Page 131 - 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 Volhard’s Method
-
Example: Determination of Cl by Volhard's method

• Chloride is first precipitated with a known excess of standard silver nitrate solution,

• Then, the excess unreacted silver is back titrated with standard thiocyanate solution.

• To detect the end point, we use ferric alum indicator, which reacts with the

thiocyanate at the E.P., forming a red complex.

-
✓ First, Cl is precipitated by excess AgNO
3
+ -
Ag (aq) + Cl (aq) ↔ AgCl(s)

(White ppt)

+ 3+
✓ Excess Ag is titrated with KSCN in the presence of Fe (indicator)

+ -
Ag (aq) + SCN (aq) ↔ AgSCN(s)

(White ppt)

+
✓ When Ag has been consumed, a red complex forms

3+ - 2+
Fe (aq) + SCN (aq) ↔ [Fe(SCN)] (aq)

(Red complex)

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