Phenolphthalein

Phenolphthalein is a sensitive pH indicator with the formula C₂₀H₁₄O₄. It turns from colourless in alkaline solutions to pale pink in acidic solutions, the changeover occuring at around pH 8.3.

Phenolphthalein is insoluble in water, so it is usually dissolved in alcohols for use in experiments such as titrations.

Phenolphthalein has been used for over a century as a laxative, but is now being removed from the market because of concerns over carcinogenity. However, the small amounts usually used in experiments are harmless.

Spelling tip: Phenolphthalein has two pHs in it!