The Chelate Formation of (o-Substituted Phenylazo)chromotropic Acids with Alkaline Earth Metals. II

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The acid dissociation constants of phenylazochromotropic acid and its o-substituted derivatives with -CH3, -OCH3, -COCH3, -COOH, -CH2COOH, -COCOOH, -OCH2COOH, and -CH(OH)COOH in the ortho position to the azo group, and the stability constants of their 1 : 1 metal chelates with alkaline earth metals have been measured by potentiometric titation. The measurements have been carried out at 25°C and at μ=0.10 with KNO3. The effects of these substituents on the acid dissociation constants of the reagents and on the stability constants of these chelates were discussed. The pKa value for the naphtholic proton of the o-substituted reagents was larger than that of phenylazochromotropic acid, and the stability constants of these chelates with alkaline earth metals decreased as follows: Mg>Ca>Sr>Ba, while for the chelates of the o-OCH2COOH reagent the order was Ca>Mg>Sr>Ba, and for the o-CH(OH)COOH reagent it was Mg≈Ca>Sr>Ba. The plots of the stability constants, log K1 against the sum of the acid dissociation constants, pKA(=pKa1+pKa2), were approximately linear except for the above two reagents with calcium, strontium, and barium. From the results obtained, the o-OCH2COOH reagent that behaves as a quadridentate ligand forms a more stable chelate with calcium, strontium, and barium, and the effect of the -OCH2COOH group, especially that of the coordination of the -O- atom, is remarkable in its calcium chelate.
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