Phenol is not a usable acid-base indicator because it absorbs in the wrong part of the spectrum, but the same principles explain why slightly larger aromatic molecules make good indicators. Alizarin, shown at the top of the next page, is a derivative of anthracene. The two double-bonded oxygens on the center ring enlarge the delocalized system from 14 atoms to 16, and shift the center of absorption from 3800 Å in the ultraviolet to 4300 Å in the indigo part of the visible spectrum. The unabsorbed wavelengths make alizarin yellow. The two -OH groups on the outer ring do not participate in the delocalized system until they dissociate, as they do in basic solutions in which III ions are scarce. When this happens, the two hydroxyl oxygens become just as much a part of the delocalization as the original double-bonded oxygens. You can see this by drawing other resonance structures that give the hydroxyl oxygens the double bonds. This enlargement shifts the main absorption to around 4800 Å in the blue re Delocalization of phenol in acid (gray tint) and phenolate ion in base (color tint) gion, and makes the solution of ions orange. Alizarin is one of the standard acid-base indicators, turning Yellow in acid and orange in base. Phenolphthalein, which is colorless in acid and deep red in base, and methyl violet, which is yellow in acid and violet in base, are other examples of aromatic acid-base indicators.