Redox Titration

1. Introduction

• Redox titration is a quantitative analytical method used to determine the concentration of an unknown analyte by reacting it with a titrant in an oxidation-reduction reaction.
• Involves electron transfer between a reducing agent (which loses electrons) and an oxidizing agent (which gains electrons).
• The endpoint is detected using a color change (self-indicating or with an external indicator) or electrochemical measurement.

2. Principle

• The reaction follows the general form:

Oxidizing Agent  +  e’s     →       Reduced Form

Reducing Agent →         Oxidized Form    +  e’s

• The equivalence point is reached when the number of electrons lost by the reducing agent equals the number of electrons gained by the oxidizing agent.

3. Types of Redox Titrations

(i) Permanganometry (Using KMnO₄)

• Potassium permanganate (KMnO₄) is a strong oxidizing agent.
• Acts in acidic medium (H₂SO₄), where MnO₄⁻ is reduced to Mn²⁺:

MnO₄⁻ +     8H⁺   + 5e⁻→   Mn²⁺   +   4H₂O

Used for:

• Estimation of Fe²⁺ in iron salts.
• Oxalates, sulphites, hydrogen peroxide, etc.
• Self-indicating (purple KMnO₄ turns colorless as Mn²⁺ forms).

(ii) Dichrometry (Using K₂Cr₂O₇)

• Potassium dichromate (K₂Cr₂O₇) is an oxidizing agent used in acidic medium: Cr₂O₇²⁻   +   14H⁺   +   6e⁻   →   2Cr³⁺   +   7H₂O
• Used for:
  • Estimation of Fe²⁺, Sn²⁺, and sulphites.
• Requires an external indicator (e.g., diphenylamine or ferroin).

(iii) Iodometry (Indirect Redox Titration)

• Based on the oxidation of iodide (I⁻) to iodine (I₂), followed by titration with sodium thiosulfate (Na₂S₂O₃).

I₂​   +   2S₂​O₃²⁻   ​→   2I⁻   +   S₄​O₆²⁻

• Used for:
  • Estimation of Cu²⁺, chlorine, and oxidizing agents.
• Starch indicator is used (blue-black complex with iodine).

(iv) Cerimetry (Using Ce⁴⁺)

• Cerium(IV) sulphate acts as an oxidizing agent:

Ce⁴⁺   +   e⁻   →   Ce³⁺

Used for:

• Estimation of Fe²⁺, Sn²⁺, and As³⁺.
• Advantages: Rapid reaction and stable standard solution.

(v) Bromatometry (Using KBrO₃)

• Based on the oxidizing action of potassium bromate (KBrO₃) in acidic medium.
• Used for the estimation of:
  • Phenols, aniline, and vitamin C.

4. Indicators Used in Redox Titrations

• Self-indicators: KMnO₄ (purple to colorless).
• External indicators:
  • Diphenylamine (for Cr₂O₇²⁻ titrations).
  • Starch (for iodometric titrations, blue-black complex with I₂).

5. Applications of Redox Titration

• Industrial Applications:
  • Determination of Fe²⁺ in steel and iron ores.
  • Estimation of hydrogen peroxide in disinfectants.
• Pharmaceutical Applications:
  • Determination of ascorbic acid (Vitamin C).
  • Analysis of drugs containing oxidizing/reducing agents.
• Environmental Applications:
  • Estimation of dissolved oxygen (DO) in water samples.
  • Determination of oxidizing/reducing pollutants in wastewater.

6. Advantages & Limitations

Advantages:

✔️ High precision and accuracy.
✔️ Simple and cost-effective method.
✔️ Suitable for both strong and weak oxidizing/reducing agents.

Limitations:

❌ Requires stable oxidation states for accurate results.
❌ Some reactions are slow and may require heating.
❌ Interference from other redox-active species can lead to errors.

Conclusion

Redox titrations are an essential tool in analytical chemistry, widely used in industries, pharmaceuticals, and environmental studies. Their accuracy and applicability make them valuable in determining the concentration of redox-active substances.