Phenolphthalein

Phenolphthalein is a sensitive pH indicator with the formula C₂₀H₁₄O₄. Often used in titrations, it turns from colourless in acidic solutions to pink in alkaline solutions, the changeover occuring at around pH 8.3. If the the concentration of indicator is particularly strong, it can appear purple.

Phenolphthalein is insoluble in water, and is usually dissolved in alcohols for use in experiments.

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!