Which of the following colour changes shown during redox titrations is not correct ?

Options

A. `Cr_(2)O_(7)^(2-)` oxidises the indicator diphenylamine to produce blue colour showing end point.
B. Iodine formed by oxidation of `I^(-)` ions gives blue colour with starch showing end point.
C. `KMnO_(4)` in the form of `MnO_(4)^(-)` ions gives pink colour showing end point.
D. Thiosulphate ions `(S_(2)O_(3)^(2-))` give blue colour showing end point.

Explanation

To determine which of the given color changes during redox titrations is incorrect, we will analyze each statement one by one.

### Step-by-Step Solution:

1. **Statement 1: Cr2O7^2- oxidizes diphenyl amine to produce blue color showing endpoint.**
- **Analysis:** In redox titrations involving dichromate ions (Cr2O7^2-), diphenyl amine is indeed used as an indicator. When the endpoint of the titration is reached, the diphenyl amine changes color from green to blue.
- **Conclusion:** This statement is correct.

2. **Statement 2: Iodine formed by oxidation of I^- ions gives blue color with starch showing endpoint.**
- **Analysis:** In iodometric titrations, iodine (I2) is produced by the oxidation of iodide ions (I^-). When iodine reacts with starch, it forms a blue complex, indicating the endpoint of the titration.
- **Conclusion:** This statement is correct.

3. **Statement 3: KMnO4 in the form of MnO4^- ions gives pink color showing endpoint.**
- **Analysis:** Potassium permanganate (KMnO4) acts as a self-indicator in redox titrations. The MnO4^- ion is purple (often referred to as pink), and it changes color to colorless (or light brown) when it is reduced, indicating the endpoint of the titration.
- **Conclusion:** This statement is correct as it describes the initial color of KMnO4 before the endpoint.

4. **Statement 4: Thiosulphate ions (S2O3^2-) give blue color showing endpoint.**
- **Analysis:** Thiosulphate ions are not indicators and do not produce any color change at the endpoint of titrations. They are typically used to reduce iodine back to iodide ions, and they do not exhibit a color change themselves.
- **Conclusion:** This statement is incorrect.

### Final Answer:
The statement that is not correct is **Statement 4: Thiosulphate ions (S2O3^2-) give blue color showing endpoint.**

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