Add 10ml dilute hydrochloric acid into the beaker, drop 2 drops of phenolphthalein solution, and gradually drop sodium hydroxide solution.

The phenomenon of reaching the midpoint is that when the last drop of sodium hydroxide is dropped, the solution suddenly turns pink and does not retreat for half a minute.

Why can PH 8 be regarded as the center, but not PH 7?

A: Although acid-base titration is a relatively accurate operation method and the calculated data are relatively reliable, all experiments have errors. Microscopically, no matter how accurate, the last drop of sodium hydroxide molecules contained in the solution are many, tens of thousands. However, in the process of changing from acidic to alkaline, the PH value of the first drop of sodium hydroxide dropped into hydrochloric acid solution changed little, but when it was close to neutral, the PH value of that drop of sodium hydroxide changed greatly. If we continue, the PH value of the last drop of sodium hydroxide will not change much when it reaches alkalinity, so it is possible to change the pH value directly from 6 to 8.

Summary: The PH value of the solution drops near the neutral point of acid-base titration changes greatly, but it has little effect on the proportion of molecules in the whole titration process (the volume of the last drop accounts for a small proportion of the whole titration volume, because after all, it is a drop, and a few milliliters may have been dropped at first, which is small enough), so the indicators of acid-base titration can be phenolphthalein and litmus. You can even titrate with methyl orange (methyl orange is generally not used, because after all, the PH of the discoloration atmosphere is too low, which is unreasonable in theory and inconvenient to observe). Acid-base titration is a scientific measurement method.