Benzene on nitration gives nitrobenzene in presence of HNO$$_3$$ and H$$_2$$SO$$_4$$ mixture, where:

In the nitration of benzene, a mixed acid of concentrated $$\text{HNO}_3$$ and concentrated $$\text{H}_2\text{SO}_4$$ is used. The role of each acid is determined by the Brønsted-Lowry concept of acids and bases.

$$\text{H}_2\text{SO}_4 + \text{HNO}_3 \rightarrow \text{HSO}_4^- + \text{H}_2\text{NO}_3^+$$

$$\text{H}_2\text{NO}_3^+ \rightarrow \text{H}_2\text{O} + \text{NO}_2^+$$

In this reaction, $$\text{H}_2\text{SO}_4$$ donates a proton to $$\text{HNO}_3$$, so $$\text{H}_2\text{SO}_4$$ acts as an acid. Since $$\text{HNO}_3$$ accepts the proton, it acts as a base. The electrophilic nitronium ion ($$\text{NO}_2^+$$) generated is the active species that attacks the benzene ring in the electrophilic aromatic substitution mechanism.

Therefore, $$\text{HNO}_3$$ acts as a base and $$\text{H}_2\text{SO}_4$$ acts as an acid.