Question

Why lead (II) can be titrated with EDTA at pH 5 but barium cannot and requires a basic pH

Answer 1

The titration reactions are all dependent on the pH value as it is related to the different stabilities of metal-EDTA complexes.
Lead Pb(2+) can be titrated in acidic conditions at pH = 5 as the effective titration happens at suitable pH in presence of indicator like xylenol orange or methylthymol blue.
But it is necessary to carry out some titrations at pH =10 likewise Barium as basic conditions are mandatory for the titration to go into completion, as already said the Barium-EDTA complex is stable at Higher pH values.
Moreover this pH is responsible for the particular color change of the complex during the titration.