Ether complexes, solvent extraction metals

Crown ethers have low soluhiUty in water hut have considerable solubility in organic solvents. Crown ether complexes of alkali metal cations are therefore quite stable in organic phases. The anions of the alkali metal cations present in the aqueous phase, Y, are simultaneously extracted into the organic phase the complex in the organic phase may be represented as (crown-M)+ y. Due to the likely absence of solvation of such anions in the organic phase, they are likely to be highly reactive. Nevertheless, crown ethers and similar host compounds have been successfully incorporated into organic solvents to extract aUcali metal ions/salts from aqueous solutions. 

In liquid-liquid extraction by solvation, the solvent itself takes part in complexation of the metal ion. This is brought about by donation of electrons by the donor atom present in the solvent molecule to the coordination sphere of the metal ion. Such an association of the solvent molecules with the metal ion results in the formation of a solvated complex that is extractable in the solvent. Thus, for example, octanol, Cg H17-OH, extracts Co2+ in the presence of perchlorate ion as [Co (C8 H17-OH)] (C104)2. Extraction by solvation generally employs oxygenated solvents such as alcohols, ethers or ketones or neutral organophosphorus compounds. These solvents stand the competition from water for solvation of the metal ion successfully because of the higher availability of electrons from their donor atoms. 

Cations that complex easily generally form stable complexes with oxygenated organic compounds, such as diethyl ether, methyl isobutyl ketone, and tributyl phosphate (TBP). The purification of uranium by solvent extraction of hexavalent uranium from nitrate solutions, with TBP forming U02(N03)2 2TBP, was described in Chap. 5. These metals are extracted most easily from aqueous solutions free of the more hi y complexing anions, such as F, P04, or S04 . 

It is interesting to remember that it was complexes of this class, neutral ion pairs, which were the first compounds of transition metals to be solvent-extracted. One of the earliest recorded uses of solvent extraction is by Rothe, who, in 1892, exploited the quantitative extraction of Fe(III) from an HCl medium into diethyl ether as the [FeCl4] [HO(C2H5)2] ion pair. The Fe ion has a high-spin d electron... 

Although non-ionic surfactants would appear to be unlikely candidates as complexing agents for metal ions, the interaction of some polyoxyethylene glycols with metal ions has recently attracted interest [90,91]. The reaction of non-cyclic polyoxyethylene derivatives with alkali and alkaline earth metals has been studied by means of solvent extraction of their thiocyanates or iodides. Polyoxyethylene dodecyl ethers with more than 7 ethylene oxide units were able to bind potassium ion in the water phase and to transfer the complexed salt to the organic phase the extracting power of Ci2Eg was about one sixth of that of a crown ether [92]. Some results are shown in Fig. 11.13. 

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