pH test paper contains methyl red, bromocresol green, thymol Blue these three indicator. Methyl red, bromocresol green, Thymol Blue and phenolphthalein are different PH value all the solutions will change color according to certain rules. The color change range of Methyl red is pH4.4 (red)-6.2 (yellow), the color change range of bromocresol green is pH3.6 (yellow)-5.4 (green), and the color change range of Thymol blue is pH6.7 (yellow) -- 7.5 (blue). Quantitative methyl red plus quantitative bromocresol green plus quantitative thymol Blue mixed indicator impregnated neutral white test paper, and the pH test paper prepared after drying can be used to measure the solution PH value it is not difficult to understand.