1-(2-pyridylazo)-2-naphthol (PAN)
sodium diethyldithiocarbamate (NaDDTC)
(see also Table 1);
∑ Electrically neutral ion-association complexes
e.g. Cu(neocuproine) 2 +, NO 3 -
(C 6 H 5 CH 2 ) 3 NH+, GaCl 4 -
[(C 2 H 5 ) 2 O] 3 H+, FeCl 4 -
(see also Table 1).The Nernst distribution law applies to metal complexes, but their distribution
ratios are determined by several interrelated equilibria. As in the case of organic
acids and bases, the efficiency of extraction of metal chelates is pH dependent,
and for some ion-association complexes, notably oxonium systems(hydrogen
ions solvated with ethers, esters or ketones), inorganic complex ions can be
extracted from concentrated solutions of mineral acids.
Reagents that form neutral metal chelate complexes (5- or 6-membered ring
structures) are weakly acidic and contain one or more additional co-ordinating
sites (O, N or S atoms). Protons are displaced according to the general equation+ nHR = + nH++ xH 2 O (14)where HR is a weakly acidic and co-ordinating reagent (ligand), and the metal
ion Mn+has a formal valency n. The removal of hydrogen ions is necessary to
drive the reaction to completion, and pH control, which is essential, is achieved
by buffering the aqueous solution. It is sometimes possible to improve the selec-
tivity of the procedure by adding an additional reagent, known as a masking
agent, that reacts preferentially with one of the metals to form a nonextractable
water-soluble complex. Typical masking agents include EDTA (ethylenedi-
aminetetraacetic acid), citrate, tartrate, fluoride, cyanide and thiourea. Solvents
commonly used to extract metal chelate complexes include trichloromethane, 4-
methyl-pentan-2-one and methylbenzene.
Electrically neutral ion-association complexes consist of cationic (positively
charged) and anionic (negatively charged) species that form an overall neutral
aggregate extractable by an organic solvent. Either the cations or the anionsMRn
extractableMn+(H 2 O)x
not extractable114 Section D – Separation techniques
Table 1. A selection of reagents and extraction systems for the solvent extraction of
metals
Reagent Type of metal complex
8-Hydroxyquinoline (oxine) Neutral metal chelate complexes,
Di-alkyldithiocarbamates extractable into organic solvents. Intense
e.g. sodium diethyldithiocarbamate color of many facilitates colorimetric
(NaDDTC) determinations.
1,10-Phenanthroline (o-phen) Ion-association complexes. Metals as
2,9-Dimethyl-1,10-phenanthroline cationic or anionic chelated complexes
(neocuproine) extracted with suitable counter ion.
Ethylenediaminetetraacetic acid (EDTA)
Oxonium systems: Ion-pairs with anionic metal halide or
i.e. protons solvated with alkyl ethers, thiocyanate complexes. Chloride complexes
ketones, esters or alcohols extractable from strong HCl solutions.