PROTONATION OF ARYLSULFONATES IN AQUEOUS SULFURIC ACID . APPARENT PKA VALUES OF AZO DYE SULFONIC ACIDS FROM SOLUBILITY MEASUREMENTS

A number of azo dye sulfonates are converted into protonated species in 0.1-3 M aqueous H2SO4 with accompanying decreases in solubility of about 100-fold. Above ca. 3 M H2SO4, the solubilities become independent of acidity. Plots of log solubility against the H0 acidity function conform quantitatively to the relationship derived for an assumed acid-base equilibrium yielding a sparingly soluble acid. The derived pKa values are more positive by several log units than values for simple arylsulfonic acids estimated by other workers from solubility and other measurements. Similar large decreases in solubility of the salts occur in neutral solutions of NaClO4 over a range of ionic strengths comparable to those of the acid solutions. Our results suggest that protonation of the azo sulfonate anions in the acid solutions occurs by salting out of a neutral protonated species, so that the apparent basicities of the salts may be a reflection of the tendency of the acids to be salted-out of a medium of varying ionic composition and structure. If this interpretation is correct, then there is a close parallel between the indicator acidity of the medium and the ability of the medium to salt out" the electrically neutral species of the organic dyes."