titration indicator
Summary
A titration indicator is a weak acid or base whose acid and conjugate-base forms differ in color, so its color change signals the endpoint of a titration. It works best when its pKa is near the pH of the equivalence point, which sits on the steep part of the curve.
Detail
Meaning: An indicator is a weak acid (HIn) or weak base whose protonated and deprotonated forms have different colors. Which form dominates depends on pH, so the color shifts over a narrow range, roughly the indicator's pKa ± 1. Only a trace is added, so it uses negligible titrant. The visible color change marks the endpoint, which should land close to the equivalence point.
Equation: pH = pKa + log([In⁻]/[HIn]), where pKa is the indicator's (unitless) and [In⁻] and [HIn] are the concentrations of its conjugate base and acid forms (mol/L). The color change is seen when the ratio is between about 1:10 and 10:1.
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. A weak acid titrated with a strong base has an equivalence pH above 7, so phenolphthalein (basic range) fits. A strong acid titrated with a weak base ends below pH 7, which suits methyl orange.
Common confusion: The endpoint is the observed color change. The equivalence point is where moles of titrant equal moles of analyte. Do not assume equivalence is at pH 7. That holds for a strong acid with a strong base, not for weak-strong titrations.
Sources
- Chemistry: The Central Science
- Chemistry: Atoms First
- General Chemistry: Principles and Modern Applications
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.