Immobilization of crown ether carboxylic acids on silica gel and their use in column concentration of alkali metal cations from dilute aqueous solutions

Eight crown ethers with pendent carboxylic acid groups are immobilized on silica gel and utilized for column concentration of alkali metal cations from dilute aqueous solutions. The column concentration selectivity and efficiency are found to be strongly influenced by (1) the cavity size of the crown ether unit, (2) conformational positioning of the proton-ionizable side arm with respect to the crown ether cavity, and (3) capping of residual silanol surface groups with trimethylsilyl functions. By use of a chromatographic stripping technique, selective column concentration of Na+, K+, (Rb+ and Cs+), and Cs+ by different functionalized silica gels has been achieved.