Hardness of water is estimated by simple complex formation titration Complex formed by cation in hard water during estimation of hardness is .

Options

A. `Na_(2)[Ca(PO_(3))_(6)]`
B. `Na_(2)[Mg(EDTA)]`
C. `[Ca(So_(4))_(2)]^(2-)`
D. `Na[Pb(EDTA)]`

Answer

To determine the complex formed by cations in hard water during the estimation of hardness, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Hard Water**:
- Hard water contains dissolved salts, primarily calcium (Ca²⁺) and magnesium (Mg²⁺) ions. These ions contribute to water hardness.

2. **Role of EDTA**:
- Ethylenediaminetetraacetic acid (EDTA) is a chelating agent used to estimate water hardness. It forms stable complexes with metal ions such as Ca²⁺ and Mg²⁺.

3. **Complex Formation**:
- When hard water is treated with EDTA, it reacts with the calcium and magnesium ions present in the water. The reaction can be represented as:
\[ \text{Ca}^{2+} + \text{EDTA}^{4-} \rightarrow \text{Ca-EDTA}^{2-} \]
\[ \text{Mg}^{2+} + \text{EDTA}^{4-} \rightarrow \text{Mg-EDTA}^{2-} \]
- This indicates that both calcium and magnesium ions form complexes with EDTA.

4. **Identifying the Correct Complex**:
- The question asks for the specific complex formed by the cation in hard water. The main complexes formed are:
- Calcium-EDTA complex (Ca-EDTA)
- Magnesium-EDTA complex (Mg-EDTA)

5. **Conclusion**:
- Since both calcium and magnesium ions are responsible for hardness, the complexes formed during the estimation of hardness in water are primarily the calcium-EDTA and magnesium-EDTA complexes. However, the question specifically asks for the complex formed by the cation in hard water, which would be either of these two complexes.

6. **Final Answer**:
- The complex formed by cation in hard water during the estimation of hardness is **Ca-EDTA or Mg-EDTA**. However, if we need to select one, we can say **option B (Mg-EDTA)** is a common representation.