A chromogenic reagent for calcium. The importance of ion-pairing in cation selection

Abstract

The chromoionophore 2 has been synthesised from the corresponding diaza 18-crown-6 in two steps in good overall yield, and the reagent 2 extracts calcium cations from aqueous solutions in the pH range 7–9 with moderately high selectivity as compared with sodium and with high selectivity as compared with other cations of biological interest; the series of apparently similar reagents 2, 3 and 4 shows very wide variation in Na⁺/Ca²⁺ selectivity.