Abstract
Abstract The acid dissociation constants of the three isomeric phosphonophenylazochromotropic acids 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. The stability order with respect to the ligands is as follows: ortho->meta->para-, and the stability constant of each ligand decreases in the order: Mg>Ca>Sr>Ba; this order is parallel to the reciprocal of the ionic radii of the metal ions. From a comparison between the chelate stability of o-phosphonophenylazochromotropic acid and that of Neo-Thorin, it has been concluded that the magnesium ion forms a more stable chelate bond with the phosphonic group than with the arsonic group.
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