Acid-base indicators

1. What is an Acid-Base Indicator?

• An indicator is a substance that changes colour depending on whether it is in an acidic or basic solution.
• Indicators are weak acids or weak bases in equilibrium with their conjugate forms.
• They help determine whether a solution is acidic or basic.

2. How Indicators Work

Indicator in Equilibrium

• Represented as: $HIn ⇌ H⁺ + In⁻$
  • $HIn =$ Indicator in one colour (acid form).
  • $In⁻ =$ Indicator in another colour (base form).

Effect of Acid and Base on an Indicator

Condition	Reaction Shift	Effect on Colour
Acid is added $(↑ H⁺)$	Shifts left	More HIn (Acid form) → Colour 1 appears
Base is added $(↑ OH⁻)$	Shifts right	More In⁻ (Base form) → Colour 2 appears

Example:

Bromothymol Blue

• $HIn$ (Yellow) $⇌ H⁺ + In⁻$ (Blue)
• In acidic solution (more $H⁺$): Colour shifts to yellow.
• In basic solution (more $OH⁻$): Colour shifts to blue.

3. Common Acid-Base Indicators

Indicator	pH Range	Colour Change
Methyl Orange	$3.1 • 4.4$	$Red → Yellow$
Methyl Red	$4.2 • 6.2$	$Red → Yellow$
Litmus	$4.5 • 8.3$	$Red → Blue$
Bromothymol Blue	$6.0 • 7.6$	$Yellow → Blue$
Phenolphthalein	$8.3 • 10.0$	$Colourless → Pink$
Universal Indicator	$3 • 11$	$Red → Orange → Yellow → Green → Blue → Violet$

4. Choosing the Right Indicator

Steps to Select an Indicator

1. Identify the strength of the acid and base ($pH$ range of the reaction).
2. Find an indicator whose $pH$ range overlaps the reaction's pH.
3. Check the colour change range to ensure visibility.

5. Summary Table for Indicator Selection

Reaction Type	Expected pH at Equivalence	Best Indicator
$Strong Acid + Strong Base$	$pH ~ 7$	Bromothymol Blue
$Strong Acid + Weak Base$	$pH < 7$	Methyl Orange
Weak Acid + Strong Base	pH > 7	Phenolphthalein