Question #0ae0f

1 Answer
Sep 15, 2016

Answer:

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.