Stability Constants of Alkaline Earth Metals with o-Arsono-o′-hydroxy Azo Compounds

Abstract

Abstract The acid dissociation constants of the o-arsono-o′-hydroxy azo compounds, and their chelate stability constants with alkaline earth metals, have been measured by the pH titration method at an ionic strength of 0.10 and at 25.0±0.1°C. These azo compounds were synthesized by the coupling reaction of diazotized o-aminophenylarsonic acid with naphthol sulfonates. As the coupling components, 1,8-dihydroxynaphthalene-3,6-disulfonic acid (chromotropic acid), 2-hydroxynaphthalene-3,6-disulfonic acid (R acid), and 1-hydroxynaphthalene-3,6-disulfonic acid were used. o-Arsonophenylazochromotropic acid (Neo-Thorin) formed the most stable chelates. It was concluded that the stabilization of the metal chelate ring of the Neo-Thorin was due to a quasi-aromaticity of the six-membered ring which was produced by the hydrogen bond between two naphtholic hydroxyl groups of chromotropic acid. The metal chelate stability order, Mg>Ca>Sr>Ba, is parallel to the reciprocal of the ionic radii of the metal ions.