The ultraviolet spectra of heterocyclic N-oxides of pyridine N-oxide series and the conjugated cationic acids (simple cations of protonated N-oxides) in acetonitrile as the representative of polar aprotic solvents, were determined. The obtained spectra of N-oxides studied (mainly tri-substituted pyridine N-oxides) and their cations obtained by the protonation of free N-oxides by the excess of perchloric acid, have been collected and discussed. Taking into account the spectroscopic results, a scheme of acid-base equilibria in polar aprotic solvents has been discussed. The influence of the traces of water on acid-base equilibria in nonaqueous media has been stressed.
