Page 69 - PC 101 practical notes 24-25..

MANSOURA NATIONAL UNIVERSIY
PHARM D- CLINICAL PHARMACY LEVEL I PHARM. ANAL. CHEM. I (PC 101)

Exp.: Determination of NaCl by Mohr's method

P Pr ri in nc ci ip pl le e : :

It depends on the titration of NaCl sample with standard AgNO 3 using

potassium chromate (K 2CrO 4) as indicator.

Titration reaction:

+
Cl + Ag → AgCl 
-
sample titrant white ppt
End point reaction:

+
2 Ag + CrO 4 → Ag 2CrO 4 
--
titrant indicator brick red ppt

- + --
In this experiment, Cl sample is titrated with Ag in presence of CrO 4
indicator where AgCl is precipitated as a white ppt before Ag 2CrO 4 THEN at

-
+
the end point (i.e. when Cl is completely reacted with Ag ), the first slight
+
--
excess of Ag will react with CrO 4 indicator giving Ag 2CrO 4 (brick red ppt)
leading to darkening of the yellow color of the solution.

N No ot te es s: :

1) Effect of pH: Mohr's method should be done in neutral or slightly

alkaline medium (pH ≈ 6.5 -9) because:

At pH  9: Ag will be precipitated as AgOH (brown to black ppt)
+
leading to Consumption of the titrant & Masking of the E.P. color.

-
Ag + OH → AgOH  (brown to black ppt)
+
While at pH  6.5: The chromate ion (CrO 4 ) changes into acid chromate
--
--
-
--
(HCrO 4 ) then to dichromate (Cr 2O 7 ). Both HCrO 4 & Cr 2O 7 form soluble
-
+
salts with Ag & so no colored ppt will be formed at the E.P.
2 CrO 4 + 2 H → 2 HCrO 4 → Cr 2O 7 + H 2O
--
+
-
--
chromate acid chromate dichromate
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