9.5.1 Selection of Indicators

To perform an acid-base titration, it is necessary to choose the correct indicator. For an acid-base titration, choose an indicator whose pH range is within the limits of the pH change of the reaction.

• pH changes rapidly from 3 to 11.
• Phenolphthalein changes colour from pH 8 to 10.
• Phenolphthalein is the best choice.

• pH value changes slightly at the end point.
• pH value changes from 6.5 to 10.
• Phenolphthalein (pH 8.3–10) is the best choice.
• Methyl orange works below pH 5.
• Example: Oxalic acid vs. sodium hydroxide.

• Example: HCl vs. Na₂CO₃.
• At the equivalent point, the pH value changes from 3.5 to 7.5.
• Methyl orange is the best indicator.
• Bromocresol green can also be used.

• No sharp change in the end point.
• Neither indicator works satisfactorily.

Table of Indicators

Intermediate Level

Titration Equation

M₁ V₁n₁ = M₂ V₂n₂

Procedure

• Acid in burette.
• Base + indicator in flask.
• Add acid until colour change.
• Record volume.
• Calculate unknown.

Example

• V₁ = 25 cm³
• M₂ = 0.12 M
• V₂ = 30 cm³
• n₁ = 1, n₂ = 1

Solution: M₁ × 301 = 0.12 × 251 M₁ = 0.1 M Strength = 0.1 × 36.5 = 3.65 g dm^-3

Selection of Indicators

Indicator Table

Important Points

• 1. Titration: Method to find unknown concentration.
• 2. Equation: M₁ V₁n₁ = M₂ V₂n₂
• 3. Acid in burette; base + indicator in flask.
• 4. End point: Indicator changes colour.
• 5. Example 9.3: M₁ = 0.1 M; strength = 3.65 g dm^-3.
• 6. Indicators: Phenolphthalein (strong/weak acid + strong base), Methyl orange (strong acid + weak base).
• 7. Weak acid + weak base: No suitable indicator.

Remember

• - Titration: Find unknown concentration
• - Equation: M₁ V₁n₁ = M₂ V₂n₂
• - Acid in burette; base in flask
• - End point: Colour change
• - Indicators: Phenolphthalein, Methyl orange
• - Example: M₁ = 0.1 M

Common Mistakes

• 1. Forgetting to use the correct n₁ and n₂: Use coefficients from balanced equation.
• 2. Mixing up acid and base volumes: Acid in burette; base in flask.
• 3. Choosing wrong indicator: Match indicator pH range to titration type.
• 4. Forgetting to record initial and final burette readings: Essential for volume calculation.

Quick Revision

• Titration: Method to find unknown concentration.
• Equation: M₁ V₁n₁ = M₂ V₂n₂
• Acid in burette; base + indicator in flask.
• End point: Indicator changes colour.
• Example 9.3: M₁ = 0.1 M; strength = 3.65 g dm^-3.
• Indicators: Phenolphthalein (strong/weak acid + strong base), Methyl orange (strong acid + weak base).
• Weak acid + weak base: No suitable indicator.

60-Second Revision

• Titration: Find unknown concentration
• Equation: M₁ V₁n₁ = M₂ V₂n₂
• Acid in burette; base in flask
• End point: Colour change
• Indicators: Phenolphthalein, Methyl orange
• Example: M₁ = 0.1 M

QUALITY VALIDATION

• Topic: 9.5 Titration
• Chapter: 9 (Acids-Bases Chemistry)

• Source re-read: PASS
• Source scope followed: PASS
• Knowledge level matched: PASS
• Important concepts covered: PASS
• Definitions checked: PASS
• Equations checked: PASS
• Derivations checked: N/A
• Examples checked: PASS
• Unsupported knowledge added: NO
• Unnecessary repetition: NO
• Notes format maintained: PASS
• Important source content omitted: NO

• Source limitations:
• NONE

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