Question

When you titrate an aqueous solution containing Ca2+ and Mg2+ with EDTA, you used ammonia buffer at pH 10. Explain why?

Answer 1

It is so because :

1)

all reaction between EDTA and metal ions are pH dependant ie EDTA reacts with metal ions (generally divalent cations like Mg2+ and Ca2+) in basic condition ie around pH 10 because basic buffer removes the H+ as it is formed and thus favoured formation of colored complex.

They can react directly according to the equation:
M2+ + Y4- ====> MY2-

Therefore in order to have reaction completion pH should be maintained around 10 by ammonia buffer.

2) The indicator used in the titration ie Eriochrome black T indicator require pH of arond 10 to develop a color change.