The visible absorption spectra of 4-(2-benzothiazolylazo)resorcinol
(BTAR) and 4-(2-benzothiazolylazo)salicylic acid (BTAS) have been
recorded in water- organic solvent mixtures in the pH range 0_5-12.0. The
organic solvents used are methanol, ethanol, n-propanot, iso-propanol,
acetone, dioxane and dimethyl formamide. The spectral changes have been
explained in terms of shifts in equilibria among different molecular and
ionic species of BTAR and BTAS existing in solution. The pKa values corresponding
to the different ion&ation steps have been determined at 25~
and I=0.1M (KN03) by graphic analysis of the absorbance-pH curves. The
results obtained are discussed in terms of the molecular structure of the
reagents and the nature of the organic co-solvent used.