Equation (4.31), which defines the extraction characteristics for any chelate system, is represented
graphically in Figure 4.2 for a mono-, di- and trivalent metal, i.e. n = 1, 2 and 3 respectively, and shows
the pH range over which a metal will be extracted. No significance should be attached to the positioning
of the curves relative to the pH scales as these are determined by the value of K*'. Thus the more acidic
the reagent or the stronger the metal complex, the lower the pH range over which the metal will be
extracted. Increased reagent concentration has a similar effect.
Figure 4.2
Extraction as a function of pH for metals of different formal valencies. (Note: the position of
each curve along the pH abscissa is not significant).The pH at which 50% of the metal is extracted, pH1/2, can be used to assess the degree of separability of
two or more metals. At E = 50, equation (4.31) reduces to
Substituting this value for log K*' in equation (4.28)
For two metals, the separation factor β is defined as D'/D'', where D' > D", or
Therefore, for extraction at a specified pH,