The secret area in acid-base titration

With phenolphthalein (in 50% EtOH) as the indicator, and base as the titrant, it is common knowledge that the solution turns from colorless to magenta.  However, with careful sub-drop addition, the solution actually first turns milky-white, then milky-pink, before it clears up to a transparent pink at the end point:

This is a titration of 1.00 M nitric acid with 1.00 M NaOH, with phenolphthalein (50% EtOH) as indicator.  The center and right-most titration was performed by Philip L.

After learning about the existence of the “secret area”, many students did the titration with utmost care.  It was fun (and pretty tense) to watch.

Chemistry-wise I am not too sure what is precipitating.  (Both the acid / base forms of phenolphthalein should be completely soluble.)  Surely I am not the first to notice the transition, but I have not seen this described in the literature.  If you have any leads please do let me know.

1 thought on “The secret area in acid-base titration

  1. Perhaps there’s a salt bridge between two molecules of phenolphthalein? Right at the pKa i’d expect pretty facile exchange of protons from one molecule to the other.

    Otherwise, there are two protonation sites. Perhaps there’s a slight gap in their pKas, such that a singly negatively charged molecule is formed?

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