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?
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.