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.
