Phenolphthalein (C20H14O4) is a weak organic acid. The conjugate base of phenolphthalein (Ph) is pink in color whereas the undissociated/acidic form (H-Ph) is a colorless solution.

The pH range of color-changing in phenolphthalein as an indicator is 8.3 to 10.5. It is colorless in acidic environments i.e. pH < 8.3, whereas it turns pink when basic conditions arise i.e. pH > 10.5.

Phenolphthalein changing color

Quinonoid theory

According to the quinoid theory, the acid/base indicators exist in tautomeric forms. Tautomeric forms usually arise due to the hydride ion shift.  In tautomerism, the atoms of a molecule arrange themselves in different arrangements such that the overall molecular formulae remain the same.

The indicators observing quinoid theory have two tautomeric structures;

  1. Benzoid form
  2. Quinonoid form

The benzoid form is the molecular tautomeric form containing benzene rings. The condition when excess electrons on functional groups, bonding with acidic protons make phenolphthalein a colorless compound. This is because the reduced forms of chemical compounds have fewer or no colors at all.

The quinonoid form instead forms a violet-pink color because, in basic conditions, the protons are not present on functional groups making them able to resonate their electrons inside benzene rings and destroying the aromaticity and resonance of benzene rings. This way, a quinonoid structure is established which is an oxidized form of phenolphthalein. As oxidized forms of chemical compounds show colors, phenolphthalein turns pink in alkaline environments.