Acid-base and optical properties of neutral red in the presence of ionic and nonionic tensides

Abstract

Changes in the optical and acid-base characteristics of neutral red in the presence of hexadecylpyridinium bromide, hexadecyltrimethylammonium bromide, α-carbethoxypentadecyltrimethylammonium bromide (Septonex), sodium dodecyl sulphate, Triton X-100, and BRIJ 35 in submicellar and micellar concentrations were studied spectrophotometrically. The conditional dissociation constant of neutral red pKai* depends on the kind and concentration of tenside, kind and concentration of inert electrolyte, and concentration of the cationic dye. Changes in the pKai* values are described in terms of formation of ionic associates of the dye with the tenside and of the inert electrolyte ions with the tenside. The validity of equations for the dependence of pKai* on the logarithm of tenside concentration was proved experimentally over a wide region of cationic tenside concentrations. The changes in pKai* can also be partly accounted for by changes in the nature of the solvent. A titrimetric method based on the changes in the conditional dissociation constant of neutral red is suggested for the determination of tensides in aqueous solutions using photometric end point indication.