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208 Modern Analytical Chemistry

7
Table .6 Selected Masking Agents
Masking Agent Species Which Can Be Masked
CN – Ag, Au, Cd, Co, Cu, Fe, Hg, Mn, Ni, Pd, Pt, Zn
SCN – Ag, Cd, Co, Cu, Fe, Ni, Pd, Pt, Zn
NH 3 Ag, Co, Cu, Fe, Pd, Pt
F – Al, Co, Cr, Mg, Mn, Sn, Zn
2–
S 2 O 3 Au, Cd, Co, Cu, Fe, Pb, Pd, Pt, Sb
tartrate Al, Ba, Bi, Ca, Ce, Co, Cr, Cu, Fe, Hg, Mn, Pb, Pd, Pt, Sb, Sn, Zn
oxalate Al, Fe, Mg, Mn, Sn
thioglycolic acid Cu, Fe, Sn

Source: Adapted from Meites, L. Handbook of Analytical Chemistry, McGraw-Hill: New York, 1963.

technique because the analyte and interferent are never physically separated from each
other. Masking can, however, be considered a pseudo-separation technique, and is in-
cluded here for that reason. A wide variety of ions and molecules have been used as
masking agent masking agents (Table 7.6), and, as a result, selectivity is usually not a problem. 13
The reagent used to bind the species to
be masked in an unreactive complex.
7
EXAMPLE .12
Suggest a masking agent for the analysis of Al in the presence of Fe. Repeat for
the analysis of Fe when Al is an interferent.

SOLUTION
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.

As shown in Example 7.13, a masking agent’s effectiveness can be judged by
considering the equilibrium constants for the analytical and masking reactions.

7 3
EXAMPLE .1
–
Show that CN is an appropriate masking agent for Ni 2+ in a method in which
nickel’s complexation with EDTA is an interference.
SOLUTION
The relevant reactions and equilibrium constants from Appendix 3C are
4–
Ni 2+ +Y ® NiY 2– K f = 4.2 ´10 18
–
Ni 2+ + 4CN ® Ni(CN) 4 2– b 4 = 1.7 ´10 30
in which Y 4– is an abbreviation for ethylenediaminetetraacetic acid (EDTA).
Cyanide is an appropriate masking agent because the formation constant for