indicator
Summary
An indicator is a weak acid or base whose protonated and deprotonated forms have different colors, so its color signals pH in a titration. It changes color near pH = its pKa, so the best choice has a pKa close to the pH at the equivalence point.
Detail
Meaning: An indicator is a weak acid (HIn) whose protonated form and conjugate base (In⁻) differ in color, so a few drops in a titration let the color report the pH. When [In⁻] = [HIn], pH equals the indicator's pKa and the color is a blend. The visible change spans roughly one pH unit on either side of the pKa. The color change marks the endpoint, which is meant to approximate the equivalence point.
Equation: pH = pKa + log([In⁻]/[HIn]). pH and pKa are dimensionless; [In⁻] and [HIn] are the concentrations of the base and acid forms in mol/L.
On the MCAT: A passage gives a titration curve and a table of indicator transition ranges. Pick the indicator whose range falls on the steep vertical part of the curve at the equivalence pH: near 7 for a strong acid with a strong base, above 7 for a weak acid with a strong base (phenolphthalein), below 7 for a weak base with a strong acid (methyl orange). You may also be asked which color appears when the pH is below or above the pKa.
Common confusion: The endpoint is the color change you observe; the equivalence point is where moles of acid and base are stoichiometrically equal. They coincide only when the indicator is well matched to the titration. Also, when pH is well below the pKa, the acid form (HIn) dominates the color, not In⁻.
Sources
- Chemistry: The Central Science
- Chemistry (Zumdahl)
- General Chemistry (Ebbing and Gammon)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.