INTERACTION BETWEEN SULPHONEPHTHALEIN DYES AND QUATERNARY AMMONIUM-IONS IN AQUEOUS-SOLUTION

The effect of cationic surfactants at concentrations below their critical micelle concentrations on the absorption spectra of the sulphonephthalein dyes was studied. The addition of a cationic surfactant to a Bromocresol Purple (BCP) solution buffered at about pH 8 resulted in a large spectral change: the color changed from blue to light-yellow. The mechanism of this color-change reaction is based on ion association of the quaternary ammonium cation with the monovalent dye anion(HBCP-) produced by the addition of a proton to BCP2- and the formation of a precipitate accompanied by ion association.