Page 196 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

6 SOLVENT EXTRACTION

6.5 ION ASSOCIATION COMPLEXES
An alternative to the formation of neutral metal chelates for solvent extraction
is that in which the species of analytical interest associates with oppositely
charged ions to form a neutral extractable species6 Such complexes may form
clusters with increasing concentration which are larger than just simple ion
pairs, particularly in organic solvents of low dielectric constant. The following
types of ion association complexes may be recognised.
1. Those formed from a reagent yielding a large organic ion, e.g. the
tetraphenylarsonium, (C,H,),As+, and tetrabutylammonium, (n-C,H,),N+
ions, which form large ion aggregates or clusters with suitable oppositely
charged ions, e.g. the perrhenate ion, ~e04. These large and bulky ions do
not have a primary hydration shell and cause disruption of the hydrogen-
bonded water structure; the larger the ion the greater the amount of disruption
and the greater the tendency for the ion association species to be pushed
into the organic phase.
These large ion extraction systems lack specificity since any relatively large
unhydrated univalent cation will extract any such large univalent anion. On
the other hand polyvalent ions, because of their greater hydration energy,
are not so easily extracted and good separations are possible between MnO4,
Re04 or Tc04 and CrOz-, MoOz- or WOz-, for example.
2. Those involving a cationic or anionic chelate complex of a metal ion.
Thus chelating agents having two uncharged donor atoms, such as
1 : 10-phenanthroline, form cationic chelate complexes which are large and
hydrocarbon-like. Tris(phenanthro1ine) iron(I1) perchlorate extracts fairly
well into chloroform, and extraction is virtually complete using large anions
such as long-chain alkyl sulphonate ions in place of C104. The determination
of anionic detergents using ferroin has been des~ribed.~
Dagnall and Wests have described the formation and extraction of a blue
ternary complex, Ag(1)-1,lO-phenanthroline-bromopyrogallol red (BPR),
as the basis of a highly sensitive spectrophotometric procedure for the
determination of traces of silver (Section 6.16). The reaction mechanism
for the formation of the blue complex in aqueous solution was investigated
by photometric and potentiometric methods and these studies led to the
conclusion that the complex is an ion association system, (Ag(phen),),BPRZ-,
i.e. involving a cationic chelate complex of a metal ion (Ag+) associated with
an anionic counter ion derived from the dyestuff (BPR). Ternary complexes
have been reviewed by Babko.,
Types (1) and (2) represent extraction systems involving coordinately
unsolvated large ions and differ in this important respect from type (3).
3. Those in which solvent molecules are directly involved in formation of the
ion association complex. Most of the solvents (ethers, esters, ketones and
alcohols) which participate in this way contain donor oxygen atoms and the
coordinating ability of the solvent is of vital significance. The coordinated
solvent molecules facilitate the solvent extraction of salts such as chlorides
and nitrates by contributing both to the size of the cation and the resemblance
of the complex to the solvent.
A class of solvents which shows very marked solvating properties for inorganic
compounds comprises the esters of phosphoric(V) (orthophosphoric) acid. The