Bromocresol green (BCG), as an important dye, is widely used in the analytical field. 1) CN201110395918.7 discloses a method for testing the content of free amine in NMP. The invention tests the content of amine in NMP by preparing bromocresol green methyl red indicator and using acid-base titration. 2) CN201811547295.9 discloses a mixed indicator test paper for measuring the concentration of ammonia in the air and its preparation and detection method, which belongs to the technical field of environmental monitoring and industrial hygiene. A method for preparing bromocresol green is as follows: (1) Dissolve 1.5 kg bromine in 1.25 L glacial acetic acid solution to obtain bromine glacial acetic acid mixture; (2) Dissolve 2kg pure m-cresol violet in 2.5L glacial acetic acid solution; (3) Slowly drop the bromine glacial acetic acid mixture obtained in step (1) into the solution in step (2) at a temperature not exceeding 80 ℃. After addition, continue to stir and keep it at 80~90 ℃ for 24 hours. After cooling, filter it. Wash the sediment twice with glacial acetic acid, dry it in the air or pump it out, and dry it at 50~60 ℃ to obtain crude bromocresol green. Moreover, the crude bromocresol green was recrystallized with benzene or glacial acetic acid to obtain pure bromocresol green, which crystallized into white, with a melting point of 218-9 ℃. Usage Cell stain Usage Acid base indicator Usage Acid base indicator, pH change range 3.8 (yellow)~5.4 (blue green) Usage Acid base indicator. Its sodium salt is commonly used in colorimetric determination of pH value. Acid base indicator, pH 3.8 (yellow)~5.4 (blue). Its sodium salt solution is used as a color developer for spectrophotometric determination of pH value. It is used as a reagent for the determination of aliphatic hydroxyacids and alkaloids by thin-layer chromatography and an extraction and separation agent for the determination of quaternary ammonium cations by spectrophotometry.