ACID-BASE AND OPTICAL PROPERTIES OF NEUTRAL RED IN THE PRESENCE OF IONIC AND NONIONIC TENSIDES

Changes in the optical and acid-base characteristics of neutral red in the presence of hexadecyl­ pyridinium bromide, hexadecyltrimethylammonium bromide, cx-carbethoxypentadecyltrimethyl­ ammonium bromide (Septonex), sodium dodecyl sulphate, Triton X-lOO, and BRIJ 35 in sub­ micellar and micellar concentrations were studied spectrophotometrically. The conditional dis­ sociation constant of neutral red pK:1 depends on the kind and concentration of tenside, kind and concentration of inert electrolyte, and concentration of the cationic dye. Changes in the pK:1 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 pKii on the logarithm of tenside concentration was proved experimentally over a wide region of cationic tenside concentrations. The changes in pKil can also be partly accounted for by changes in the nature of the solvent. A titrimetric method based on the changes in the condi­ tional dissociation constant of neutral red is suggested for the determination of tensides in aqueous solutions using photometric end point indication. It has been known that tensides and coloured organic reagents form ionic associates of stoichiometric composition. The stable electroneutral associates are low soluble in water and in polar organic solvents and well extractable into low polar organic solvents. The formation of ionic associates is employed for the extraction-photo­ metric determination of tensides 1 or their determination by a two-phase titration with visual or photometric end point indication using dye indicators2 • Stoichiometric ionic associates are solubilized at higher total concentrations of tensides. This is accompanied by appreciable changes in the acid-base and optical properties of the coloured organic reagents. The interaction is associated with shifts of the absorption bands of the reagent or appearance of a new absorption band of the ionic associate. The conditional dissociation constants of the acid-base transi­ tions are varied as well, the changes being dependent on the kind and concentration of tenside, kind and concentration of inert electrolyte, and total concentration of the organic analytical reagent3 - 21 • The changes are most marked in the region of sub­ micellar concentrations of the cationic tenside whereas in a range close to the critical micellar concentration the changes are very low or nil 3 - 7.