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