Synergistic effects in solvent-extraction systems based on alkylsalicylic acids .2. Extraction of nickel, cobalt, cadmium and zinc in the presence of some neutral N-, O- and S-donor compounds

Nine alkyl- and dialkyl-substituted salicylic acids were prepared and their solvent extraction behaviour towards divalent nickel, cobalt, cadmium and zinc in nitrate media was examined, both in the absence and in the presence of some neutral N-, O- and S-donor compounds (n-octyl 3-pyridinecarboxylate and octanal oxime as N-donors, tri-n-butylphosphine oxide as O-donor, and tri-n-butylphosphine sulphide as S-donor). It was found that the pH(50) values for extraction of these metals by the salicylic acids themselves can be correlated with the respective pK(a) and steric parameters of the extractants. Addition of the N-donor compounds caused synergistic shifts in the pH(50) values for all four metals, these being especially marked for nickel (up to almost 3 pH units). Addition of the S-donor compound caused substantial shifts only for cadmium (up to 2.5 pH units), and the O-donor only for zinc and cadmium (up to 1.5 pH units). The size of the synergistic shift depends on the identity of the alkylsalicylic acid, and can be correlated with variations in steric and electronic factors through the series of compounds studied. Most significantly, the shifts increase with increasing steric bulk of the alkylsalicylic acid used. Continuous variation and saturation loading methods were used to show that the complexes extracted by mixtures of 3,5-diisopropylsalicylic acid (HA) and the neutral ligands (L) have the probable stoichiometries MA(2)L(4) (M = Ni, Co, Cd) and ZnA(2)L(2) when L = octanal oxime; MA(2)L(2) and MA(2)(HA)(2)L(2) (M = Cd, Zn) when L = n-octyl 3-pyridinecarboxylate; CdA(2)(HA)L(3) and CdA(2)(HA)(2)L(2) when L = tri-n-butylphosphine sulphide; and ZnA(2)L when L = tri-n-butylphosphine oxide.