🧪 Theory of Indicators: How Acid‑Base Indicators Work

Indicators are organic dyes (weak acids or weak bases) that change colour over a specific pH range. In acid‑base titrations, they signal the endpoint — the moment when the amount of titrant exactly neutralises the analyte. The theory of indicators explains why these colour changes occur and how the dissociation equilibrium or structural rearrangement of the indicator molecule responds to pH variations.

📌 What is an Indicator? Types & Principles

An indicator is typically a weak organic acid or base whose undissociated form has a different colour from its dissociated (ionised) form. Indicators are classified into two main categories:

• Weak acid indicators (e.g., phenolphthalein, bromothymol blue)
• Weak base indicators (e.g., methyl orange, methyl red)

Additionally, a universal indicator is a mixture of several indicators that produces a continuous colour change across a wide pH range (typically pH 4–10), allowing approximate pH determination. It works on the same principle: each component responds to different pH intervals.

🎨 Common Indicators and Their pH Ranges

📖 Two Major Theories of Indicator Action

Two complementary theories explain the mechanism of colour change:

1️⃣ Ostwald’s Theory (Ionisation Theory)

Proposed by Friedrich Wilhelm Ostwald, this theory states that the colour change is due to the ionisation equilibrium of the indicator. Postulates:

• The indicator exists in two forms: undissociated (molecular) and dissociated (ionic).
• These two forms have distinctly different colours.
• In acidic solution, the equilibrium shifts toward the undissociated form; in basic solution, it shifts toward the dissociated form.
• For a weak acid indicator (HIn): HIn ⇌ H⁺ + In⁻. Colour₁ (HIn) ≠ Colour₂ (In⁻).
• For a weak base indicator (InOH): InOH ⇌ In⁺ + OH⁻. Again, two different colours.

Example – Phenolphthalein (weak acid indicator): Represented as HPh. In acidic medium, high [H⁺] suppresses dissociation (Le Chatelier), so colourless HPh dominates. In alkaline medium, H⁺ is removed, equilibrium shifts right, producing pink Ph⁻ ions.

Example – Methyl orange (weak base indicator): Represented as MeOH. In acidic medium, OH⁻ ions are neutralised, shifting equilibrium toward the red Me⁺ form. In alkaline medium, excess OH⁻ pushes equilibrium left, giving yellow unionised MeOH.

2️⃣ Quinonoid Theory (Resonance / Tautomeric Theory)

This theory explains colour changes through structural isomerism (tautomerism). Key postulates:

• Indicators can exist in two tautomeric forms: benzenoid (colourless or pale) and quinonoid (highly coloured).
• The two forms are in dynamic equilibrium.
• Acidic or alkaline conditions favour one form over the other, causing a visible colour shift.
• The quinonoid form has a conjugated double‑bond system that absorbs visible light.

Phenolphthalein: In acidic/neutral medium, it adopts the colourless benzenoid structure. In alkaline medium, it rearranges to the pink quinonoid form.

Methyl orange: In alkaline solution, the benzenoid form (yellow) is stable. In acidic solution, protonation leads to a quinonoid structure (red).

⚖️ Comparison of Ostwald and Quinonoid Theories

🧪 Practical Applications & Selection of Indicators

Choosing the correct indicator is crucial for accurate titrations. The indicator’s pH range must bracket the equivalence point pH. For strong acid–strong base titrations (equivalence pH = 7), phenolphthalein (8–10) or methyl orange (3–4) work, but phenolphthalein gives a sharper endpoint. For weak acid–strong base, use phenolphthalein; for strong acid–weak base, use methyl orange.

📚 Summary of Key Equations & Concepts