Page 138 - Pharmaceutical_Analytical_Chemistry_1_Theoretical_Notes_Level_1

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

• The pH of the medium should be adjusted to keep the indicator in the dissociated

form.

• If not, it will not be adsorbed and it is known that the color change depends on the

adsorption of the ion of the indicator.

• Examples:

+ -
• Fluorescein (Na In ) is the sodium salt of a very weak acid and it's hydrolyzed in

water to give its acid (HIn) where its dissociation is suppressed in acidic medium
due to common ion effect.

+ -
HIn ↔ H + In

+
H

So, fluorescein is used in neutral or slightly alkaline medium (pH=7-10).

• After adsorption, fluorescein color changes from yellowish green to pink.

• Examples:

+ -
Eosin (Na In ) is the sodium salt of a stronger acid (tetrabromo derivative of

fluorescein), so its dissociation is not affected by acidic medium. So, it can be used at

pH 2. The colour change is sharpest in an acetic acid solution.

• After adsorption, eosin color changes from orange red to reddish violet.

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