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DrX Whiz Niraj
Acid-Base Titration: Theories & Curves
Comprehensive exam notes on Acid-Base Indicators (Ostwald & Quinonoid Theories), Neutralization Curves, and pH jumps.
Acid-Base titration (also known as Neutralization titration) is a quantitative analytical technique used to determine the unknown concentration of an acid or base by neutralizing it with a standard solution of a base or acid.
Principle of Neutralization:
The fundamental reaction involves the combination of Hydrogen ions (H⁺) from the acid with Hydroxide ions (OH⁻) from the base to form unionized water molecules.
Reaction: H⁺(aq) + OH⁻(aq) → H₂O(l)
An indicator is a substance that changes its color in response to a chemical change (change in pH). There are two main theories explaining how indicators work:
1. Ostwald's Theory (Ionic Theory)
According to this theory, acid-base indicators are either weak organic acids (like Phenolphthalein) or weak organic bases (like Methyl orange).
- The unionized form has a different color than the ionized form.
- Phenolphthalein (HIn) is colorless in unionized form (in acid) and Pink in ionized form (In⁻) in basic medium.
2. Quinonoid (Resonance) Theory
This theory states that an indicator exists in two tautomeric forms having different structures. One form exists in acidic medium, and the other in basic medium.
- Benzenoid Form: Usually lighter in color or colorless.
- Quinonoid Form: Highly conjugated, usually deeply colored.
A neutralization curve is a graph plotted between the pH of the solution (Y-axis) and the volume of titrant added (X-axis). It helps in selecting the correct indicator for a specific titration. The steep vertical portion of the curve indicates the pH jump at the equivalence point.
| Type of Titration | pH Range at Equivalence Point (Jump) | Suitable Indicator |
|---|---|---|
| 1. Strong Acid vs Strong Base (e.g., HCl vs NaOH) |
pH changes rapidly from ~4 to 10. Equivalence point = 7.0 |
Any indicator (Methyl Orange, Methyl Red, Phenolphthalein) |
| 2. Weak Acid vs Strong Base (e.g., CH₃COOH vs NaOH) |
pH changes rapidly from ~7.5 to 10.5. Equivalence point > 7 (Basic) |
Phenolphthalein (pH range 8.3 - 10) |
| 3. Strong Acid vs Weak Base (e.g., HCl vs NH₄OH) |
pH changes rapidly from ~3.5 to 6.5. Equivalence point < 7 (Acidic) |
Methyl Orange (pH range 3.1 - 4.4) |
| 4. Weak Acid vs Weak Base (e.g., CH₃COOH vs NH₄OH) |
No sharp pH jump. Very gradual curve. | No simple indicator is suitable. (Use Mixed indicator or conductometry) |
Practical Alert: Why Non-Aqueous Titration?
When dealing with Very Weak Acids or Very Weak Bases (like many organic drugs e.g., Adrenaline, Ephedrine), their titration in water (aqueous medium) does not give a sharp end-point. Water acts as a competing acid/base. Hence, we must use Non-Aqueous solvents (like Glacial Acetic Acid) to sharpen the end-point and make the titration possible.