Question

The reagent used to determine hardness of water titrimetrically is

• A. Oxalic acid
• B. EDTA
• C. Sodium citrate
• D. Sodium thiosulphate

Answer 1

The correct option is B EDTA

Calcium and magnesium salts dissolved in water cause water hardness. Water hardness can be measured using a titration with EDTA. EDTA dissolved in water forms a colourless solution. At pH 10, calcium and magnesium ions form colourless, water soluble, $\underset{–}{complexes}$ with EDTA.calcium ion complexed by EDTA :
​$CaEDT{A}^{2-}$ magnesium ion complexed by EDTA : $MgEDT{A}^{2-}$ An indicator, known as a metal ion indicator, is required for the titration. This indicator can be used to detect the presence or absence of free EDTA ions in solution.Endpoint of the titration is when all the $C{a}^{2+}andM{g}^{2+}$ ions have been complexed by the EDTA. Before the endpoint, $C{a}^{2+}andM{g}^{2+}$ are in excess, there is no free EDTA in solution.
Immediately after the endpoint, there is an excess of EDTA. colour changes to blue