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 ______.
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → sodium salt of EDTA
The hardness of water is due to dissolved \( \ce{Ca^2+} \) and \( \ce{Mg^2+} \) ions. It is estimated by complexometric titration: the water is titrated against a standard solution of the disodium salt of EDTA (ethylenediaminetetraacetic acid). The EDTA ion is a hexadentate ligand; it binds one metal ion through its two nitrogen atoms and four carboxylate oxygen atoms, giving a very stable 1:1 chelate:
\( \displaystyle \ce{Ca^2+ + EDTA^4- -> [Ca(EDTA)]^2-} \)
\( \displaystyle \ce{Mg^2+ + EDTA^4- -> [Mg(EDTA)]^2-} \)
The volume of EDTA solution used gives the total amount of \( \ce{Ca^2+} \) and \( \ce{Mg^2+} \). So hardness is estimated by titration with the sodium salt of EDTA.
NCERT (2026-27 edition):
Importance and Applications of Coordination Compounds – …
