Indicators and Polyprotic Acids

An indicator is itself a weak acid, HIn, that sits in equilibrium with its conjugate base In-. HIn is one color, In- is another. Which form dominates depends on pH, exactly the way it depends for any weak acid: when pH is well below the indicator's pKa, HIn dominates and you see its color; well above pKa, In- dominates. Near pH = pKa, both forms are present in comparable amounts and you see the color changing over a narrow range, typically about two pH units wide. That range is fixed by the indicator's chemistry, not by the titration you're running, which is why you have to pick the right indicator for the right job.

To choose well, match the indicator's color-change range to the pH of the equivalence point, not to pH 7. A strong acid-strong base titration has its equivalence point at pH 7, but a weak acid titrated with a strong base lands above 7, and a weak base titrated with a strong acid lands below 7. The color change only approximates the equivalence point, because the indicator's switch happens over its own equilibrium range, set by its Ka, which rarely centers exactly on the equivalence pH.

Polyprotic acids like carbonic acid, H_2CO₃, lose protons one at a time, each with its own Ka. Titrating H_2CO₃ with NaOH shows two separate equivalence point jumps on the curve, one for removing the first proton to form HCO₃-, and a second, usually less sharp, for removing the second proton to form CO₃²-. Each jump needs its own indicator if you want to detect it by color.