- 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.
1. Masking :
- 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.
2. Demasking:
- 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.
Masking and Demasking Reagents
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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.
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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.
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