Quite a number of crown ethers have been prepared since the initial synthesis by Peterson [1]. Crown ethers or cryptands are known as selective ligands for various alkaline or alkaline earth metal ions and some of the cationic metal complexes are readily extractable from the aqueous into an organic phase. The selectivity principally depends on the cavity size within the ligand molecule [2-5]. Chromogenic complexing reagents that show a distinct colour ...
