Masking and demasking reagents

Masking and demasking are techniques used to selectively protect or reveal certain ions or functional groups during chemical reactions or analysis.
These techniques are particularly useful in complex mixtures to control the reactivity of specific components and prevent unwanted side reactions.

Masking involves temporarily protecting a specific ion or group by forming a stable, less reactive complex with a masking agent.
This allows selective reactions to occur without interference from the masked species.
Example: In complexometric titrations, a masking agent can bind certain metal ions, preventing them from reacting with the titrant while allowing other ions to be titrated.

Demasking is the reverse process where the masked ion or group is released by altering conditions (e.g., pH) or adding a demasking agent.
Example: After completing the titration of unmasked ions, demasking agents can be added to release the previously masked ions, allowing them to be titrated separately.

Common Masking Reagents:
- Ethylenediaminetetraacetic acid (EDTA): Masks many metal ions by forming stable complexes.
- Cyanide ions (CN⁻): Masks metals like silver (Ag⁺) and copper (Cu²⁺) by forming cyanide complexes.
- Dimethylglyoxime (DMG): Masks nickel (Ni²⁺) by forming a stable red complex.
- 1,10-Phenanthroline: Masks iron (Fe²⁺) by forming a stable complex.
Common Demasking Reagents:
- Hydrogen peroxide (H₂O₂): Demasks chromium (Cr³⁺) from its EDTA complex by oxidation.
- Oxalic acid: Demasks calcium (Ca²⁺) from fluoride by forming calcium oxalate.
- Ammonium hydroxide (NH₄OH): Demasks aluminium (Al³⁺) and iron (Fe³⁺) from fluoride complexes by raising the pH.
- Ascorbic acid: Demasks copper (Cu²⁺) by reducing it to Cu⁺, releasing it from cyanide complexes.