Unit 8 Notes Topic 11 AP CHEMISTRY Acid-Base Indicators

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🎨 Acid-Base Indicators Explained

Unit 8 | Topic 11 – How Color Reveals pH

🔍 What is an Acid-Base Indicator?

An acid-base indicator is a special type of weak acid or weak base that changes color depending on the pH of the solution. It's not just a dye — it's a chemical that shifts its structure (and color) based on whether it's protonated or deprotonated.

HIn (acid form, color 1) ⇌ H⁺ + In⁻ (base form, color 2)

Color Change Logic:

• ✅ In acidic solution → More HIn → You see Color 1
• ✅ In basic solution → More In⁻ → You see Color 2

🔁 It's All About Equilibrium Shifting!

Indicators behave just like any other equilibrium system.

• If you add acid (H⁺): Equilibrium shifts left → More HIn → Color 1 appears
• If you add base (OH⁻): OH⁻ removes H⁺ → Equilibrium shifts right → More In⁻ → Color 2 appears

The transition between colors doesn’t happen all at once. It happens gradually over a pH range centered around the indicator’s pKa.

📍 When Does the Color Change?

The color change is most noticeable when the concentrations of HIn and In⁻ are about equal. That’s when:

pKa = pH

Since it’s a gradual shift, indicators typically change color over a range of:

pH = pKa ± 1

🎯 Choosing the Right Indicator

You should match the indicator’s pH range to the steep vertical section of the titration curve. That’s where the biggest pH jump happens during neutralization.

If the indicator changes color during that jump → You see a clear, visible endpoint.

🧪 Indicator Table

Indicator	Color in Acid	Color in Alkali	pKa	Color Change Range
Thymol Blue	Red	Yellow	1.7	1.2 – 2.8
Methyl Orange	Red	Yellow	3.7	3.1 – 4.4
Bromophenol Blue	Yellow	Blue	4.1	3.4 – 4.6
Methyl Red	Red	Yellow	5.1	4.4 – 6.2
Phenolphthalein	Colorless	Pink	9.3	8.3 – 10.0

💡 Indicator Tips by Titration Type

• Strong Acid + Strong Base: pH shifts from 4–10 → Use Methyl Red or Phenolphthalein
• Strong Acid + Weak Base: pH shifts from 4–7 → Use Methyl Red (best match)
• Weak Acid + Strong Base: pH shifts from 7–10 → Use Phenolphthalein (ideal range)
• Weak Acid + Weak Base: ❌ No sharp jump — no indicator works! Use pH meter instead

⚠️ Common Mistake: Students choose indicators randomly or just pick the one with the "nicest color" 😅 — Always match it to the equivalence pH zone!