216 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS ionic from other than the free base form, say the chloride form, the order of affinities is different',: fluoride chloride bromide •- iodide---- .acetate phosphate nitrate tartrate citrate chromate ,, sulphate hydroxide .... • APPLICATIONS The applications of the materials can be broadly listed under three headings: (1) Complete removal of ions from a solution: ,(a) 'undesirab!e ions from a valuable solution , (b) valuable' iohs from an otherwise valueless • Solution. I• I Separation and concentr•hon of electrolytes. Substitution of specific ions it/solution by other ions: Removal of undesirable •ions, from a valuable solution can be achieved comparatively simply if the valuable substance is not itself to, ken up by the exchanger, as will be the case if it is non-ionic or it is present.as a com- plex or if it is ionised but is too large to pass down the resin pores. • For the complete. removal of electrolytes from •a solution the demi•neralisation technique, using first a hydrogen exchanger and then an union exchanger to remove the acids so .formed, would be used. Alternatively,, the,mixed bed technique can be used, in which the cation and union exchangers are, mixed together and the solution to be demineralised.is .flowed through• a single column _of the mixture. Examples are the removal of electrolytes ,from sugar solutions, from glycerine solutions and from solutions of other non- ionised organic substances. Mercuri c .chloride is largely unionised in solution and impurities can be remove, d by passing the solution through a hydrogen exchanger• which .scarcely affects the mercury salt. The mixed bed technique is also useful for removing all the/suits pres•ent in tap water, and water conforming to the B.P. standard for distilled water can be obtained in this way. Indeed, in the latest edition of the Pharma- copoeia the previous item "distilled water" has now been' amended to "purified water," and this includes water produced from a mixed bed of ion exchange resinss., The removal of valuable' ions from an otherwise valueless solution can be carried out by using the appropriate resin which is chosen such that the valuable ions are taken up by the resin and the impurities pass through. On regenerating or eluting the resin' the cation resin with acid and the anionresinwith alkali--the valuable ions appear in the reienerant effluents in concentrated form and can be recovered, for example, by, a suitable chemical method. An example of this,is,the recovery of streptomycin on a carboxylic resia. The mould that produces st•reptomycin is grown in a nutrient. salt solution in the presence of air, and ,when it has finished its work it,is filtered off from the solution, which now contains th e streptomycin along with the inorganic nutrient salts. ,.This solution is passed. through a car•boxylic resin

in the sodium form which takes up the streptomycin and allows everything else to pass, through. The streptomycin is •then eluted: from. the column with dilute HC1 or dilute sulphuric acid to give streptomycin hydrochloride or sulphate respectively. The separation of ions is carried out by taking advantage of the different affinities of the various ions for the resin. This technique is used a great deal in chemical analysis, mainly in the analysis of inorganic substances since these are more frequently ionised than organic substances. However, organic acids and bases can be separated by using the appropriate resin. This is the principle of ion exchange chromatography. Perhaps the single field in which the most work has b•een carried out is that of amino acids. Some extremely elegant separations of amino acid mixtures containing as many as 50. amino acids have been carried out by Moore and Stein 9 in the U.S.A. and by Partridge •ø in this country. Generally a resin is used of finer particle size than that for ordinary simple ion exchange procedures where 16-50 mesh particles are. normal, and although sometimes 100-200 mesh material is used, most commonly it is less than 200 mesh. This is essential because only by the use of such fine particles combined with low flow rates can the near-equilibrium conditions be attained that are so essential to the production of sharp boundaries. Any of the techniques of frontal analysis, displacement development or elution can be used, just as in conventional chromatography on, say, alumina. In frontal analysis the solution containing the ions to be separated is flowed continuously through the column of resin, the ions of higher affinity displacing those of lower after an initial period in which all the ions are taken up--see Fig. 5. Only the. ion of lowest affinity A + is .obtair•ed pure, and in consequence the method is only used when one component (and that having the lowest affinity) is required 'pure. An example of its use is in the separa- tion of a weak and a strong acid, e.g., • acetic and hydrochloric, on a weakly basic resin. In displacement development the mixture of ions to be separated is absorbed'as a band at the top of the column. A displacing solution is then flowed through the column, this solution containing an exchanging ion that has a higher affinity for the resin than any component of the original rpixture. The ions ultimately move down and out of the column in the order of their increasing affinities, that •with the least affinity appearing firstwsee Fig. 6. This is the method used by Partridge. Its advantages are that the separated ions can be obtained in high concentration, and loadings as high as 50 per cent of the total capacity of the column can be used (i.e. the band extends half-way down the column). It can, accordingly, be used as • preparative method.