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So, I've looked it up and Wikipedia claims that two protons are released by Phenolphthalein when pH is over 8.2, but I've seen other sources that claim only one proton is lost. So, is it really just one or two? Which form is responsible for its colour? (I would expect a fully deprotonated PhPh in more basic solutions, so I guess the PhPh that released two protons is the one responsible for the solution's colour)

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  • $\begingroup$ Can you please cite the "other sources"? Their context may be different, for example, they may lose only one proton in stronger acidic conditions. $\endgroup$ – Gaurang Tandon May 3 '18 at 1:37
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ChemSpider phenolphthalein's $\mathrm{p}K_\mathrm{a}$ value: 9.7 (at room temperature). Here they've is only given one $\mathrm{p}K_\mathrm{a}$ value, but it is still reasonable to assume that there are other forms in other pH conditions, like in this image:

Phenolphthalein Wikipedia image

(source: Wikipedia)

In other sources like academic ones, it is always shown with 2 forms, and these are associated with the supposedly (especially back titration from pink to colourless is very hard to catch) sharp color change in pH 8-10 range. If one is to use an indicator for that pH region, phenolphthalein is chosen but not for the other regions so no one is interested in possible other forms.

It is also understandable if people actually don't even know the other forms, or try to use them also since pH meter(also possibly UV-Vis spectrometry) is used for a long time to track the pH, I have only seen these indicators in student laboratories for teaching purposes, not in any of research laboratory.

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