214 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS These resins are also used for ordinary neutral exchange, e.g., sodium for calcium or potassium. The weakly acidic resins--the carboxylic resins--show a very low capacity towards neutral salts but their hydrogen ions can be exchanged for the cations of salts of weak acids as, for example, bicarbonates. This is useful in the treatment of water for cooling systems where it is required to remove the scale-forming hardness (bicarbonates of calcium and magnesium), but where the permanent hardness (chl?r•ides and sulphates of calcium and magnesium) is not particularly undesirable: 2Resin-COOH + Ca(HCO3)2--• (Res•n-COO)2Ca + 2H•O + 2CO, Resin-COOH + CaCI• --• no action Weakly basic anion exchange resins will take up strong acids like the mineral acids and, to a lesser extent, weak organic acids like acetic acid. They have no capacity in alkaline solution. The strongly basic resins, on the other hand, will take up not only strong acids and the ordinary weak acids to a high capacity, but such feeble acids as H•S, boric acid, CO•, silica and phenols. They will also exchange anions in neutral solution just as the strongly acidic cation resins will exchange cations in neutral solution: Resin. C1.- + NaNO• • Resin. NOs- + NaC1 and will exchange OH- ions just as the sulphonic resins will exchange H + ions: Resin. OH + NaNO• • Resin. NOa + NaOH The earlier resins were prepared by condensing formaldehyde with a phenol • or a phenol sulphonic acid s to yield cation exchangers, and with an amine 4 to yield an anion exchanger. Generally the condensation was per- formed with a compound cohtaining the active exchange groups that the final resin was desired to carry. More recently, however, resins have been prepared by polymerising unsaturated compounds like styrene and then attaching the appropriate groups to the polymer by suitable chemical treat- ment. All four types have been prepared in this way. The nature of these polymers is such that, in general, a larger number of active groups can be introduced into them per unit weight than into the older type of condensation polymers. This means that the newer types have higher capacities than the old and in addition they are more stable. Since the ion exchange groups (NH•, NR4, SO,H, COOH) are hydrophilic groups, the tendency is for them to pass into solution when the resin is brought into contact with water. As the groups are attached to the polymeric resin structure, they tend to draw the whole resin into solation also. To prevent this, the polymer structure' has to be tied !ogether with cross links in order to form a three-dimensional molecule. These cross links restrain the tendency of the resin to dissolvd and the result is that the ion exchange

•o• •XCI•A•G• R•S•S 215 resin merely swells when the dry substance is placed in water. If the degree of cross linking is too low a further change in volume will occur during the exchange of one ion for another if the cross linking is too high the exchange of ions becomes undesirably slow. In practice sufficient cross linking is used to strike a balance between these two opposing effects. Ion exchange in resins is not merely a surface phenomenon and the whole of the•particles is available for exchange, the exchanging ions being able to travel to the centres of the particles in a comparatively short time. The ions reach the centre by moving along the water-filled pores of the resin and these pores are of molecular dimensions, generally between about 5 and 15 •k diameter according to the resin. Ions larger than this cannot pass down the pores and a limited exchange capacity is observed with such ions. In fact, this can be exploited to separate small ions from large ones. The position of equilibrium is determined by the "affinities" for the resin of each of the two ions taking part in the exchange. For instance, in the exchange between H + on a strongly acidic resin with Na + in solution the equilibrium lies rather more than half-way to tb.e right, indicating that the sodium /on has a slightly higher affinity for the resin than the hydrogen ion. By measuring the position of equilibrium reached with a resin in some standard ionic form--the hydrogen form, for example--and solutions con- taining a series of salts with other cations, a table of affinities can be con- structed. Similarly, by taking an anion exchange resin in, say, the chloride form, an• allowing it to come to equilibrium with a series of salts containing ott•er anions, a table of affinities for anion exchange can be obtained. The order of affinities of some cations from 0-1N chloride solution for a phenol sulphonate resin* is: Hg ++ Li + H + Na + K+= NH4 + Cd ++ Ag+(NOa) Mn++ Mg++: Zn ++ Cu++ _• Ni++ Co ++ Ca ++ • Sr ++ Pb++(NO•) Ba ++ A1 +++ Th++++(NO•). The order of affinities of some anions for a quaternary ammonium anion exchange resin• is: Fluoride = hydroxide acetate formate bicarbonate chloride nitrite bisulphite cyanide bromide nitrate bisulphate iodide salicylate. As already mentioned, a weakly basic anion exchange resin in the free base form will not take up anions appreciably from neutral salts but only from free acids. The affinities under these circumstances will be largely determined by the strength of the acid (as measured by its, dissociation constant) and its basicity, and the order of affinities of a few acids for a resin of this type is HC1 = HNO• H•SO• H•P04, and benzoic oxalic formic acetic: citric salicylic. If the resin is converted to an