Question

The reagent commonly used to determine the hardness of water is:
[A] Oxalic acid.
[B] Disodium salt of EDTA.
[C] Sodium citrate.
[D] Sodium thiosulphate.

Answer 1

Hint: The reagent commonly used in determination of hardness of water is a chelating agent. It is an acid and it forms chelating complexes with the hardness causing salts present in water.

Complete answer:
We know that hardness of water is caused due to the mineral impurities like calcium and magnesium in water.
Presence of bicarbonates, chlorides and sulphates of calcium and magnesium makes the water hard. We can measure this hardness of water through various chemical processes.
The most common method that we use for the measurement of hardness of water is titration through EDTA.
EDTA is ethylenediaminetetraacetic acid and it is used as a chelating agent. We know that chelating agents are chemical compounds which react with metal ions and form a water soluble complex.
We can measure the hardness of water using EDTA through complexometric titration. At a pH of 10, the dissolved salts of calcium and magnesium form complexes with EDTA and the solution is colourless. We use an indication named Erio T which is wine red coloured in hard water and the end point of the titration, the solution turns blue.
Before reaching the end point, there is excess of $C{{a}^{2+}}\text{ and M}{{\text{g}}^{2+}}$ and when we add EDTA to the solution, it starts forming a chelating complex with the ions and eventually there is no free $C{{a}^{2+}}\text{ and M}{{\text{g}}^{2+}}$ left in the solution and EDTA is present in excess which marks the end point by turning the solution blue. We can write the reaction as-
     \[\begin{align}
  & C{{a}^{2+}}+{{H}_{2}}EDT{{A}^{2-}}\to CaEDT{{A}^{2-}}+2{{H}^{+}} \\
 & M{{g}^{2+}}+{{H}_{2}}EDT{{A}^{2-}}\to MgEDT{{A}^{2-}}+2{{H}^{+}} \\
\end{align}\]

Therefore, the correct answer is option [B] Disodium salt of EDTA.

Note:
Complexometric titration is a form of volumetric analysis where the end point is indicated by colour change. EDTA is commonly used in similar titrations to identify metal ions in a solution. It is also used to remove heavy metals from blood.