Acid-Base Indicators

Phenolphthalein is a common indicator with a color change region of 8 to 10. While its structure is relatively complex, its reaction can be simply expressed:

PhH Ph- + H+

Placing phenolphthalein in an acidic or weakly basic solution (pH < 8) is like adding H+ to this equilibrium. According to LeChatelier's Principle, the reaction will shift left to oppose this change, so we see the acid-form of phenolphthalein (PhH) which is colorless. If phenolphthalein is placed in a basic solution (large amount of OH- present) the reaction will shift to the right as the OH- reacts and removes the H+ ion. We will see the basic-form of phenolphthalein (Ph-) which is pink.

IMPORTANT: What the indicator's color tells us