Page 134 - Pharmaceutical_Analytical_Chemistry_1_Theoretical_Notes_Level_1

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

3- The end point can occur too soon:

+
Due to adsorption of Ag on the freshly precipitated AgSCN, therefore vigorous
shaking near the end point and titrating till permanent color are necessary.

4- For the estimation of iodide, ferric alum indicator should not be added until excess
3+ 3+
of silver is present, since the dissolved iodide reacts with Fe ions: i.e. Fe oxidize
iodide into iodine.

3. Fajans’ Method

• Def.: It is a type of precipitation titration that depends on the use of an adsorption

indicator which is adsorbed on the surface of the formed ppt at the E.P. leading to

change in the ppt color (a colored adsorption compound).

Sample: - - - -
Generally, it can be used for Cl, Br, I & SCN

Type of titration: Direct titration

Standard Solution: Standard AgNO
3
Indicator Different adsorption indicators (dyes) can be used but
the most common indicators are:

Fluorescein Eosin

- - - - - -
is used for Cl, Br, I & is used for Br , I & SCN (Not for
- -
SCN Cl )

pH neutral or slightly Acidic
alkaline
at pH down to 2
(pH = 7 - 10)

Color at E.P.: Pink ppt Reddish violet ppt

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