Analysis of the principle of PH meter and PH test paper

The application of ph meter test paper is very extensive. The reaction principle of ph meter (acidity meter) is based on the ph meter indicator method. At present, the general ph meter analysis test paper contains methyl red [ph meter 4.2 (red) ~ 6.2 ( yellow)], bromocresol green [ph 3.6 (yellow) ~ 5.4 (green)] bromothymol blue [ph 6.7 (yellow) ~ 7.5 (blue)], the appropriate amount of these mixed acid-base indicators can reflect The variation range of ph meter is 4.5~9.0. Take the color change of phenolphthalein under different acid and alkali conditions as an example to illustrate the color change principle of pH meter test paper: When phenolphthalein is dropped into concentrated H2SO4, it will turn orange, and the color will not change no matter how long it is shaken. If the orange liquid is mixed with a large amount of water, a colorless liquid is obtained. When phenolphthalein is dropped into dilute NaOH solution, it turns purple. When phenolphthalein is dropped into concentrated NaOH solution (concentration greater than 2mol/l), some purple red will appear on the surface of the concentrated NaOH solution where phenolphthalein is dropped. After a little oscillation, the purple color will disappear immediately. In fact, this is the four corresponding discolorations of phenolphthalein due to the change of its structure under different acid-base conditions. There are three indicators on the test paper of the pH meter: methyl red, bromocresol green, and thymol blue. Like methyl red, bromocresol green, thymol blue and phenolphthalein, they all change color according to certain rules in solutions with different pH values. The discoloration range of methyl red is ph 4.2 (red) - 6.2 (yellow), the discoloration range of bromocresol green is 3.6 (yellow) - 5.4 (green) in pH, and the discoloration range of thymol blue is ph Total 6.7 (yellow) - 7.5 (blue). It is not difficult to understand that the neutral white test paper is impregnated with the mixed indicator of quantitative methyl red plus quantitative bromocresol green plus quantitative thymol blue, and the ph meter test paper prepared after drying can be used to measure the ph value of the solution.