ION EXCHANGE RESINS By T. R. E. KRESSMAN, ProD., D.I.C., F.R.I.C.* A lecture delivered before the Society on Friday, March 1 st, 1957. IN Ta•s account of ion exchange resins it is proposed to discuss the nature and properties of the resins and the principles underlying their action, and then to indicate how these properties can be applied to a few specific problems. This is a better approach than trying to present simply a cata- logue of applications, because if the principles of the resins are known it is comparatively easy to see how they can be applied to one's own particular problems. An ion exchange resin can be defined as an insoluble organic polymer containing labile ions that will reversibly exchange with other ions in a surrounding solution. The resins were discovered by Adams and Holmes in 1935. • The first of these contained phenolic OH groups leading to cation exchahgers, and aromatic amino groups leading to anion exchangers. The exchange of ions took place through these groups thus R.OH + NaC1 ,_2 R.ONa + HC1 R.Nt{• + HC1 r -• R.NH•C1 The capacities of these materials were low for two reasons' 1. The concentration of exchange groups in the resins was not very high (obviously a high capacity will result from a high concentration of groups). 2. The resins behave as weak acids and bases so that the equilibria lie rather to the left. Even when they were used in the form of columns and the reaction product--the HC1, say--was removed as fast as it was formed, the unfavourable equilibrium still resulted in a low working capacity. Development of the resins, therefore, followed the lines of increasing the strength of the exchange groups and of increasing the number of the groups per unit weight of resin. Increasing the strength was a comparatively simple matter and cation resins, for example, were quickly developed that contained SO •H groups, while anion exchange resins containing aliphatic amino groups followed shortly afterwards. Later, resins containing rather less strongly acidic groups in the form of COOH groups were developed and still later artion exchange resins containing strongly basic groups--quaternary ammonium groups--appeared. Thus each type of exchanger--cation and anion--now has its counter- part of weakly and strongly ionised groups, and their properties can be illustrated by their titration curves (see Figs. 1-4). It should be noted that * The Permutit Company Limited, Chiswick, W.4. 212

ION EXCHANGE RESINS 213 •H Fig. 1. Titration curve of weakly basic resin. 2 Fig. 2. Titration curve of weakly acidic resin. Fig. 3. Titration curve of strongly basic resin. pH Fig. 4. Titration curve of strongly acidic resin. these are roughly mirror images in pairs, and the two anion exchange resins behave on the one side of neutrality exactly as the two cation resins behave on the other side. , The sulphonic resins are used where it is required to exchange all cations in solution for, say, hydrogen ions because the reaction R.SO•H + NaCI• R.SO3Na + HC1 goes sufficiently far to the right that if we work in a column so that the HC1 is removed as fast as it is formed, complete exchange will occur and the reaction can be written in effect R.SO•H + NaC1---•R.SO•Na + HC1