Some Recent Developments in Methods of Separation

Separation methods are being applied more extensively than ever before, particularly in trace analysis. Some of the more interesting procedures which have recently been proposed in solvent extraction and ion-exchange chromatography are reviewed, together with other new procedures for separation based on classical techniques. The possibilities of gas chromatographic separation of metal chelates are briefly discussed. Despite notable advances in the development of selective reagents, masking agents and selective instrumental methods, the literature on methods of separation is still expanding rapidly. This is partly because the special circumstances which demand prior separations are multiplying, and partly because more efficient separation methods have become available and are being applied widely. Solvent extraction seems to be the preferred approach and some notable advances have been made during the last few years; however, other methods have not been neglected and many possess interesting features. This account is influenced by the interests of the author, but the selected methods which are presented probably give a fair idea of recent progress. Particularly noteworthy is the fact that efficiency of separation is so often checked nowadays by radiochemical methods. SOLVENT EXTRACTION A new departure in liquid—liquid extraction has been described: liquid— liquid extraction is followed by solid—liquid extraction. The determination of copper as its 8-hydroxyquinoline complex serves to illustrate the technique'. Reagent pellets are prepared by melting together 8-hydroxyquinoline and diphenyl. The solid pellet is dropped into the test solution, which is heated to above the melting-point of the pellet (90°C), and then the container is well shaken. It is allowed to cool and the solidified pellet now contains the copper complex. After removal of the aqueous layer, the pellet is dissolved in chloroform and the absorbance is measured. This procedure overcomes difficulties associated with slow attainment of the distribution equilibrium and with poor separation of phases. Other metals which have been determined by such processes are zinc and magnesium (as oxinates with naphthalene as solvent)2.