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-DONOR, O-DONOR AND S-DONOR COMPOUNDS

Nine alkyl- and dialkyl-substituted salicylic acids were prepared and their solvent extraction behaviour towards divalent nickel, cobalt, ca dmium 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-butylphosph ine oxide as O-donor, and tri-n-butylphosphine sulphide as S-donor). I t was found that the pH(50) values for extraction of these metals by t he 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 un its). 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 ca dmium (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 com pounds studied. Most significantly, the shifts increase with increasin g steric bulk of the alkylsalicylic acid used. Continuous variation an d saturation loading methods were used to show that the complexes extr acted by mixtures of 3,5-diisopropylsalicylic acid (HA) and the neutra l 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 Z nA(2)L when L = tri-n-butylphosphine oxide.