Reverse reaction chelation/extraction

Certain lead(II) complexes were shown to display reversible redox behavior on the CV scale. Examples are Pb(II) complexes of 2(o-hydroxyphenyliminomethyl)-pyrrole and 2(o-hydroxyphenyliminomethyl)-thiophene. Their stability constants were also determined by polarography140. Anodic exchange reactions of extracted metal chelates were carried out with 1-pyrrolidinecarbodithioic acid in isobutyl methyl ketone141. 

In the second method, extracted metal-ligand complexes in SF were dissociated in a back-extraction device containing nitric acid solution by a reverse reaction of chelation/extraction (13,27), which is generically shown as follows ... 

A study of the kinetics of the extraction of Ga3+ from 0.1 mol 1 1 aqueous C104 solutions into CHC13 using thenoyltrifluoroacetone (HA), and of the back-extraction of the tris-chelate GaA3, shows that the rate-determining step in the former process involves association of the species GaOH2+ with A in the aqueous phase.518 The back-reaction is thought to be the exact reverse. 

In mass-transfer-controlled systems in which extensive complexing or association takes place in the bulk phases, a proper mass transfer model must account for transport of all species. Otherwise, the transport model will not be consistent with a chemical model of phase equilibrium. For example. Fig. 8.4-4 indicates schematically the species concentration profiles established during the extraction of copper from ammonia-ammonium sulfate solution by a chelating agent such as LIX. In most such cases the reversible homogeneous reactions, like copper complexation by ammonia, will be fast and locally equilibrated. The method of Olandei can be applied in this case to compute individual species profiles and concentrations at the interfiice for use in an equilibrium or rate equation. This has been done in the rate analyses of several of the chloride and ammonia systems cited above. ... 

Electrodeposition (reduction of metal ions to metal by electrons at an electrode), reverse osmosis (see Section 5.10), and ion exchange are frequently employed for metal ranoval. Solvent extraction using organic-soluble chelating substances can be used to remove many metals. Cementation, a process by which a metal deposits by reaction of its ion with a more readily oxidized metal, can... 

---