Iodometry & Iodimetry
Redox Titrations involving Iodine | Basic Concepts of Chemistry
1. Introduction
Titrations involving Iodine ($I_2$) are essential redox analyses. Iodine acts as a mild oxidizing agent. These titrations are broadly classified into two types: Iodimetry and Iodometry.
Common Titrant: Sodium Thiosulphate ($Na_2S_2O_3 \cdot 5H_2O$), commonly known as Hypo, is used to titrate iodine.
2. Iodimetry (Direct Titration)
This is a direct titration method. A standard solution of Iodine ($I_2$) is used to titrate a reducing agent.
- Process: The reducing agent (analyte) is directly titrated with Iodine solution.
- Analytes: Reducing agents like Sulphides ($S^{2-}$), Sulphites ($SO_3^{2-}$), Thiosulphates ($S_2O_3^{2-}$), Arsenites.
- Reaction Condition: Usually carried out in neutral or mildly acidic/alkaline medium.
$$ 2S_2O_3^{2-} + I_2 \rightarrow S_4O_6^{2-} + 2I^- $$
Here, $S_2O_3^{2-}$ (Thiosulphate) is oxidized to $S_4O_6^{2-}$ (Tetrathionate), and $I_2$ is reduced to Iodide ($I^-$).
3. Iodometry (Indirect Titration)
This is an indirect titration method. It is used to estimate oxidizing agents.
- Step 1: An oxidizing agent (analyte) reacts with excess Potassium Iodide ($KI$) in acidic or neutral medium to liberate free Iodine ($I_2$). $$ 2Cu^{2+} + 4I^- \rightarrow Cu_2I_2 + I_2 $$
- Step 2: The liberated Iodine is then titrated against a standard Hypo solution ($Na_2S_2O_3$). $$ I_2 + 2S_2O_3^{2-} \rightarrow 2I^- + S_4O_6^{2-} $$
Analytes: Oxidizing agents like $KMnO_4, K_2Cr_2O_7, CuSO_4, H_2O_2, Cl_2$.
4. Comparison Table
| Feature | Iodimetry (Direct) | Iodometry (Indirect) |
|---|---|---|
| Analyte Nature | Reducing Agent | Oxidizing Agent |
| Role of Iodine | Titrant (Standard Solution) | Product (Liberated then titrated) |
| Number of Steps | Single Step | Two Steps |
5. Starch Indicator
Starch solution is used as an indicator near the end point.
- Free Iodine + Starch $\rightarrow$ Intense Blue-Black Complex.
- In Iodometry: Starch is added near the end point (when solution turns pale yellow). Adding it too early can cause iodine to get trapped in starch, leading to error.
- End Point: Disappearance of blue color (Blue $\rightarrow$ Colorless).
Practice Quiz
Test your understanding of Iodine Titrations.
Finally my concept got cleared. Wonderful explanation.
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