This is a follow-up to the question here. There is no mono-anionic stage shown, does it "disproportionate" to the neutral molecule and the di-anion?

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    $\begingroup$ It just means second pKa is not significantly higher then first. $\endgroup$
    – Mithoron
    Commented May 4, 2018 at 22:49
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    $\begingroup$ So maybe when we form the mono-anion the conjugation of the two phenolic rings favors driving the next proton off to form a highly stabilized "quinone-aromatic" structure, thus lowering that pKa? $\endgroup$ Commented May 5, 2018 at 0:55

1 Answer 1


This is an answer to a question of mine, which I posted a few days ago, Deprotonated form of phenolphthalein.

I found this paper (it's in Japanese, but the relevant things are legible) https://www.jstage.jst.go.jp/article/yakushi1947/117/10-11/117_10-11_764/_pdf, which says that phenolphthalein has 2 pKas, pKa1 = 9.05 and pKa2 = 9.50. It also says that the pink form is the twice deprotonated; the monoanionic form exists, but is colourless.


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