Discussion
The EDTA titration of hardness is buffered at pH 10 because at a lower pH
Answer: B
Correct answer: the metal-EDTA complexes are unstable
EDTA is a weak acid, and at low pH its donor groups are protonated so it cannot chelate effectively - the metal complexes dissociate and the end point is lost. A pH 10 ammonia buffer keeps enough of the EDTA deprotonated for the complexes to be stable, while remaining low enough that magnesium hydroxide does not precipitate.
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