A solution containing Mg2 was buffered to pH 10 and titrated with EDTA. Eriochro

Question

A solution containing Mg2 was buffered to pH 10 and titrated with EDTA. Eriochrome black T, with pKa values of 6.3 (pKa2) and 11.6 (pKa3), was used as the indicator. What is the predominant form of eriochrome black T in solution at pH 10?

A. HIn2-

B. ln3-

C. H2ln-

D. H3ln

What color is the solution before the equivalence point?

blue

yellow

orange

wine red

what color is the solution after the equivalence point?

blue

orange

wine red

yellow

hint:

Eriochrome black T is a triprotic acid. The fully protonated form of eriochrome black T is represented as H3In.

Metal ion indicators used in EDTA titrations are also acid-base indicators. The predominant species of eriochrome black T is dependent on where the pH lies in relation to the pKa values. Metal ion indicators change color when they are complexed to a metal ion. Consider whether the indicator is complexed to the metal ion before or after the equivalence point of the titration.

Explanation / Answer

At pH 10 the major species in solution would be,

In3-

The color of the solution beyond equivalence point is,

blue

Erichrome black T indicator is bound to the metal ion before the equivalence point. Before the equivalence point the color is pink for MIn complex. After equivelance point indicator is free of metal and gives a blue color.

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