Consider the nitration of benzene using mixed conc. H2SO4 and HNO3. If a large amount of KHSO4 is added to the mixture, the rate of nitration will be:

During nitration of benzene with a mixture of nitric acid and sulphuric acid, when a large amount of KHSO4 is added, the rate of nitration will be slower. Sulphuric acid helps in the formation of the electrophile +NO2 ion. HONO2 + 2H2SO4 ⇌ 2[HSO4]⁻ + H3O⁺ + NO2⁺. When a large amount of KHSO4 is added, it will provide HSO4⁻ (common ion) which will decrease the concentration of NO2⁺.