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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 occurring between pH 8 and pH 10. If the the concentration of indicator is particularly strong, it can appear purple.

In strongly alkaline solutions, phenolphthalein's pink color undergoes a rather slow fading reaction and becomes colorless again. In other words the molecule has three forms:

        H2P      <->      P2-      <->      POH3-
       acidic          alkaline       strong alkaline
      no color           pink            no color

The fading reaction is sometimes used in undergraduate classes for the study of reaction kinetics.

Phenolphthalein is insoluble in water, and is usually dissolved in alcohols for use in experiments. It is itself a weak acid, which can lose an H+ ions in solution. The phenolphthalein atom is colourless. However, the ion is a pink/purple colour. When an alkaline is added to the phenolphthalein, the atom<->ions equilibrium shifts to the ionisation because H+ ions are removed, (by Le Chatelier's principle).

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!

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