Masking agents table

Demasking methods for a number of masking agents are enumerated in Table 11.38. 

TABLE 11.36 Masking Agents for Various Elements Continued)... 

TABLE 11.37 Masking Agents for Anions and Neutral Molecules... 

To find a suitable masking agent, we look for a species that binds with the interferent but does not bind with the analyte. Oxalate, for example, is an inappropriate choice because it binds with both Al and Fe. From Table 7.6 we find that thioglycolic acid is a selective masking agent for Fe in the presence of Al and that F is a selective masking agent for Al in the presence of Fe. 

A widely applicable masking agent is sodium triphosphate, Na5P3Oi0-6H2O, which readily complexes with a very wide variety of cations in all groups of the Periodic Table, preventing their precipitation by alkali hydroxide, ammonia, phosphate, carbonate or borate. It is used commercially as Calgon to mask calcium which cannot then form precipitates with citrate, fluoride or oxalate ions and in many other instances (see Table 3). 

Table 5 Use of Masking Agents in the Extraction of Metal Dithizonates7 8...

In many cases the separation of mixtures must be preceded by conversion of the main component into a stable complex, in order to retain it in the solution and prevent its extraction along with the separated microcomponents. The masking agents most appropriate for individual elements are shown in Table 3.1. 

Similarly, the use of cyanide as a masking agent will permit the selective extraction of AP+ by 8-quinolinol in the presence of such transition-metal ions as Cu " and Fe ", as well as Ag" AP" does not form a CN complex, whereas the other metals form strong complexes. Other examples of successful masking can be predicted with the help of Table 20.3. Masking such as this is also useful in improving the selectivity of ion-exchange separations. 

Table 20.3. Values of Masking Factor (- log aufrom Eqn. 7Q. 6)for Representative Metal Ions and Masking Agents at Various pH Values... 

Table 15 Influence of various hydride-forming elements on the determination of antimony, and the effect of masking agents. (Source L. H. J. Lajunen, T. Merkkiniemi, and H. Hdyrynen, Talanta, 1984, 31, 709)...

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