Question #0ae0f

1 Answer
anor277
Sep 15, 2016

Because you use a particular indicator, the delightfully named Eriochrome Black T.

Explanation:

When you add the indicator it binds to the metal cations present. As you add "EDTA" by titration, the "EDTA" binds to the metal complex present, displacing the BOUND "Eriochrome Black T".

When the indicator was bound as a complex, the colour was red; the free base is blue. "Eriochrome Black T" is simply another indicator, whose acid and base forms are differently coloured.

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