Question

Assertion :Hardness of water is determined by tinting it with disodium salt of $EDTA$ Reason: The indicator used in the titration is Erichrome black-$T$ at $pH=10$

Read the above assertion and reason and choose the correct option regarding it.

• A. Both Assertion and Reason are correct and Reason is the correct explanation for Assertion
• B. Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion
• C. Assertion is correct but Reason is incorrect
• D. Both Assertion and Reason are incorrect

Answer 1

The correct option is A Both Assertion and Reason are correct and Reason is the correct explanation for Assertion

R.E.F image

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 $\left(H_2{Y}^{2-}\right)$. The titration reaction is :

$C{a}^{2+}\left(aq\right)+H_2{Y}^{2-}\left(aq\right)\to Ca{Y}^{2-}\left(aq\right)+2H^{+}\left(aq\right)$

Eriochrome Black T is commonly used as indicator for the above titration. At pH 10,

$C{a}^{2+}\left(aq\right)$ ion first complexes with the indicator as $CaI{n}^{+}\left(aq\right)$ which is wine red. As the

stronger ligand EDTA is added, the $CaI{n}^{+}\left(aq\right)$ complex is replaced by the $Ca{Y}^{2-}\left(aq\right)$

complex which is blue. The end point of titration is indicated by a sharp colour change

from wine red to blue.