Title: Fluorescin
CAS Registry Number: 518-44-5
CAS Name: 2-(3,6-Dihydroxy-9H-xanthen-9-yl)benzoic acid
Additional Names: resorcinolphthalin
Molecular Formula: C20H14O5
Molecular Weight: 334.32
Percent Composition: C 71.85%, H 4.22%, O 23.93%
Literature References: Obtained by heating fluorescein with NaOH and zinc dust. Formation by Pseudomonas aeruginosa: King et al., Can. J. Res. 26C, 514 (1948); Totter, Moseley, J. Bacteriol. 65, 45 (1953).
Properties: Bright yellow powder, mp 125-127°. Readily oxidizes to fluorescein. Practically insol in water. Sol in alkali hydroxides or carbonates, alcohol, ether. Keep well closed.
Melting point: mp 125-127°
Use: Reagent for oxidases, peroxides.
Fluoresone Fluoridamid Fluorine Dioxide Fluorine Monoxide Fluorine Nitrate

Skeletal formula
Ball-and-stick model
Sample of dark red powder
CAS number 2321-07-5 YesY
PubChem 16850
ChemSpider 15968 YesY
EC number 219-031-8
DrugBank DB00693
KEGG D01261 YesY
MeSH Fluorescein
ChEBI CHEBI:31624 YesY
ATC code S01JA01
Jmol-3D images Image 1
Molecular formula C20H12O5
Molar mass 332.31 g mol−1
Density 1.602 g/mL
Melting point 314 to 316 °C; 597 to 601 °F; 587 to 589 K
Solubility in water Slightly
S-phrases S26 S36
 N (verify) (what is: YesY/N?)
Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
Infobox references

Fluorescein is a synthetic organic compound available as a dark orange/red powder slightly soluble in water and alcohol. It is widely used as a fluorescent tracer for many applications.

Fluorescein is a fluorophore commonly used in microscopy, in a type of dye laser as the gain medium, in forensics and serology to detect latent blood stains, and in dye tracing. Fluorescein has an absorption maximum at 494 nm and emission maximum of 521 nm (in water). The major derivatives are fluorescein isothiocyanate (FITC) and, in oligonucleotide synthesis, 6-FAM phosphoramidite.

Fluorescein also has an isosbestic point (equal absorption for all pH values) at 460 nm. Fluorescein is also known as a color additive (D&C Yellow no. 7). The disodium salt form of fluorescein is known as uranine or D&C Yellow no. 8.

The color of its aqueous solution varies from green to orange as a function of the way it is observed: by reflection or by transmission, as it can be noticed in bubble levels in which fluorescein is added as a colorant to the alcohol filling the tube to increase the visibility of the air bubble and the precision of the instrument. More concentrated solutions of fluorescein can even appear red.