The Chelating ligand used for the estimation of hardness of water is .....

EDTA

The correct answer is Option (3) → EDTA

Hardness of water is due to $\text{Ca}^{2+}$ and $\text{Mg}^{2+}$ ions. Their estimation is done by complexometric titration, where a chelating ligand forms a stable complex with these metal ions.

The standard ligand used is EDTA (Ethylenediaminetetraacetic acid), a hexadentate ligand that tightly binds $\text{Ca}^{2+}$ and $\text{Mg}^{2+}$.

Explanation of each option

1. DMG (Dimethylglyoxime)

DMG is a bidentate ligand mainly used for nickel ($\text{Ni}^{2+}$) detection, forming a red precipitate of $\text{Ni(DMG)}_2$. It is not suitable for $\text{Ca}^{2+}$ and $\text{Mg}^{2+}$ estimation, so not used in water hardness.

2. DMSO (Dimethyl sulfoxide)

DMSO is primarily a polar aprotic solvent, not a strong chelating ligand. It does not form stable complexes with $\text{Ca}^{2+}/\text{Mg}^{2+}$ for titrimetric estimation.

3. EDTA (Ethylenediaminetetraacetic acid)

EDTA is a hexadentate chelating ligand. It forms very stable $1:1$ complexes with $\text{Ca}^{2+}$ and $\text{Mg}^{2+}$. This property makes it ideal for EDTA titration, the standard method for determining water hardness.

4. Ethylenediamine

This is a bidentate ligand, forming weaker complexes than EDTA. Not used for hardness estimation because it does not form sufficiently stable complexes for accurate titration.