Page 133 - Pharmaceutical_Analytical_Chemistry_1_Theoretical_Notes_Level_1

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

Quiz

1. Q. Why do we must make filtration during determination of chloride
by Volhard’s?

2. Q. Filtration is not necessary during determination of bromide or

iodide by Volhard's, Why?

Precautions of Volhard’s Method
1- Volume of indicator:

The volume of the indicator used is not critical since the colored complex formed

at the end point is soluble and is not a precipitate as in case of Mohr׳s method.

2- pH in Volhard’s method must be made acidic using nitric acid (0.2-1.6 N).

Why? Role of nitric acid??

+ 3+
a) HNO prevents the precipitation of Ag & Fe ions as AgOH & Fe(OH) which
3 3
takes place in alkaline medium.

+
b) HNO prevents the precipitation of other Ag salts like carbonate, phosphate and
3
sulfide because these salts are soluble in HNO .
3
c) HNO helps the coagulation of AgCl ppt & that facilitates its filtration or coating.
3
BUT, too high concentration of HNO should be avoided because it destroys
3
the colored ferric thiocyanate complex formed at the E.P.

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