Question

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

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

Answer 1

The correct option is B Disodium salt of EDTA

The solutions of metal ions in case of complexometric titrations or volumetric analysis can be titrated against the solutions of the polydentate ligands in the presence of a suitable buffer and the end point can be detected by the use of a suitable indicator.
Polydentate ligands are those ligands which contain more than two donor atoms or coordinating groups in their molecules which can coordinate to the central metal atom or ion.
The most common polydentate ligand is the hexadentate ligand called edta or the ethylenediaminetetraacetic acid ligand. It is also abbreviated as EDTA. It has a total of 6 donor atoms. There are two nitrogen atoms and four oxygen atoms of the four carboxylic acid groups which are all capable of bonding to the metal atom. It is usually represented as $H_{4}Y$ .
Its disodium salt is most commonly used in volumetric titrations as it has better solubility in water than the acid. It is represented as $N{a}_2{H}_{2}Y$. It ionizes as follows:
$N{a}_2{H}_{2}Y\to 2N{a}^{+}+H_2{Y}^{2-}$