Why is EBT the indicator used for EDTA complexometric titration in determining water hardness?

Question

Why is EBT the indicator used for EDTA complexometric titration in determining water hardness?

Answer 1

Eriochrome Black produces a sharp end-point in this titration, though it is not the only indicator suitable for the experiment.

It combines initially with the calcium/magnesium ions to give a red-coloured complex.  EDTA is a much 'better' ligand than EBT and so gradually displaces it.  When all the calcium/magnesium ions have reacted with the EDTA, the blue colour of the uncombined EBT marks the end-point.