Fluorescein

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Fluorescein
Names
Systematic IUPAC name
3',6'-Dihydroxy-3H-spiro[2-benzofuran-1,9'-xanthen]-3-one
Other names
Fluorescein, Resorcinolphthalein, diresorcinolphthalein, D&C yellow no. 7, Japan yellow 201, soap yellow
Properties
C20H12O5
Molar mass 332.31
Appearance Dark red powder
Density 1.602 g/cm3
Melting point 314 to 316 °C (597 to 601 °F; 587 to 589 K)
Soluble in basic aqueous solutions, acetone and diethyl ether, slightly soluble in water
Acidity (pKa) 6.4
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references

Fluorescein (also known as resorcinophthalein or D&C yellow no. 7) is an organic compound that is a brick-red solid at room temperature. It dissolves slightly in water to give bright yellow, strongly fluorescent solutions. Sodium fluorescein, also known as uranine, has a much higher solubility in water.

Despite its name, fluorescein is NOT a fluorine compound.

Properties

Chemical

Fluorescein will react with bicarbonates to form fluorescein salts.

Physical

Fluorescein is a dark red powder, slightly soluble in water. However, when dissolved in a very large amount of water, it will turn bright green, with the color getting stronger the more it dissolves (up to a point obviously), a property which gives it useful applications in various domains.

Fluorescence

Fluorescein has an absorption peak at 494 nm and an emission peak at 512 nm. Its isosbestic point (wavelength that is equally absorbed at all pH values) is 464 nm.

Availability

Fluorescein is sold by chemical suppliers. It can also be found on eBay and Amazon, at various prices.

One seller offers 100 g of fluorescein at $18.81.

Obtaining

Extraction

Certain brands of highlighters use a solution of fluorescein in water with a water-soluble binder. The ink cartridges of these highlighters may be extracted with isopropanol to give a solution of pure fluorescein. Some highlighters use pyranine instead, which has a strong blue fluorescence at high pH.

Bubble levels are often very dilute fluorescein solutions.

Synthesis

Fluorescein is easily prepared from resorcinol and phthalic anhydride through a Friedel-Crafts reaction with an acid catalyst, most often concentrated sulfuric acid. The product may be diluted into a basic solution to obtain a dark sodium fluorescein stock solution, which works for most demonstrations.

Projects

  • Water flow indicator

See also

Handling

Safety

Diluted fluorescein solutions are considered safe and are commonly used in eye examinations. Ingestion of concentrated solutions however can lead to nausea.

Storage

Solid fluorescein should be stored in closed bottles, away from moisture or corrosive vapors. Solutions should be kept in bottles, away from light.

Disposal

No special disposal is required.

References

Relevant Sciencemadness threads