5.11:
Masking and Demasking Agents
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.
5.11:
Masking and Demasking Agents
EDTA titrations may necessitate masking and demasking agents to temporarily protect a particular metal ion in a mixture from the EDTA reaction. These agents facilitate the sequential analysis of the metal ions by forming stable complexes with some—but not all—metal ions during certain steps.
There are many masking agents, such as cyanide, fluoride, triethanolamine, thiourea, and 2,3-bis(sulfanyl)propan-1-ol (formerly 2,3-dimercapto-1-propanol), with the masking agent chosen based on the metal ions involved. For example, a cyanide mask is used during the EDTA titration of a lead and cadmium mixture. Cyanide only masks the cadmium ion, so the lead ion still reacts with EDTA.
Demasking agents are used to release the metal ions from masking agents. For example, formaldehyde acts as a demasking agent for cyanide complexes.