In the iodometric estimation in the laboratory which process is involved?

Options

A. `Cr_2O_7^(2-)+H^(o+)+I^(ɵ)to2Cr^(3+)I_2`
`I_2+S_2O_3^(2-)toS_4O_6^(4-)+I^(ɵ)`
B. `MnO_4^(ɵ)+H^(o+)+I^(ɵ)toMn^(2+)+I_2`
`I_2+S_2O_3^(ɵ)toS_4O_6^(2-)I^(ɵ)`
C. `MnO_4^(ɵ)+OH^(ɵ)+I^(ɵ)toMnO_2+I_2`
`I_2+S_2O_3^(2-)toS_4O_6^(2-)+I^(ɵ)`
D. `Cr_2O_7^(2-)+OH^(ɵ)+I^(ɵ)to2Cr^(3+)+I_2`
`I_2+S_2O_3^(2-)toS_4O_6^(2-)+I^(ɵ)`

Answer

### Step-by-Step Solution:

1. **Understanding Iodometric Estimation**:
Iodometric estimation is a quantitative analytical method used to determine the concentration of a substance by measuring the amount of iodine that reacts with it. The process involves redox reactions where iodine is liberated and then titrated.

2. **First Step - Reaction of Oxidizing Agents**:
- In iodometric estimation, an oxidizing agent reacts with potassium iodide (KI) in a neutral or weakly acidic medium.
- Common oxidizing agents used include potassium permanganate (KMnO4), potassium dichromate (K2Cr2O7), and hydrogen peroxide.

3. **Example Reaction**:
- For potassium dichromate (K2Cr2O7) in the presence of sulfuric acid (H2SO4), the reaction can be represented as:
\[ K2Cr2O7 + 6KI + 7H2SO4 \rightarrow 2Cr2(SO4)3 + 3I2 + 7H2O + K2SO4 \]
- Here, iodine (I2) is liberated as a product of the reaction.

4. **Second Step - Titration with Sodium Thiosulfate**:
- The liberated iodine (I2) is then titrated with a standard solution of sodium thiosulfate (Na2S2O3).
- Starch is used as an indicator during this titration. The blue-black color indicates the presence of iodine.

5. **End Point of Titration**:
- At the end point of the titration, the blue-black color disappears, indicating that all the iodine has reacted with the thiosulfate to form iodide ions (I−).
- The reaction can be represented as:
\[ I2 + 2S2O3^{2-} \rightarrow 2I^{-} + S4O6^{2-} \]

6. **Conclusion**:
- The process involved in iodometric estimation is primarily the reaction of an oxidizing agent with potassium iodide to liberate iodine, followed by the titration of the liberated iodine with sodium thiosulfate.