An acid-base indicator. Changes colour from clear and colourless in acid to pink in base over the pH range of 8.0 to 9.6.

-adapted from Quantitative Chemical Analysis, by Daniel C. Harris

Also is a VERY strong laxative. Used to be the main ingredient in a well known over-the-counter laxative until people complained it was too harsh.

I have it under good authority that the addition of small (and I do mean SMALL) amounts of phenolphthalein to a batch of oatmeal raisin cookies adds a slight flavour of licorice.

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