Studies on Imidazole Derivatives as Chelating Agents. II. Spectrophotometric and Potentiometric Investigations of Acid-base Equilibria of Azoimidazoles

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Spectrophotometric and potentiometric investigations of the acid-base equilibria of the following azoimidazoles in aqueous media were carried out with a view to get some basic knowledge for their applications to chelatometric titrations as metallochromic indicators and for the calculations of the stability constants of their metal chelates : 1-[4(5)-methyl-5(4)-imidazolylazo]-2-naphthol ; 2-[4(5)-methyl-5(4)-imidazolylazo]-4-meth-oxyphenol; 4(5)-methyl-5(4)-(p-dimethylaminophenylazo) imidazole ; 3-dimethylamino-6-[4(5)-methyl-5(4)-imidazolylazo] phenol. pH Titrations of these azoimidazoles in 50v/v% dioxane-H_2O at 25±0.1°afforded their acid dissociation constants (pKa). By comparing with the pKa values of 2-phenyl-azoimidazole (3.2) and 2-methyl-4(5)-phenylazoimidazole (4.5), assignments of the pKa values, pK<N__H>+_(CH3)_2,pK__<NH+>, pK__<OH>, and pK__<NH+>, which refer to the pKa values of dimethyl-amino group, pyridine nitrogen, hydroxy group, and imino group, respectively, were attempted : pK<N__H>+_(CH3)_2< 2; pK__<NH>+=3.8〜5.1; pK__<OH>=11.0〜11.8; pK__<NH>>12. The pK__<NH+> of I and the pK<N__H>+_(CH3)_2,pk__<NH+>, an dpK__<NH> of III were also obtained spectrophotometrically by plotting the absorbances of I (50 v/v% EtOH-H_2O) and III (2 v/v% EtOH-H_2O) against pH.
社団法人日本薬学会の論文
1966-09-25