The reagent commonly used to determine hardness of water titrimetrically is `:`

Options

A. Oxalic acid
B. Disodium salt of EDTA
C. Sodium citrate
D. Sodium thiosulphate.

Answer

B

Explanation

Disodium salt of ethylene diamine tetracetic acid has been used for the estimation of `M^(2+)` metal ions like `Mg^(2+), Ca^(2+)` etc. by complexometric titrations. In this titration, indicator ions (`D^(3-)` ions) form red-metal indicator complex , `[MD^-] " with M^(2+)` ions.
`underset("metal ion" (Mg^(2+) or Ca^(2+)))(M^(2+))+underset"indicator ion (blue )"(D^(3-)) overset"pH=10"iff underset"Metal indicator complex (red)"([M^(2+)D]^-)`
`underset"(red)"([M^(2+)D]^-) + underset"EDTA ion"(EDTA^(4-))iff underset"metal -EDTA complex "([M^(2+)(EDTA)]^(2+))+ underset"(blue)"(D^(3-))`
The release of the free indicator ion is marked by a change from red (colour of `[M^(2+)D]^-` complex) to blue (colour of `D^(3-)` ion ) colour.