The Dissociation Phenomena of o-Phenol- and m-Chloro-o-phenolazochromotropic Acids and the Formation Constants of Magnesium and Calcium Chelates

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Abstract
The acid dissociation constants of o-phenol-, m-chloro-o-phenol- and o-anisolazochromotropic acids have been measured by potentiometric titration. The measurements have been carried out at 25.0±0.1°C and μ=0.10 (by KNO3). From these constants, the dissociation of the hydroxyl groups in the reagents has been discussed. The phenolic hydroxyl group was first dissociated, and then one of the naphtholic hydroxyl groups; the other naphtholic group was not dissociated under ordinary experimental conditions. The pKa1 and pKa2 values were 7.60 and 10.60 for o-phenolazochromotropic acid, and 7.56 and 10.35 for m-chloro-o-phenolazochromotropic acid. o-Phenol- and m-chloro-o-phenolazochromotropic acids formed 1 : 1 chelates with calcium and magnesium. The formation constants were measured potentiometrically; the logK’s for calcium were 5.01 and 5.22, and for magnesium, 6.15 and 6.22.
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