Hardness of water is estimated by titration with Na2EDTA, and the Ca2+ and Mg2+ ions can be estimated selectively. A difference in which of the following makes this selective estimation possible?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → The stability constants of the calcium and magnesium complexes
Both Ca2+ and Mg2+ form stable complexes with EDTA, which is why Na2EDTA can be used to estimate the hardness of water. The two complexes are not equally stable: their stability constants differ, and this difference lets the two ions be estimated separately.
Why the other options are incorrect
Option (1) The colours of the calcium and magnesium complexes — Incorrect. Ca2+ ([Ar]) and Mg2+ ([Ne]) have no d electrons, so no d–d transition can give their complexes a colour.
Option (2) The magnetic moments of the calcium and magnesium complexes — Incorrect. With noble gas configurations, both ions have no unpaired electrons; both complexes are diamagnetic.
Option (4) The oxidation states of calcium and magnesium in the complexes — Incorrect. Both metals are present as +2 ions, so there is no difference to use.
NCERT (2026-27 edition):
5.7 Importance and Applications of Coordination Compounds – “Hardness of water is estimated by simple titration with Na2EDTA. The Ca2+ and Mg2+ ions form stable complexes with EDTA. The selective estimation of these ions can be done due to difference in the stability constants of calcium and magnesium complexes.”