An acid-base indicator is used to mark the end point of a titration, usually by changing color. Most acid-base indicators are substances which, upon protonation or de-protonation, absorb light differently and change colors as a consequence. The equation below represents the dissociation of an indicator.
The equation can be written in the form of an equilibrium expression.
which can also be expressed by the so-called Henderson-Hasselbalch equation. This form is derived by taking the log of the equilibrium expression and rearranging it.
Indicators change over a range of about 1.5 pH units. In addition, indicators vary from one another at the pH in which the color changes.
Indicators are chosen based upon color occurring in the pH range which coincides with the equivalence point or very near the equivalence point of a titration.
When two weak acids are mixed and titrated using a strong base, the stronger acid is titrated first, as in the Figure below.
The volume of titrant used is related to the amount (moles) of each acid. Suppose the amount of the weaker acid is reduced. The Figure above shows that, as the amount of weaker acid is lowered, the volume of base titrant required is reduced. pH indicators are weak acids. In acid form, they have one color; in base, another. The red arrows show the volume required for the weaker acid. Indicators are chosen from intensely colored substances such that the volume required to traverse the entire curve for the indicator is one or less drops of titrant. Any theory of how indicators work can be rooted in exactly the same notions discussed for other weak acids. The goals for indicator selection include two: to choose an indicator whose pKa is the same as the equivalence point of the titration, and to choose one so intensely colored that titrating the full amount of the indicator requires a fraction of a drop of titrant.
A more complete listing of acid-base [local] indicators is listed. Indicators change color owing to the fact that the number of conjugated double bonds change in the more acid environment compared to the more basic environment.
