4-Biphenylsulfonic acid

31.2 g (0.2 mol) of biphenyl was dissolved in 200 ml of o-nitro ethylbenzene at a controlled temperature of 30 to 40C, 35 g of chlorosulfonic acid was slowly added dropwise and reacted at this temperature for 10 to 15 minutes, then the reaction mixture was heated to 80C and kept at 80C for 2 hours. Cooled down to room temperature, filtered and dried to obtain a dry product weighing 49.2 g biphenyl-4-sulfonic acid content of 98.5%.