In the nitration of benzene, the electrophile, nitronium ion ($\text{NO}_2^+$), is produced by the transfer of a proton from concentrated sulphuric acid to concentrated nitric acid. In this process of generating the nitronium ion, sulphuric acid donates a proton (serving as an acid) and nitric acid accepts the proton (serving as a base) . Therefore, nitric acid acts as a base in the nitrating mixture.