Page 135 - Pharmaceutical_Analytical_Chemistry_1_Theoretical_Notes_Level_1

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

❖ An adsorption indicator is an organic compound that tends to be adsorbed on the

surface of the solid in a precipitation titration.

❖ The action of these indicators is due to the fact that at the equivalence point, the
indicator is adsorbed by the precipitate, and leads to a substance of different colour.

• Types of adsorption indicators:

Adsorption indicators may be :

as fluorescein, eosin, dichloro-fluorescein, and di-iodo-dimethyl-
a) Acid dyes:
fluorescein.

b) Basic dyes: as Rhodamine 6G.

Theory of Fajans’ Method

Theory of Adsorption Indicator

➢ Silver halide precipitates are usually colloidal in nature.

➢ The theory of the action of adsorption indicators is based upon the electrical

properties of colloids.

➢ This can be explained by the electrical double layer theory.

➢ According to this theory, a double layer of ions appear at the surface of solid particles

in solutions.

➢ The first adsorbed layer consists of ions preferentially adsorbed (one of the ions

constituting the ppt. and present in excess) that imparts charge to colloidal

particles.

➢ The second layer consists of ions from the solution of an opposite charge to the

first layer.

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