The pH Effect in EDTA Complexing

The success of EDTA titration depends on the high Kf values for most metal ions (Table 9-2). Since the ligand is taken to be a rather strong base, the pH will be important in determining its effective concentration. The [Y ] is given by aoy times the EDTA not in MY, the metal ion complex. 

We can now get the minimum pH condition for quantitative reaction with EDTA (99.9%) using the Kf expression. With 0.01 M final metal ion, one wants only 10 m uncomplexed (0. 1 %). At the 

Most of the transition metal ions have log Kf above 14 for EDTA and can be titrated in acidic solutions. This has advantages in the choices of indicators it makes possible. 

Masking, competition of other ligands for the metal ion in EDTA methods, can be treated by similar steps. 

Example 4. Show why EDTA titration is found to work for Hg(N03)2 solutions under slightly acid conditions, but does not work if chloride is present. At what pH should EDTA complex Hg(II) if unbound chloride is 0.01 M  

---