New metallochromic indicators for compleximetric titration of calcium

Seven o,o'-dihydroxyazo dyes were synthesized and tested as metallochromic indicators for calcium: 1-(2-hydroxy-3-sulfo-5-chloro-1-phenylazo)-2-hydroxy-3-naphthoic acid (I), 1-(2-hydroxy-3-sulfo-5-chloro-1-phenylazo)-2-naphthol-3,6-disulfonic acid (II), 1-(2-hydroxy-6-sulfo-1-naphthylazo)-2-naphthol-3,6-disulfonic acid (III), 2,2'-dihydroxy-1, 1'-azonaphthalene-3,3', 6,6'-tetrasulfonic acid (IV), 1-(2-hydroxy-3-carboxy-1-naphthylazo)-2-naphthol-3,6-disulfonic acid (V), 1-(2,3-dihydroxy-1-naphthylazo)-2-naphthol-3, 6-disulfonic acid (VI), 1-(2,3-dihydroxy-6-sulfo-1-naphthylazo)-2-naphthol-3,6-disulfonic acid (VII). The acid dissociation constants and the formation constants of the calcium and magnesium chelates of these dyes were determined. The rate of the decomposition of these dyes and their calcium chelates in alkaline solutions was also examined. From the results of photometric and visual titrations of calcium in the presence of magnesium, indicators III and IV provide the sharpest end-points and the best stability in alkaline solutions.