ION EXCHANGE RESINS 219 Elution analysis is similar to displacement development except that the eluting solution contains an ion of relatively low affinity for the resin. The ions move out of the column and the elution curve resembles Fig. 7. The separated components are always contaminated with the exchanging ions of the eluting agent. For this reason an eluting agent is chosen such that it can be easily removed from the solution. Ammonia is frequently used, and so is formic acid, or ammonium formate. This method is the one used by Moore and Stein, and it is particularly useful for the separation of micro quantities of ions. It is not really suitable for preparative work since both the concentration and the loading are somewhat low. Z ELUTING IONS VOLUME OF EFFLUENT Fig. 7. Effluent composition in elution analysis. The third heading--Substitution of Specific Ions in Solution by Other Ions--is generally simple and straightforward. The most common example is probably that of water softening, where calcium and magnesium ions are replaced in the solution by sodium ions 2R.SO3Na + CaCt•--(R.SO3)2Ca + 2NaC1 An important factor here is the relative affinity of the two ions in question. If the equilibrium lies strongly to one side, i.e., the ion initially on the resin has a high affinity relative to that of the ion in solution, complete exchange will rarely occur. For instance, it is impossible to form a calcium salt by passing a dilute sodium salt solution over a calcium resin. (It is, however, possible to displace calcium by the use of a concentrated sodium salt solution

220 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS because in this circumstance the relative affÉnities of sodium and calcium are reversed, and the sodium now has the higher affinity.) Likewise, it is impos- sible to form a solution of sodium phosphate by passing a dilute solution of sodium chloride over an union exchange resin in the ph6sphate form. This exchange'of one ion for another is useful for preparing salts and acids that cannot be' conveniently prepared by other methods. For a particular study, it was once required to prepare the tetra-ethyl-ammonium salt of Chlorazol Sky Blue. The quaternary ion was available as the bromide salt and the dye as the sodium salt. A solution of the quaternary bromide was passed through a strong base union exchgnger in the free' base form Resin. OH- + Net4Br-• Resin. Br- + NEt•OH and the dye salt through a sulphonic resin R.SOaH + Na-dye - R.SO,Na + H-dye and the free base and free acid solutions were then mixed until the pH was 7, as indicated by a pH meter. The direct method R.SO,H + NEt•Br-- R.SO•NEt4 + HBr R.SO•NEt• + Na-dye -- R.SOsNa + NEtcdye could not be used because the affinity of NEt• + is so high compared With that of sodium that the last reaction does not, in fact, occur quantitatively. A special case exists here when a solution of a free base is passed throhgh a hydrogen resin, e.g., R.SOaIt + NaOH -• R.SOaNa + H•O The effect is that the base has been completely removed. from the solution. Similarly, acids can be removed, wi,'th an union exchange resin R.NH2 + HC1 - R.NHaC1 In addition to these more-or-less standard applications there are a few somewhat specialised, 'applications, 'Mostly th•se depend on the simple replacement of one ion by another. For example, resins have been proposed as ingredients in antiperspirant and deodorant formulations. The resin will absorb fatty acids, ammonia and other products found in 'sweat, and they are mixed n a finely,powdered form with •a hydrophilic carrier sub- stance. A typical formulation is 20 per cent carboxylic resin (in the form of particles --200 mesh) and 15 per cent aluminJure phenol sulphonate dispersed in an oil-in-water emulsion. • Mixtures of cation and union exchange resins have also been proposed for use in the same way, both as deodorants and to relieve dermatitis resulting from contact with plants like poison ivy." The resins have also been suggested as carriers for medicaments in ointments. For example, ointments comprising a carboxylic resin containing