During nitration of benzene using a mixture of conc. ${\mathrm{HNO}}_3$ and conc. ${\mathrm{H}}_2{\mathrm{SO}}_4$ function of conc. ${\mathrm{H}}_2{\mathrm{SO}}_4$ is to increase the rate of reaction by increasing the concentration of ${\mathrm{NO}}_2^{+}$ according to following reaction.

Here nitric acid acts as