Effect of Resin Capacity

The capacity factors for metal cations were measured with 0.75 M perchloric acid eluent on resins of 6.1,13.8, and 24.9 pequiv/g exchange capacity. The results, shown in Table 5.9, show a substantial increase in capacity factors with increased resin capacity. According to Eq. 5.19, the logarithm of k should vary linearly with the logarithm of resin capacity, C. Linear plots are indeed obtained but the slopes are lower than the theoretical values and they vary from one metal ion to another. 

An explanation of these observations might be that sulfonation of the resins proceeds inwardly from the outside of the resin beads. Thus, resins of higher capacity are apt to have thicker sulfonation layers. The microscopic selectivity coefficient may well change in various parts of the sulfonation layer. The selectivity coefficients measured would be an average of different microscopic values and might not follow the behavior predicted by Eq. 5.19. 

Cation Eluent HCIO4 NaC104 EnH-% PhenH z Mg(C104)2 

---

This last equation is very fundamental for ion chromatography. It predicts a linear decrease in log k as log E is increased. At fixed eluent concentration, the effect of resin capacity on log k may also be predicted. The preceding equations are derived for equihbria where the two ions S and E both have the same charge. A plot of log k against log [E] will have a slope of -1. If S is divalent, such as sulfate, and E has a 1- charge, the slope of the plot will be -2. The predicted slope will be -1/2 if S is 1- and E is 2-. 

---