Question

why can lead (II) be titrated with EDTA at ph 5 but magnesium cannot and requires a basic ph

Answer 1

The complex forming tendancy of metal ions with EDTA depends upon the pH of solution.

so metal ions form strong complexes with EDTA at low pH (like Lead(II)) while most of the other metal ions form more stable complex with EDTA at higher pH (like Mg+2).

The variation in the pH range is due to variation in the formation constant of complex between EDTA and any metal ion.

The formation of strong complex is desirable as we generally use an indicator which should have less affinity for metal ion as compared to EDTA.

For example : Erichrome Black -T