The selectivity of EDTA reactions is improved by masking and demasking agents.

A masking agent chelates with a potential interfering ion to form a more stable complex than the unwanted EDTA complex, keeping the ion from participating in the reaction.

Cyanide, fluoride, triethanolamine, and thiourea are common masking agents.

A demasking agent makes the masked metal ion available to the reaction.

Masking and demasking also allow the successive titration of several metal ions.

Consider the titration of a mixture of lead, magnesium, and zinc ions with EDTA. Excess sodium cyanide masks the zinc ion, preventing the reaction with EDTA.

Then, the lead and magnesium ions are titrated together with standard EDTA. 

After the equivalence point, the bidentate ligand 2,3-bis(sulfanyl)propan-1-ol, also known as British anti-Lewisite, selectively reacts with lead, forming a more stable complex than the lead–EDTA complex.

The resulting free EDTA is titrated with a standard magnesium-ion solution.

Lastly, formaldehyde demasks the zinc ions, which are titrated with standard EDTA.