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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?

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    $\begingroup$ There probably is no one answer, but consider this: commercial chlorine bleach can be expected to react with indicators which are organic dyes. $\endgroup$ – Oscar Lanzi Oct 10 '19 at 0:42
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    $\begingroup$ 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. $\endgroup$ – Poutnik Oct 10 '19 at 1:16
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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 $\mathrm{pH}= 13$.

wikipedia

So the strong alkali is very probable reason, but bleachers or other compounds reacting with phph cannot be eliminated.

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  • $\begingroup$ One limitation in this answer is any alkali can do this, all are sufficiently soluble. The problem is actually the question, which is not well posed unless multiple choices are attached (and missing here). $\endgroup$ – Oscar Lanzi Oct 10 '19 at 2:06
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    $\begingroup$ @Oscar Lanzi No objection, but quality of answers should not be expected better than the quality of the question. The general alkali is as good answer as sodium hydroxide. Sure, bleachers or other compounds reactions with phph cannot be eliminated. $\endgroup$ – Poutnik Oct 10 '19 at 2:11
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The indicator may slowly disappear after the titration because it is reacting with the air. As it reacts with chemicals in the air (CO 2 for example) which causes the indicator to be slowly disappear from the solution.

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