Fluorescein

Fluorescein is an <a href="/facts/Organic_compound/gqCNaN45">organic compound</a> and <a href="/facts/Dye/9tHyErlh">dye</a> based on the <a href="/facts/Xanthene/Dd1jDgBu">xanthene</a> tricyclic structural motif, formally belonging to <a href="/facts/Triarylmethane_dye/PnPvyEoF">triarylmethine</a> dyes family. It is available as a dark orange/red powder slightly soluble in water and alcohol. It is used as a <a href="/facts/Fluorescent/qsJVT2qP">fluorescent</a> <a href="/facts/Flow_tracer/ssA1CASQ">tracer</a> in many applications.
The color of its aqueous solutions is green by reflection and orange by transmission (its spectral properties are dependent on <a href="/facts/PH/RnSv8D7y">pH</a> of the solution), as can be noticed in <a href="/facts/Spirit_level/fXQH69hY">bubble levels</a>, for example, in which fluorescein is added as a <a href="/facts/Colorant/uqSBnEWe">colorant</a> to the <a href="/facts/Alcohol_(chemistry)/Y0SCTDkh">alcohol</a> filling the tube in order to increase the visibility of the air bubble contained within. More concentrated solutions of fluorescein can even appear red (because under these conditions nearly all incident emission is re-absorbed by the solution).
It is on the <a href="/facts/WHO_Model_List_of_Essential_Medicines/6kmu81Xm">World Health Organization's List of Essential Medicines</a>.

Fluorescein open-in-new

Fluorescein