GATE CE 2021 Set 2 — Question 25
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Environmental Engineering → Water Quality → pH, Turbidity, Hardness & Alkalinity
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Question
The hardness of a water sample is measured directly by titration with solution of ethylenediamine tetraacetic acid (EDTA) using eriochrome black T (EBT) as an indicator. The EBT reacts and forms complexes with divalent metallic cations present in the water. During titration, the EDTA replaces the EBT in the complex. When the replacement of EBT is complete at the end point of the titration, the colour of the solution changes from
Correct answer
(D) wine red to blue
Solution
In the EDTA titration method for determining water hardness:
1.The indicator Eriochrome Black T (EBT) is added to the water sample buffered at pH 10.
2.EBT forms a complex with divalent metal ions (like and ), which has a characteristic wine red color.
3.As EDTA is added during titration, it displaces the EBT from the metal-indicator complex because EDTA forms a more stable complex with the metal ions.
4.At the end point, when all metal ions are sequestered by EDTA, the free EBT indicator is released into the solution. The free form of EBT at pH 10 is blue.
Thus, the color change at the end point is from wine red to blue.Continue learning with Success Tracker
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