In the nitrating mixture, ${\mathrm{HNO}}_3$acts as

${\mathrm{HNO}}_3$acts as a base. It accepts proton(As per Bronsted) from ${\mathrm{H}}_2{\mathrm{SO}}_4$${\mathrm{HNO}}_3+2{\mathrm{H}}_2{\mathrm{SO}}_4\rightleftharpoons {\mathrm{H}}_3{\mathrm{O}}^{+}+2{\mathrm{HSO}}_4^{-}+{\mathrm{NO}}_2^{+}$.