Page 126 - Pharmaceutical_Analytical_Chemistry_1_Theoretical_Notes_Level_1

Mansoura National University
Pharm D-Clinical Pharmacy Program Level 1 Pharm. Anal. Chem. 1 (PC 101)

Principle of Mohr’s Method

• Depends on solubility differences between two insoluble silver salts

+ -
Ag (aq) + Cl (aq) ↔ AgCl(s)

primary ppt
Titration reaction: Titrant Sample
(White)
-10
K = 1.8 X 10
sp

+ 2-
2Ag (aq)+ CrO (aq) ↔ Ag CrO (s)
Indicator reaction 4 2 4
Titrant Indicator secondary ppt
(E.P.):
-12 (Brick-red)
K = 1.1 X 10
sp

• The colored indicating precipitate must be more soluble than the main precipitate

forming during titration.

• Ksp Ag CrO < Ksp AgCl, BUT Ag CrO is more soluble than AgCl, so, no Ag CrO
2 4 2 4 2 4
-
will precipitate until all Cl ions are precipitated.

• Ag CrO is brick red in color, so a color change is observed at the endpoint.
2 4

121