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DrX Whiz Niraj
Types of Redox Titrations: Principles & Applications
Comprehensive exam notes on Iodimetry vs Iodometry, Cerimetry, Bromatometry, Dichrometry, and Potassium Iodate (Andrews) Titrations.
Both methods involve Iodine, but their principles and procedures are entirely opposite. This is the most frequently asked question in university exams.
| Parameter | Iodimetry (Direct) | Iodometry (Indirect) |
|---|---|---|
| Definition | Direct titration of a reducing sample with a standard Iodine solution. | Titration of liberated Iodine (produced by reacting an oxidizing sample with excess KI) against standard Sodium Thiosulphate. |
| Titrant in Burette | Standard Iodine (I₂) solution. | Standard Sodium Thiosulphate (Na₂S₂O₃) solution. |
| Analyte Nature | Analyte is a Reducing Agent. | Analyte is an Oxidizing Agent. |
| Addition of Starch | Added at the beginning of the titration. | Added near the end point (when iodine color fades to pale yellow). |
Practical Alert: Starch Indicator Rule
In Iodometry, the starch indicator is NEVER added at the beginning. If added when iodine concentration is very high, starch forms a permanent, water-insoluble complex with iodine, ruining the titration.
It must be added near the end point, when the dark brown color of iodine fades to a pale yellow (straw) color. Once added, the solution turns dark blue, and titration continues until it becomes colorless.
Cerimetry
Utilizes Ceric Ammonium Sulphate as a strong oxidizing agent. In acidic medium, Cerium (IV) is reduced to Cerium (III).
- Equation: Ce⁴⁺ + e⁻ → Ce³⁺
- Indicator: Ferroin (Changes from Red to Pale Blue).
- Advantage: Solutions are highly stable and can be used in the presence of high HCl concentrations (unlike KMnO₄).
- Application: Assay of Ferrous sulphate, Paracetamol.
Dichrometry
Utilizes Potassium Dichromate (K₂Cr₂O₇) as a moderately strong oxidizing agent. It is a primary standard.
- Equation: Cr₂O₇²⁻ + 14H⁺ + 6e⁻ → 2Cr³⁺ + 7H₂O
- Indicator: Diphenylamine (Turns Purple/Violet at end point).
- Application: Analysis of iron ores and alloys.
Bromatometry
Uses Potassium Bromate (KBrO₃) as a strong oxidizing and brominating agent. In acidic medium, it liberates free bromine (Br₂).
- Indicator: Methyl orange or Methyl red. (The free bromine irreversibly decolorizes/bleaches the indicator at the end point).
- Application: Assay of Phenol, Isoniazid (Anti-TB drug).
Potassium Iodate (Andrews Titration)
Uses KIO₃ in a strongly acidic medium (high HCl concentration). The iodine formed initially is further oxidized to Iodine monochloride (ICl).
- Indicator: Organic solvent layer (Chloroform or CCl₄).
- End Point: The free iodine gives the chloroform layer a purple color, which completely disappears (becomes colorless) when all I₂ is converted to ICl.
- Application: Assay of Potassium iodide, Ascorbic acid.