Chapter 14 - Ion exchange capacity

Abstract

This chapter explains the ion exchange capacity in zeolites. In a traditional aluminosilicate zeolite the source of the ion exchange capacity is the extent of isomorphous substitution of Al for Si in the tetrahedral framework. The theoretical exchange capacity thus can be derived from the elemental composition. To estimate the CEC (cation exchange capacity, meq/g) in a natural zeolite it is usual to observe the uptake of the ammonium cation at room temperature when equilibrium conditions are known to have been attained in the presence of a IM ammonium salt solution. When synthetic materials are to be characterized, prior careful elemental analysis will provide the expected, theoretical cation capacity. The most sensitive analytical technique available is to use radioisotopes to prelabel the A cation and follow its replacement by increase in solution activity with time. (Isotope dilution technique). This will be possible for the as synthesized cations Na, K, Rb, Cs, Ca, Sr and Ba. In the absence of radiochemical facilities, flame photometry, atomic absorption are perfectly adequate.