The applicability of Hammett's acidity function H0 to solutions in methanol‐water mixtures

The dissociation constants of 4‐aminoazobenzene‐4′‐sulphonic acid, 4‐(methylamino)azobenzene‐4′‐sulphonic acid, 4‐(phenylamino)‐azobenzene‐4′‐sulphonic acid, 4‐(dimethylamino)azobenzene‐4′‐sulphonic acid and p‐nitrophenol were determined in water and in several mixtures of methanol and water at 25°. The influence of the percentage of methanol on the dissociation constant differs greatly for the various indicators, even for indicators of the same charge type. From this it follows that Hammett's acidity function H0 for solutions in mixtures of methanol and water depends on the indicator used for determining H0. Thus, for solutions in mixtures of methanol and water and presumably also in mixtures of water and other non‐electrolytes, H0 is not a useful, unambiguous measure of the acidity.