Which one of the following is not present in the nitration mixture?

A mixture of concentrated nitric acid with organic or inorganic compounds containing  ${{\mathrm{NO}}_2}^{+}$ ion is called nitration mixture.

$2{\mathrm{H}}_2{\mathrm{SO}}_4 + {\mathrm{HNO}}_3 \to {{\mathrm{NO}}_2}^{+ } + {\mathrm{H}}_3{\mathrm{O}}^{+} + {{\mathrm{HSO}}_4}^{-}$

So, ${{\mathrm{SO}}_4}^{2-}$ ions are not present in nitration mixture.