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GATE 2021 CE (CE2) – Question 25

Water and Waste Water Quality and Treatment · Water quality standards, parameters and water quality index · 1 mark · Multiple choice

In an EDTA hardness titration with eriochrome black T indicator, EDTA replaces the indicator in divalent-metal complexes. What colour change marks the endpoint?

  1. blue-green to reddish brown
  2. blue to colourless
  3. reddish brown to pinkish yellow
  4. wine red to blue

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Correct answer: (D) wine red to blue

Explanation

The metal–EBT complex is wine red. EDTA binds the hardness ions more strongly and releases the free indicator, which is blue at the buffered titration pH. Complete displacement therefore produces a **wine-red to blue** endpoint, **D**.