Page 77 - Pharmaceutical Analytical Chemistry II - Pharm D Clinical- 07-PA202

Volumetric Precipitation Titrations
(Precipitimetry)

Precipitation titrations are volumetric methods based on the
formation of a slightly soluble precipitate. They are in many ways simpler
than gravimetric methods. The precipitate needs not to be separated, and
needs not to be pure, as long as the impurity does not consume titrant.
The substance is determined simply by converting it into an insoluble
form of known composition by adding a standard solution of the titrant.
The equivalence point is reached when an equivalent amount of the titrant
has been added. From the volume of the latter, the amount of the
substance is calculated. The precipitate must be sufficiently insoluble to
ensure completion of the reaction and to ensure a marked change in the
concentration of the ions of precipitate at the equivalence point of the
titration.

When silver nitrate is used as a titrant, the method is called
argentometric titration. These procedures were and still are routinely
used for the analysis of silver ion as well as for such anions as chloride
(Cl-), bromide (Br-), iodide (I-) and thiocyanate (SCN-)

Argentometric titration is divided into two

types:

1. Direct titration: when silver nitrate (AgNO3) is used as titrant.
2. Indirect determination: In which an excess of standard silver

nitrate is added to the analyte, and back titrate the unconsumed

76