The wikipedia page states clearly that the coloured form of phenolphthalein is the one with two protons removed. What is the evidence that phenolphthalein loses both protons in basic solution? And does the only once deprotonated form ever appear?
Answer
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 deporotonated; the 1- form exists, but is colourless.
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