Extraction of lead picrate by 18-crown-6 ether into various diluents: Examples of sub-analysis of overall extraction equilibrium based on component equilibria

Extraction constants for lead picrate, PbPic2, with 18-crown-6 ether (L) from aqueous solutions at a narrow range of ionic strength (I) into eleven diluents were determined at 298 K by AAS measurements. These constants were defined as Kex = [PbLPic2]o/([Pb2 +][L]o[Pic−]2) and Kex ± = [PbLPic+]o[Pic−]o/([Pb2 +][L]o[Pic−]2), where the subscript “o” denotes the organic phase (or diluent). The ratio, Kex/Kex ±, gave an ion-pair formation constant for PbLPic+o+Pic−o⇌PbLPic2,o at the averaged value of ionic strength (Io) of the organic phase. A plot of log Kex ± versus log KD,Pic yielded a straight line with the slope of 2.1 in a series of the diluents employed; KD,Pic refers to an apparent distribution constant of Pic− between the aqueous and organic phases and its values were experimentally determined. The slope indicated that Kex ± is mainly controlled by the square of KD,Pic when a thermodynamic cycle is assumed. The extraction constant based on the cycle was expressed as Kex ± = (KD,Pic)2KD,MKML,orgK1,org, where KD,MKML,orgK1,org denotes the product of other component equilibrium-constants. Moreover, the relation of log KD,Pic = alog (Io/I) + b was experimentally found at a ≈ 1 and then an example of b was expressed theoretically. It was shown that Pb(II) can be separated from mixtures with Cd(II) in unit operation.