Use of p-nitroaniline to determine protonation equilibrium constants of oxygen-containing molecules in aqueous solution

Abstract

The criticisms contained in a recent paper¹ of the interpretation in terms of protonation equilibria^2,3 involving (H₂O)₄H⁺_aq of the changes in optical density of acidic solutions of p-nitroaniline on addition of organic oxygen containing substrates are discussed. It is concluded that the effects of activity coefficients are too small to influence the interpretation, and it is indicated how attacking molecules distinguish between protonated and unprotonated molecules of substrate in oxidation reactions and complexing equilibria. Good agreement is obtained for protonation equilibrium constants between values from this spectrophotometric method and values obtained using a range of other independent techniques.