Indicators and Titration Curves

Indicators

What are Acid-Base Indicators?

An acid-base indicator is a substance, usually a weak acid or weak base, that changes colour depending on the pH of the solution.

Indicators help identify the endpoint of a titration by visibly showing when neutralisation has occurred.

• In acidic conditions, an indicator exists predominantly in its protonated form (often a different colour).
• In basic conditions, the deprotonated form becomes more common, causing a colour change.

Common Indicators

Indicator	Colour in Acid	Colour in Base	pH Range	Suitable For
Litmus	Red	Blue	5 • 8	Strong acid/strong base
Methyl orange	Red	Yellow	3 • 5	Strong acid/weak base
Phenolphthalein	Colourless	Pink	8 • 10	Weak acid/strong base

How Indicators Work: The Theory

Indicators function through the reversible dissociation of a weak acid or weak base.

For a weak acid indicator ($HIn$):

$$\text{HIn} \rightleftharpoons \text{H}^+ + \text{In}^-$$

• $HIn$ (the undissociated form) and $In⁻$(the conjugate base) have different colours.
• The colour change depends on the relative concentrations of these two species, which is influenced by the pH of the solution.

Le Chatelier's Principle in Indicators

If an acid is added:

$\text{[H}^+ \text{]}$ increases, pushing the equilibrium to the left, resulting in more $\text{HIn}$, so the solution appears the colour of the acid form.

If a base is added:

$\text{[OH}^- \text{]}$ reacts with $\text{H}^+$, decreasing $\text{[H}^+ \text{]}$, shifting the equilibrium right, producing more $\text{In}^-$, which causes the base colour to dominate.

Choosing the Right Indicator

To choose a suitable indicator for a titration, the following must be considered:

• The pH at the endpoint of the titration.
• The pH range over which the indicator changes colour. The indicator must change colour sharply at the equivalence point, where neutralisation occurs.

Titration Curves

A titration curve shows how the pH of a solution changes as a titrant (e.g., acid or base) is added.

The shape of the curve depends on the strength of the acid and base involved in the titration.

Key Features of Titration Curves

• Initial pH: The starting pH before any titrant is added.
• Equivalence Point: Where equal amounts of acid and base have reacted. The pH at this point varies depending on the acid and base's strength.
• Buffer Region: A section of the curve where pH changes more gradually.
• Steep Slope: Indicates the rapid pH change near the endpoint.

Types of Titrations

Strong Acid vs. Strong Base

Weak Acid vs. Strong Base

Strong Acid vs. Weak Base

Weak Acid vs. Weak Base