How to use vinegar to remove dirt

White vinegar 1. Absorb a small amount of white vinegar with a dropper, drop 1 ~ 2 drops on the pH test paper, and compare with the standard colorimetric chart to determine the pH value of white vinegar. 2. Empty the old thermos bottle with scale, add 30 ml ~ 40 ml of white vinegar, cover the cork, put the bottle flat, and let it stand 15min. 3. Rotate the bottle once every 15 minutes, so that the white vinegar can immerse other parts of the inner wall of the bottle. 4. After about 1h, pour out the white vinegar, observe the color change and determine the pH value. Just wash the thermos bottle with water several times.