Studies of reagents for spectrophotometric measurement of ionic magnesium

Four mono-azo aryl dyes having the o-carboxy-o′-hydroxy functionality were investigated as reagents for the measurement of ionic Mg 2+ in solutions containing complexing ligands. 2-(2′-Carboxyl-1′-benzeneazo)-1,8-dihydroxynaphthalene-3,6-disulfonic acid came closest to the ideal of having absorbance unaffected by pH over the pH range of interest and of differentiating between Ca 2+ and Mg 2+ complex formation. Ligand acid dissociation constants and formation constants for the metal-ligand complexes were measured.