Weak acids or bases whose color differs between protonated and deprotonated forms.
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An acid-base indicator is a weak organic acid or base whose protonated and deprotonated forms have different colors. Its equilibrium is HIn <-> H+ + In-, and the color changes over a range of about one pH unit centered on its pKa. Because the color depends on the ratio of the two forms, a practical indicator is one whose transition range falls on the steep part of the titration curve. Indicators are used in very small amounts, so they do not significantly disturb the equilibrium being measured. They are chosen to match the expected equivalence pH.
Key ideas: Indicators are weak acids HIn whose acid and base forms differ in color. The color changes over pH = pKa(In) plus or minus 1, since the ratio [In-]/[HIn] runs from 0.1 to 10. Examples: methyl orange 3.1-4.4, methyl red 4.4-6.2, bromothymol blue 6.0-7.6, phenolphthalein 8.2-10.0 (colorless to pink).
Common mistakes and connections: Students use too much indicator, which consumes titrant and shifts the end point. Another mistake is memorizing colors without ranges. The change is gradual, not abrupt. Choosing the indicator relies on the equivalence-point pH; connects to Henderson-Hasselbalch and spectroscopy of colored species.
An indicator HIn is a weak acid: it changes color as the ratio of HIn to In- shifts, with the midpoint at its pKa (about 9.3 for phenolphthalein). The interactive lesson shows this as a live animation or chart.
Methyl orange changes color between pH 3.1 and 4.4, while phenolphthalein changes between pH 8.2 and 10.
Weak acids or bases whose color differs between protonated and deprotonated forms.
Acid-Base Indicators is a beginner topic in Acids, Bases & Electrochemistry. It needs no earlier chemistry lessons and is a good starting point.
Study these earlier topics in Acids, Bases & Electrochemistry first: Titration Curve, Strong Acid Strong Base Titration, Weak Acid Strong Base Titration.
The next topics in Acids, Bases & Electrochemistry are Universal Indicator and Choosing Indicators.
Yes. The interactive lesson includes a self-check question on Acid-Base Indicators, and the Acids, Bases & Electrochemistry quiz draws on every topic in this area.
Acid-Base Indicators is topic 28 of 105 in Acids, Bases & Electrochemistry.