NEW CHELATING-AGENTS FOR COLORIMETRIC DETERMINATION OF CALCIUM

Two new calcium chromoionophores 2 and 3 are described and their poten tial application to colorimetric determination of calcium is assessed. Chromogenic compound 2 bears two acetic acid chelating groups and a 4 -(4'-nitrophenyl)azophenol chromophore. Its complexation of Ca2+ at pH 9.0 is accompanied by a 109-nm bathochromic shift and increase in the absorptivity. Triaryl chromoionophore 3 bears two phosphonic acid che lating groups and the same azophenol chromophore. Compound 3 exhibits a 107-nm bathochromic shift of the absorption maximum, an increase in the absorptivity upon complexation with Ca2+, and good selectivity for Ca2+ over Mg2+ at pH 7.0. Both chromoionophores exhibit linear respon ses from 0 to 4 x 10(-3) M Ca2+.