutes with very small distribution ratios (<1) can be achieved only by continu-
ously exposing the sample solution to fresh solvent that is recycled by refluxing
in a specially designed apparatus.
Broad classes of organic compounds, such as acids and bases, can be sepa-
rated by pH control, and trace metal ions complexed with organic reagents can
be separated or concentrated prior to spectrometric analysis (Topic E10).

Extraction of organic acids and bases

Organic compounds with acidic or basic functionalities dissociate or protonate

in aqueous solutions according to the pH of the solution. Their extraction can,

therefore, be optimized by pH adjustments. The relation between pH and the

distribution ratio, D, of a weak acid can be derived in the following way:

A weak acid, HA, dissociates in water according to the equation

HA =H++A- (7)

The acid dissociation constant, Ka, is defined as

Ka (8)

Only the undissociated form, [HA], can be extracted into a nonpolar or slightly

polar solvent such as diethyl ether, the distribution or partition coefficient, KD,

being given by

KD (9)

However, the distribution ratio takes account of both the dissociated and

undissociated forms of the acid in the aqueous phase and is given by

D (10)

Re-arrangement of equation (8) and substitution for [A-]aqin equation (10) gives

[HA]ether





([HA]aq[A-]aq)

[HA]ether




[HA]aq

[H]aq[A-]aq





[HA]aq

112 Section D – Separation techniques