Mohr's method

Formation of coloured ppt
Mohr's method of determination of chlorides and bromides by titration with silver nitrate.

PRINCIPLE:

The method involving end point detection by formation of an orange red precipitate of silver chromate (after the precipitation of AgCl is complete) using potassium chromate as an indicator is called Mohr's Precipitation titration method.
chloride ion sample ---> dissolved in water ---> titrate with AgNO3 ---> formation of white AgCl ppt.
At the end point, excess drop of silver nitrate reacts with indicator and forms the orange red coloured ppt of Silver chromate.
The concentration of the indicator is important.
The silver chromate should start precipitating at the equivalence point, where we have the saturated solution of AgCl.
From Ksp, the concentration of Ag+ at the equivalence point is 10¯⁵ M.
Ksp for Ag2CrO4 = 1.1 x 10¯¹²

Ksp = [Ag+]² . [CrO4¯²] (∵Ag₂CrO4)

∴ 1.1 x 10¯¹² = (10¯⁵)² . [CrO4¯²]

∴ [CrO4¯²] = 1.1 x 10¯² M

Hence the concentration of chromate ion required to start precipitation is 0.011 M.
If the concentration is greater than this, Ag₂CrO4 will begin to precipitate when [Ag+] is less than 10¯⁵ M.
If the concentration is less than this, then the [Ag+] will have to exceed 10¯⁵ M before precipitation of Ag₂CrO4 begins.

AgNO3 + KCl ----> AgCl ↓ + KNO3

2AgNO3 + K2CrO4 -----> Ag2CrO4

Limitations of Mohr's Method:

It is carried out is neutral to slightly alkaline media i.e. pH range of 6-10.
In highly acidic media, chromate ions reacts with H+ ions and forms HCrO4¯¹, which is slightly ionized , thus, reduce the concentration of chromate ion.

2H+ + 2CrO4¯² ----> 2HCrO4¯¹ -----> Cr2O7¯² + H2O

In alkaline media, silver reacts with hydroxide ion, precipitating in the form of Ag(OH)2 and AgO2 . Both processes interfere with the determination of the accuracy.
This method is only applicable to Chloride and Bromide but not applicable to iodide and thiocynate due to, AgI and AgSCN adsorb chromate ion and hence false result is obtained and titration is not feasible.
This method is applicable at room temperature as the change of temperature also changes the solubility of AgCl and Ag2CrO4 .

Example:

Preparation and standardization of 0.1 M silver nitrate solution:

Principle:

Silver nitrate when exposed to atmospheric air, gets darken due to reduction to metallic silver. Hence silver nitrate solution should be standardize.
Standardization of silver nitrate is carried out by its titration with primary standard compound NaCl which forms a ppt of AgCl.
when all the chloride has been titrated the very next drop of silver nitrate reacts with potassium chromate (indicator) and gives orange red coloured silver chromate complex, indicating the end point.

Reactions:

AgNO3 + NaCl ----> AgCl ↓ + NaNO3

2AgNO3 + K2CrO4 -----> Ag2CrO4

red orange ppt