Correct option (b) disodium salt of EDTA
Explanation:
Disodium salt of ethylenediaminetetraacetic acid has been used for the estimation of M2+ metal ions like Mg2+ Ca2+ etc. by complexometric titrations. In this titration, indicator ions (D3- ions) form red- metal indicator complex, [MD-] with [M2+] ions.
The release of the free indicator ion is marked by a change from red (colour of M2+ D]- complex) to blue (colour of D3- ion) colour.
