Example:

An example of usefulness of masking agent is provided by the extraction of aluminium in the presence of iron with 8 - quinolinol, in chloroform. Iron interference is avoided by the addition of alkali cyanide prior to extraction to form very stable ferrocyanide ion. Nickel may be extracted with dimethylglyoxine in the presence of cobalt if CN^-   is added to mask cobalt. Ni (CN)₄^2-  is destroyed by hydrogen peroxide or formaldehyde  but Co(CN)₄ ^2-  is  very stable. The CNS ^-   and S₂O₃^2- have been used to increase the selectivity in dithizonate extractions. EDTA is one of the most useful masking agents. It forms anionic metal complexes with quite a few metal ions and has been applied with success to extractions by dithizone, 8 - quinolinol, carboxylic acids, acetylacetone and many more. The possibility that a longer equilibrium time may be required in the presence of masking agent can be explored for separations. EDTA forms chelates whose rates of dissociation are low. This has been observed in the extraction of iron and copper 8 - quinolinates and copper thiocarbamate where the presence of EDTA made necessary either longer extraction time or higher pH for complete extraction. There are examples where interfering anions may be masked by using masking cations. Thus, the interference of F^-   in UO₂²⁺ extraction may be masked by adding an excess of Al³⁺ or B³⁺.