Experimental phenomenon of phenolphthalein test solution dropping into sodium hydroxide solution

The test solution is red and becomes colorless after a period of time. This is because phenolphthalein is a weak organic acid, which has colorless lactone structure in solution with pH 8.2. The quinone type or quinone acid salt of phenolphthalein is very unstable in alkaline medium and will slowly transform into colorless carboxylate; When it meets a strong alkali solution, it will immediately become a colorless carboxylate form. Therefore, when phenolphthalein reagent drops into concentrated alkali solution, phenolphthalein begins to turn red, and soon the red color recedes and becomes colorless.