Disodium hydrogen phosphate is used to test :

Options

A. `Mg^(2+)`
B. `Na^(o+)`
C. `Ca^(2+)`
D. All of these

Answer

To solve the question regarding the use of disodium hydrogen phosphate, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Compound**: Disodium hydrogen phosphate is represented chemically as Na₂HPO₄. It is important to know its composition and how it behaves in solution.

2. **Identify the Test**: Disodium hydrogen phosphate is used to test for the presence of certain metal ions in solution. Specifically, it is known to react with magnesium ions (Mg²⁺) in a specific buffer solution.

3. **Buffer Solution**: The test is conducted in an ammonia-ammonium chloride buffer. This buffer maintains a specific pH that is conducive for the precipitation reaction to occur.

4. **Reaction with Magnesium Ions**: When disodium hydrogen phosphate is added to a solution containing magnesium ions in the presence of the buffer, it reacts to form magnesium ammonium phosphate (MgNH₄PO₄). This compound precipitates out of the solution as a characteristic white crystalline solid.

5. **Conclusion**: Since disodium hydrogen phosphate specifically produces a precipitate with magnesium ions, it is primarily used to test for Mg²⁺ ions. The other options (sodium ions and calcium ions) do not produce a precipitate with disodium hydrogen phosphate under these conditions.

6. **Final Answer**: Therefore, the correct answer to the question is that disodium hydrogen phosphate is used to test for **Magnesium 2 plus (Mg²⁺)** ions.