Page 73 - Pharmaceutical Analytical Chemistry II - Pharm D Clinical- 07-PA202

IV. Indirect titrations (determination of anions)
Anions that form precipitates with certain metal ions may be

analyzed with EDTA by indirect titration. For example, sulfate can be
analyzed by precipitation with excess Ba2+. The BaSO4 precipitate is
boiled with excess EDTA. The Ba2+ is released back into solution as
Ba(EDTA)2-. The excess EDTA is back-titrated with Mg2+.

EDTA Selectivity

EDTA is a very unselective chelating agent because it reacts with
numerous bi-, tri-, and tetravalent metals. However, the selectivity can be
highly increased by:

(a) Masking and demasking
Masking is the addition of a reagent that protects some component

of the analyte from the reaction with EDTA. Masking agent is an
auxiliary ligand that preferentially forms highly stable complexes with
some components of the sample. Cyanide is a common masking agent
that forms complexes with Cd2+, Zn2+, Hg2+, Co2+, Cu2+, Ag+, Ni2+, Pd2+,
Pt2+, Fe2+, and Fe3+, but not with Mg2+, Ca2+, Mn2+, or Pb2+. If cyanide is
first added to a solution containing Cd2+ and Pb2+, only the Pb2+ is then
able to react with EDTA, while Cd2+ forms a sufficiently stable cyanide
complex to prevent reaction with EDTA.

Demasking refers to the release of a metal ion from a masking agent.
Cyanide complexes can be demasked by treatment with formaldehyde in
acetic acid medium

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