AbstractSelectivity numbers for a synthetic cation exchanger of the carboxylic type (IRC-50) and the natural cation exchanger, attapulgite clay mineral, were determined for different pairs of cations. The results in IRC-50 for Na+-K+, Na+-Rb+, and Na+-Cs+ systems showed no marked selectivity. The resin phase was considered to act as a concentrated salt solution. Hence, differences in the selectivity numbers (K) were attributed to the differences in the activity coefficients in the concentrated resin-phase solution. In the alkaline-earth systems (Mg++-Ca++, Mg++-Sr++, and Mg++-Ba++) the resin showed more selectivity between its members than between those of the alkali series. The selectivity numbers were functions of the percentage of Mg in the resin. In the Mg++-Ca++ system, Mg++ appeared more strongly held at low percentages of Mg, whereas in the Mg++-Ba++ system, Ba++ appeared more strongly held at all percentages of Mg in the resin. These results were explained on the basis of formation of slightly dissociated complexes of the alkaline-earth metals with the carboxylic groups of the exchanger. The results in attapulgite clay for the Na+-K+, Na+-Rb+, and Na+-Cs+ systems showed marked selectivity. The selectivity numbers were functions of the percentage of Na in the clay. The differences in selectivity were greater in the Na+-Cs+ system than in the Na+-Rb+ system and, in the Na+-Rb+ system, greater than in the Na+-K+ system. The results were broadly understood on the basis of the bifunctional character of the clay. The differences in the selectivity numbers for the alkaline-earth metals in the clay (Mg, Ca, Sr, and Ba) are small as compared with those of the alkali metals. In general, the resin behaved as a moderately strong electrolyte with respect to the alkali metals. It behaved like a weak electrolyte with respect to the alkaline-earth metals. Attapulgite clay behaved like a weak electrolyte with respect to both the alkali metals and alkaline-earth metals. The results of the selectivitynumber curves for the clay resembled those of a bifunctional cation exchanger and a high cross-linkage resin.
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