Fluorescein synthesis

Equipment

Porcelain bowl or small beaker (50 ml), crucible tongs, Bunsen burner, beaker (400 ml), if available: UV lamp.

Chemicals

Resorcinol (Xi), phthalic anhydride (Xi), concentrated sulfuric acid (C), dilute sodium hydroxide solution (w = 5%, C), dilute hydrochloric acid (c = 0.1 mol / l, Xi), possibly ethanol (F).

How-to

A spatula tip each of resorcinol and phthalic anhydride are mixed and a drop of concentrated sulfuric acid is added. It is heated carefully until a viscous black-red liquid forms. The melt is poured into a beaker with water. A yellowish solution forms. When adding sodium hydroxide solution, a noticeable fluorescence is observed (sunlight, UV lamp). The fluorescence can be reversibly changed using hydrochloric acid.

The rest of the melt is dissolved in ethanol and recrystallized or used directly for acid-base titration.

What happened?

The fluorescein synthesis starting from phthalic anhydride and resorcinol
The fluorescein synthesis starting from phthalic anhydride and resorcinol

The colorless compounds resorcinol and phthalic anhydride combine under the influence of sulfuric acid to form dark red fluorescein. This substance fluoresces under UV-light.