Page 195 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

SYNERGlSTlC EXTRACTION 6.4

apart, then excellent separation can be achieved by controlling the pH of
extraction. It is often helpful to plot the extraction curves of metal chelates. If
one takes as the criterion of a successful single-stage separation of two metals
by pH control a 99 per cent extraction of one with a maximum of 1 per cent
extraction of the other, for bivalent metals a difference of two pH units would
be necessary between the two pHll: values; the difference is less for tervalent
metals. Some figures for the extraction of metal dithizonates in chloroform are
given in Table 6.1. If the pH is controlled by a buffer solution, then those metals
with pH values in this region, together with al1 metals having smaller pH
values, will be extracted. The pHll2 values may be altered (and the selectivity
of the extraction thus increased) by the use of a competitive complexing agent
or of masking agents. Thus in the separation of mercury and copper by extraction
with dithizone in carbon tetrachloride at pH 2, the addition of EDTA forms a
water-soluble complex which completely masks the copper but does not affect
the mercury extraction. Cyanides raise the pH values of mercury, copper,
zinc, and cadmium in dithizone extraction with carbon tetrachloride.

Table 6.1
Metal ion Cu(I1) Hg(I1) Ag Sn(I1) Co Ni, Zn Pb
Optimum pH of extraction 1 1-2 1-2 6-9 7-9 8 8.5-11

6.4 SYNERGlSTlC EXTRACTION
The phenomenon in which two reagents, when used together, extract a metal
ion with enhanced efficiency compared to their individual action is called
synergism. A common form of synergistic extraction is that in which a metal
ion, Mn+, is extracted by a mixture of an acidic chelating reagent, HR, and an
uncharged basic reagent, S. The joint action of the reagents is especially
pronounced in those cases where the coordination capacity of the metal ion is
not fully achieved in the MR, chelate; then the extractant S gives a mixed
complex, MRnS,, which is extracted with much greater efficiency than the parent
chelate. This concept has been usefully applied in the case of the reagent dithizone
by using it in combination with bases such as pyridine and 1,lO-phenanthroline.
Thus, although the complex formed by manganese(I1) with dithizone alone is
of no analytical value because it decomposes rapidly, the red complex formed
by manganese(I1) with dithizone and pyridine is sufficiently stable to light and
oxidation for it to be used in a sensitive photometric procedure for determining
trace amounts of mangane~e.~ Again the normally very slow reaction of
nickel(I1) with dithizone, H ,Dz, is greatly accelerated by the addition of nitrogen
bases such as 1,lO-phenanthroline (phen). The complex formed according to
the equation:

is the basis of a very sensitive extraction-photometric method for nickel3 (see
also Section 6.17).
In addition to systems of the above type, i.e. involving adduct formation,
various other types of synergistic extraction systems are recognised and have
been re~iewed.~ An example is the synergistic influence of zinc in the extraction
and AAS determination of trace cadmium in water.'