Question

Question #0ae0f

Answer 1

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.