The authors reported in a previous paper on the extraction and separation of divalent metals, such as Cd
2+, Pb
2+, Ni
2+, and Zn
2+, by using an ionic liquid, 1-butyl-3-methylimidazolium hexafluorophosphate ([Bmim][PF
6]), including the extractant of 2-pyridinealdoxime (2-PA). However, the extraction equilibrium of Ni
2+in the 2-PA/[Bmim][PF
6] extraction system remained unresolved. Then, the present study was undertaken to elucidate the extraction mechanism of Ni
2+ in the 2-PA/[Bmim][PF
6] extraction system, as well as the effect of masking reagent on the metal extraction. The chemical form of the extracted Ni-complex was examined by analyzing the slope for the relationship between log
D and the pH and between log
D and log [2-PA] in [Bmim][PF
6]. Taking into account the facts that the slopes for the above-mentioned relationships were both close to 2.0 and the Ni
2+ ion was mainly extracted in the acidic pH region in the vicinity of a p
Ka1 of 2-PA (3.73) the extraction equilibrium of Ni
2+ with 2-PA into [Bmim][PF
6] phase was concluded to be Ni
2+(aq)+2H
2L
+(aq)+2Bmim
+•PF
6−(org)↔Ni(HL)2
2+•2PF
6−(org)+2H
++2Bmim
+(aq), where HL and Bmim•PF
6 denote 2-PA and 1-butyl-3-methylimidazolium hexafluorophosphate and the subscript (org) and (aq) means the organic phase and aqueous phase, respectively. That is, the pyridinium form of 2-PA, H
2L
+, in the aqueous phase reacted with the Ni
2+(aq) ion to form a nickel-2-PA complex cation, Ni(HL)
2+(aq), while repelling two H
+ ions. Then, the formed Ni(HL)
2+(aq) ion was distributed into the organic phase by ion exchanging with the 1-butyl-3-methylimidazolium cation in the organic phase.
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