`NO_(2)^(-)` and `NO_(3)^(-)` can be distinguished by which of the following reagent?

Options

A. Dil. `H_(2)SO_(4)`
B. Conc. `H_(2)SO_(4)`
C. Devarda's alloy `+` conc. `NaOH`
D. None of the above

Answer

To distinguish between `NO2^(-)` (nitrite) and `NO3^(-)` (nitrate), we can utilize the following step-by-step approach:

### Step 1: Understanding the Reagents
We need to identify the reagents that can differentiate between nitrite and nitrate ions. The two main reagents we will consider are dilute sulfuric acid (H2SO4) and concentrated sulfuric acid.

### Step 2: Reaction with Dilute H2SO4
When we add dilute H2SO4 to a solution containing nitrite ions (`NO2^(-)`), the following reaction occurs:
- The nitrite ions react with the hydrogen ions from dilute sulfuric acid to form nitrous acid (`HNO2`).
- This nitrous acid can decompose to produce nitrogen trioxide (`N2O3`), water, and some remaining `HNO2`.

The key observation here is that the solution will turn pale blue due to the formation of `N2O3`.

### Step 3: Reaction with Concentrated H2SO4
In contrast, when concentrated sulfuric acid is added to a solution containing nitrate ions (`NO3^(-)`), the following reaction occurs:
- The nitrate ions react with concentrated sulfuric acid to produce nitrogen dioxide (`NO2`), oxygen (`O2`), sulfate ions, and water.
- The nitrogen dioxide produced is characterized by reddish-brown fumes.

### Step 4: Observing Color Changes
- If we observe a **pale blue color**, it indicates the presence of nitrite ions (`NO2^(-)`).
- If we observe **reddish-brown fumes**, it indicates the presence of nitrate ions (`NO3^(-)`).

### Conclusion
Thus, the distinguishing reagent for `NO2^(-)` and `NO3^(-)` is **dilute sulfuric acid (H2SO4)**, as it will react specifically with nitrite ions to produce a pale blue solution, while nitrate ions will not react with dilute H2SO4.

### Final Answer
The correct answer is **dilute sulfuric acid (H2SO4)**.

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