The last several years have seen a reawakening appreciation of the value of solvent extraction separations processes both on analytical and process scales. Separations by solvent extraction are simple, convenient, clean, and applicable equally well to carrier-free tracer and macro amounts of materials. They usually require apparatus as uncomplicated as a separatory funnel and may readily be adapted to remote handling procedures.A variety of separations techniques have been applied to the problems of isolation and radiochemical purification of nuclides. General descriptions and recent surveys of radiochemical separations can be found in references (1) through (5). Although all of the separation methods mentioned are of great importance to the radiochemist, solvent extraction is particularly well suited to the purification of many radioisotopes since, after several extractions of a substance, the final product is usually relatively free from extraneous im purities which may be present when other separation procedures are em ployed. Also, if the isotope of interest has a short half life, a rapid separation is essential and extraction procedures have proved invaluable. Most of the extractions employed in radiochemistry are based on previously developed conventional analytical extractions, and the recent book by the authors (6) offers a comprehensive treatment of the subject of extraction as applied to inorganic analysis. In addition to covering the principles and practical as pects of the technique, procedures for the extraction of the elements are presented. The review by the authors (7) in Analytical Chemistry surveys the latest developments in the field. CLASSIFICATION OF SOLVENT EXTRACTION SYSTEMSMost metal salts are strong electrolytes that are relatively insoluble in organic solvents and soluble in water because of its ability to solvate the ions as well as because of its high dielectric constant. Since it is usually necessary to transform the hydrated metal ion into an uncharged complex in order to extract it into an organic solvent, it is convenient to classify metal extraction systems on the basis of the nature of the extractable metal 1 The survey of literature pertaining to this review was completed in December, 1958.2 Among the abbreviations used in this chapter are: EDTA (ethylenediamine tetraacetic acid); HDGP (dioctylphenyl orthophosphoric acid); TBP (tributyl phos phate); TTA (thenoyltrifluoracetone).

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