The direct nitration of aniline with nitric acid and sulfuric acid, according to one source, results in a 50/50 mixture of "para"- and "meta"-nitroaniline isomers. In this reaction the fast-reacting and activating aniline (ArNH2) exists in equilibrium with the more abundant but less reactive (deactivated) anilinium ion (ArNH3+), which may explain this reaction product distribution. According to another source, a more controlled nitration of aniline starts with the formation of acetanilide by reaction with acetic anhydride followed by the actual nitration. Because the amide is a regular activating group the products formed are the para and ortho isomers. Heating the reaction mixture is sufficient to hydrolyze the amide back to the nitrated aniline.