The pH indicator paper contains three indicators, methyl red, bromocresol green, and thymol blue. Like methyl red, bromocresol green, thymol blue and phenolphthalein, they will change according to a certain rule in solutions with different pH values. The color change range of methyl red is pH 4.4 (red) - 6.2 (yellow), the color change range of bromocresol green is pH 3.6 (yellow) - 5.4 (green), and the color change range of thymol blue is pH 6 .7 (yellow) - 7.5 (blue). It is not difficult to understand that a pH test paper prepared by impregnating a neutral white test paper with a certain amount of methyl red plus quantitative bromocresol green plus quantitative thymol blue indicator can be used to determine the pH of the solution.