Page 75 - Pharmaceutical Analytical Chemistry II - Pharm D Clinical- 07-PA202

Titrations involving unidentate ligands

Because of the stepwise formation of successive complexes,
unidentate ligands are only rarely suitable for the titration of metal ions.
However, there are a few examples of important titrations based upon
such ligands, and we shall consider briefly the two best-known cases.

1- Cyanometric titrations

It is the titration of cyanide ion using silver nitrate.

Liebeg method

The titration of a solution containing cyanide ion with silver nitrate

solution to causes the formation of the soluble dicyanoargentate ion,

Ag(CN)2-. After sufficient silver has been added to react with all the

cyanide by this reaction, the first excess will react to form insoluble silver

dicyanorgentate, Ag[Ag(CN)2], which is often referred to and written as

silver cyanide, AgCN. The ionic equations for the reaction are:

Ag+ + 2CN- Ag(CN) -
2
Ag+ + Ag(CN) -
Ag[Ag(CN) ]
2
2

As a consequence, the appearance of a permanent precipitate
indicates the end point.

Deniges modification of Liebeg method

As the end point in the Liebeg method is not sharp because local excess
of silver nitrate causes precipitation of silver dicyanoargentate which is
slow to redissolve; in the Deniges method iodide is used as indicator and
ammonia is added to prevent precipitation of silver dicyanoargentate.

2- Mercurimetric titrations

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