7.2.10 Acid-Base Indicators

How Indicators Work

An acid-base indicator is actually a Weak Acid itself () that has a different colour from its conjugate base ()

The equilibrium equation is:

The Mechanism (Le Chatelier’s Principle)

1. In Acidic Solution (High )
   • The concentration of pushes the equilibrium to the LEFT.
   • The solution takes the colour of the weak acid ().
2. In Alkaline Solution (Low )
   • The from the alkaline solution reacts with the , lowering its concentration.
   • The equilibrium shifts to the RIGHT to replace the .
   • The solution takes the colour of the conjugate base ().

---

The pH Range of an Indicator

• Indicators do not change colour instantly at pH 7.
• They change colour over a specific pH Range, typically spanning about 2 pH units.

The pH at which the colour change occurs depends on the (the of the indicator):

Indicator	Acid Colour	Base Colour	pH Range	Use Case
Methyl Orange	Red	Yellow	3.1 - 4.4	Strong Acid titrations
Phenolphthalein	Colourless	Pink	8.3 - 10.0	Strong Alkali titrations

The Math: Why pH ≈ pKin?

The midpoint of an indicator’s color change occurs when the concentration of the Acid form () equals the Base form ().

Using the equilibrium expression:

If , they cancel out:

Taking negative logs of both sides gives the rule:

This explains why different indicators work at different pH levels—it depends entirely on their specific value.

---

Selecting the Correct Indicator

To get an accurate result, the indicator must change colour during the vertical section of the titration curve.

The Rule: The pH Range of the indicator must fall completely within the Vertical Section of the titration graph.

Matching Indicators to titrations

Strong Acid + Strong Alkali

Suitable Indicator: Both

pH curve of strong acid + strong alkali titration

• Vertical Section: pH 3 10
• Suitable Indicator: Both
• Explanation: Both ranges (3.1-4.4 and 8.3-10.0) fall inside the huge vertical jump.

Weak Acid + Strong Alkali

Suitable Indicator: Phenolphthalein

pH curve of weak acid + strong alkali titration

• Vertical Section: pH 7 10
• Suitable Indicator: Phenolphthalein
• Explanation: Vertical section is high. Methyl Orange (3.1-4.4) would change too early.

Strong Acid + Weak Alkali

Suitable Indicator: Methyl Orange

pH curve of strong acid + weak alkali titration

• Vertical Section: pH 3 7
• Suitable Indicator: Methyl Orange
• Explanation: Vertical section is low. Phenolphthalein (8.3-10.0) would change too late.

Weak Acid + Weak Alkali

Suitable Indicator: None

pH curve of weak acid + weak alkali titration

• Vertical Section: None
• Suitable Indicator: None
• Explanation: There is no vertical section. No indicator gives a sharp endpoint.

Note: A pH meter can be used instead