Page 130 - Pharmaceutical_Analytical_Chemistry_1_Theoretical_Notes_Level_1

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

2. Volhard’s Method

✓ It is not always possible to use Mohr’s method to determine concentration of

halides.

✓ For example, Mohr’s method requires a neutral solution, but in many cases, the

solution has to be acidic, to prevent the precipitation of metal hydroxides which are

coloured (Hg, Cu, Co, Ni).

✓ In such cases, we can use Volhard’s method, which is not sensitive to low pH.
✓ Volhard’s method is also used if the reaction is too slow or if there is no appropriate

indicator selected for determining the equivalence point.
✓ So, this method can be applied to the determination of chlorides, bromides, and

iodides in acid solution.

✓ Volhard’s method is a back or residual titration.

✓ Excess of standard silver nitrate solution is added, and the excess unused is back

titrated with standard thiocyanate solution using ferric alum (ferric ammonium

sulphate) as indicator.

✓ Def.: It is a type of precipitation titration that depends on the formation of a colored

soluble complex at the end point.

Sample: Cl , Br and I
-
-
-
Type of titration: Back titration.

Standard Solution: Standard AgNO 3 and standard NH 4SCN.

pH: Acidic medium.

Indicator: ferric nitrate or ferric ammonium sulphate (ferric alum).

Color at E.P.: Red soluble complex of ferric thiocyanate.

125