# Phenolphthalein Indicator mixed with base that first turns pink and then clear

What occurs in the following situation: After adding 2 drops of Phenolphthalein Indicator (an indicator that turns bases pink and stays clear for acids,) into a unknown solution, the solution turns pink. However, within a few minutes, the pink slowly fades and the solution becomes colorless. What is the unknown solution?

• There probably is no one answer, but consider this: commercial chlorine bleach can be expected to react with indicators which are organic dyes. – Oscar Lanzi Oct 10 '19 at 0:42
• There is the 2nd acid/base transition at high pH, turning phenolphthalein colourless again, but I suppose it should be fast reaction, not taking minutes. – Poutnik Oct 10 '19 at 1:16

In strongly basic solutions, phenolphthalein is converted to its $$\ce{In(OH)^3-}$$ form, and its pink color undergoes a rather slow fading reaction and becomes completely colorless above $$\mathrm{pH}= 13$$.