Question

The hardness of a water sample is measured directly by titration with 0.01 M 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

• A. blue-green to reddish brown
• B. blue to colourless
• C. reddish brown to pinkish yellow
• D. wine red to blue

Hint:

In EDTA titrations using Eriochrome Black T (EBT), the color change from wine red to blue signals the complete complexation of metal ions by EDTA.

Answer 1

The Correct Option is D

The EDTA titration is commonly used for measuring the hardness of water, which is due to the presence of divalent metal ions such as calcium (Ca²⁺) and magnesium (Mg²⁺). The titration process is aided by an indicator called Eriochrome Black T (EBT). Initially, EBT forms a complex with the metal ions present in the water sample. This complex is wine red in color, which indicates that metal ions are present.

As EDTA is gradually added, it reacts with the metal ions and replaces the EBT indicator from the complex, forming a stable EDTA-metal ion complex. This replacement is complete at the endpoint of the titration, and the color changes from wine red to blue. The blue color indicates that the metal ions have been completely chelated by EDTA, leaving no metal ions available to bind with the EBT indicator.

Therefore, the correct color change at the end of the titration is from wine red to blue, which corresponds to option (D).

Final Answer: wine red to blue