Conc `HNO_(3)` is added before proceeding to test for group III. This is to

Options

A. Convert `Fe^(+2)` ion `Fe^(+3)` ion
B. Oxidise any remaining `H_(2)S`
C. From nitrate which give granular precipitate
D. Increases ionisation of `NH_(4)OH`

Answer

To understand why concentrated HNO3 is added before testing for group III cations, we can break down the reasoning step by step:

### Step-by-Step Solution:

1. **Understanding Group III Cations**:
Group III cations in qualitative analysis typically include ions like Fe²⁺, Al³⁺, and others. The behavior of these ions during testing is crucial for proper identification.

2. **Role of Concentrated HNO3**:
Concentrated nitric acid (HNO3) is a strong oxidizing agent. When added to a solution containing Fe²⁺ ions, it serves to oxidize these ions to a higher oxidation state, specifically to Fe³⁺ ions.

3. **Oxidation of Fe²⁺ to Fe³⁺**:
The oxidation reaction can be represented as:
\[ \text{Fe}^{2+} \xrightarrow{\text{HNO}_3} \text{Fe}^{3+} \]
This conversion is important because Fe²⁺ ions are less stable and more soluble than Fe³⁺ ions.

4. **Solubility Considerations**:
Ferrous hydroxide (Fe(OH)₂) is more soluble in water compared to ferric hydroxide (Fe(OH)₃). Thus, if Fe²⁺ ions remain in the solution, they may not precipitate effectively during subsequent tests. By converting them to Fe³⁺, we ensure that they will precipitate as Fe(OH)₃, which is less soluble.

5. **Elimination of H₂S**:
Concentrated HNO3 also helps to oxidize any remaining hydrogen sulfide (H₂S) in the solution. This is important because H₂S can interfere with the precipitation reactions and the identification of cations.

6. **Increasing Ionization of NH4OH**:
While NH₄OH (ammonium hydroxide) is used in qualitative analysis to precipitate hydroxides, the addition of concentrated HNO3 primarily focuses on the oxidation of Fe²⁺ to Fe³⁺ and the removal of H₂S. The ionization of NH₄OH is not significantly affected by the addition of HNO3 in this context.

7. **Conclusion**:
Therefore, the primary reason for adding concentrated HNO3 before testing for group III cations is to convert Fe²⁺ ions to Fe³⁺ ions, ensuring effective precipitation and accurate identification of the cations.

### Final Answer:
Concentrated HNO3 is added before proceeding to test for group III to convert Fe²⁺ ions to Fe³⁺ ions, ensuring effective precipitation of ferric hydroxide.

---