Page 83 - Pharmaceutical Analytical Chemistry II - Pharm D- 02-06-07102

Many methods are utilized in determining end points in precipitation reactions

such as:

❖ Formation of colored secondary precipitate (Mohr method)
❖ Formation of colored product (Volhard method)
❖ Formation of colored adsorption compound (Fajans method)

A- Formation of a colored precipitate (Mohr method)
An example for the use of the formation of a secondary highly

colored precipitate for the detection of end point is the Mohr method for
the determination of chloride and bromide ions with silver nitrate.

Here, chromate ion is the indicator, the end point is detected by the
appearance of brick red silver chromate (Ag2CrO4) in neutral medium. It
is necessary that precipitation of the indicator occurs at or near the end
point of the titration. Silver chromate is more soluble (molar solubility of
Ag2CrO4 is 7.94x10-5 mol/L; Ksp = 1.1x10-12) than AgCl (molar solubility
of AgCl is 1×10-5 mol/L; Ksp = 1.1×10-10). As a result, AgCl precipitates
first in the titration flask. If silver ions are added to a solution containing
a large concentration of chloride ions and a small concentration of
chromate ions, AgCl will precipitate first. Ag2CrO4 will not form until
[Ag+] increases to a large enough value to exceed the Ksp of Ag2CrO4.
This can be easily determined as follows:

At equivalence point:

[Ag+] = [Cl-] =
[Ag+] = 1.05×10-5 M

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