Read the passage carefully and answer the questions.

The coordination compounds are of great importance. These compounds are widely present in the mineral, plants and animal world and are known to play many important functions in the area of analytical chemistry, metallurgy, biological systems, industry and medicine. For example, coordination compounds find use in many qualitative and quantitative chemical analysis. Some important metal extraction and purification make use of complex formation. Hardness of water can be estimated by stable complex formation of $ \text{Ca}^{2+}$ and $\text{Mg}^{2+} $

Hardness of water can be estimated by titration with ______.

sodium salt of EDTA

The correct answer is Option (3) → sodium salt of EDTA

Hardness of water is due to the presence of calcium ($Ca^{2+}$) and magnesium ($Mg^{2+}$) ions.

To estimate hardness, a complexometric titration is performed using EDTA (Ethylenediaminetetraacetic acid).

The sodium salt of EDTA is used because it readily forms stable, soluble complexes with $Ca^{2+}$ and $Mg^{2+}$ ions.