[1] The color of its aqueous solutions is green by reflection and orange by transmission (its spectral properties are dependent on pH of the solution),[2] as can be noticed in bubble levels, for example, in which fluorescein is added as a colorant to the alcohol filling the tube in order to increase the visibility of the air bubble contained within.

It is available as sterile single-use sachets containing lint-free paper applicators soaked in fluorescein sodium solution.

[1] Oral and intravenous use of fluorescein can cause adverse reactions, including nausea, vomiting, hives, acute hypotension, anaphylaxis and related anaphylactoid reaction,[6][7] causing cardiac arrest[8] and sudden death due to anaphylactic shock.

[15] The risk can be reduced with prior (prophylactic) use of antihistamines[17] and prompt emergency management of any ensuing anaphylaxis.

[citation needed] Fluorescein has a pKa of 6.4,[2] and its ionization equilibrium leads to pH-dependent absorption and emission over the range of 5 to 9.

Also, the fluorescence lifetimes of the protonated and deprotonated forms of fluorescein are approximately 3 and 4 ns, which allows for pH determination from nonintensity based measurements.

Upon exhaustive irradiation with visible light fluorescein decomposes to release phthalic and formic acids and carbon monoxide, effectively acting as a photoCORM.

The remaining resorcinol rings react with singlet oxygen formed in situ to give oxidized, ring-opened products.

The extent to which fluorescein dilaurate is broken down to yield lauric acid can be detected as a measure of pancreatic esterase activity.

The method involves the fusion of phthalic anhydride and resorcinol,[1] similar to the route described by Adolf von Baeyer in 1871.

In cellular biology, the isothiocyanate derivative of fluorescein is often used to label and track cells in fluorescence microscopy applications (for example, flow cytometry).

Additional biologically active molecules (such as antibodies) may also be attached to fluorescein, allowing biologists to target the fluorophore to specific proteins or structures within cells.

This is because fluorescein is xylem-mobile and unable to cross plasma membranes, making it particularly useful in tracking water movement through the xylem.