Pyranine is a hydrophilic, pH-sensitive fluorescent dye from the group of chemicals known as arylsulfonates.

[2][3] Pyranine is soluble in water and is used as a coloring agent, biological stain, optical detecting reagent, and pH indicator.

[4][5] Pyranine is also used in yellow highlighters to provide their characteristic fluorescence and bright yellow-green colour.

[6] Pyranine is synthesized from pyrenetetrasulfonic acid and a solution of sodium hydroxide in water under reflux.

[7] The trisodium salt crystallizes as yellow needles when adding an aqueous solution of sodium chloride.