Consider the nitration of benzene using mixed concentrated ${\text{H}}_2{\text{SO}}_4$ and ${\text{HNO}}_3$. If a large amount of ${\text{KHSO}}_4$ is added to the mixture, the rate of nitration will be:

Nitration involves the formation of the nitronium ion (${\text{NO}}_2^{\oplus }$):
$${\text{HNO}}_3+2{\text{H}}_2{\text{SO}}_4\rightleftharpoons {\text{NO}}_2^{\oplus }+{\text{H}}_3{\text{O}}^{\oplus }+2{\text{HSO}}_4^{\ominus }$$
Adding ${\text{KHSO}}_4$ increases the concentration of ${\text{HSO}}_4^{\ominus }$. According to Le Chatelier's Principle, the equilibrium shifts to the left, decreasing the concentration of ${\text{NO}}_2^{\oplus }$, thereby slowing down the rate.