Question

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

• A. $N{a}_2\left[Ca\right(P{O}_3{)}_6]$
• B. $N{a}_2\left[Mg\right(EDTA\left)\right]$
• C. $N{a}_2\left[Pb\right(EDTA\left)\right]$
• D. $\left[Ca\right(S{O}_4{)}_2{]}^{2-}$

Answer 1

Answer: (B)

$N{a}_2\left[Mg\right(EDTA\left)\right]$

The ions involved in water hardness, i,e., $C{a}^{2+}\left(aq\right)$ and $M{g}^{2+}\left(aq\right)$, can be determined by titration with a chelating agent, ethylenediaminetetraacetic acid(EDTA), usually in the form of disodium salt $\left(N{a}_2{Y}^{2-}\right)$.

The titration reaction is:

$M{g}^{2+}\left(aq\right)+N{a}_{2}EDT{A}^{2-}\left(aq\right)\to N{a}_2\left[Mg\right(EDTA\left)\right]\left(aq\right)$

Eriochrome Black T is commonly used as indicator for the above titration

Since lead is not involved in water hardness so it won't form a complex with EDTA.

Hence the correct option is B.