MARTYN F. CHILLMAID / SCIENCE PHOTO LIBRARY MARTYN F. CHILLMAID / SCIENCE PHOTO LIBRARY
Phenolphthalein is used as a single indicator in acid-base titrations. In a base solution, as here at pH11, phenolphthalein is a pink colour. Adding a dilute acid turns it gradually colourless. At pH 9.3 the colour is barely perceivable and by pH 8 it is colourless.
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