The Chelate Formation of o-Substituted Phenylazochromotropic Acids with Alkaline Earth Metals

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Abstract
The acid dissociation constants of phenyl, o-, m-, and p-sulfophenyl-, o-carboxyphenyl-, o-phenol- and o-arsonophenylazochromotropic acid and the formation constants of their 1:1 metal chelates with alkaline earth metals have been measured by potentiometric titration. The measurements have been carried out at 25°C and μ=0.10 (KNO3). The influence of these substituents on the acid dissociation constants of the reagents, and on the chelate formation constants was discussed. The dissociation constants of the naphtholic protons of the o-substituted reagents increase as follows: o-OH<o-AsO(OH)2<o-COOH<o-SO3Na<o-H. The metal chelate stabilities of o-phenol-, phenyl- and o-arsonophenylazochromotropic acids decrease as follows: Mg>Ca>Sr>Ba, while for chelates of o-carboxyphenyl- and o-sulfophenylazochromotorpic acids the order is Mg>Ca>Sr≈Ba. The sulfonic group in o-sulfophenylazochromotropic acid, in spite of its remarkable electron-withdrawing effect, depress the acid dissociation, and form more stable barium chelate than phenyl-, m- and p-sulfophenylazochromotropic acid do. The plot of the formation constants, logK, against the sum of the dissociation constants, pKA(=pka1+pka2+pka3), was approximately linear.