Pyranine

Pyranine is a hydrophilic, pH-sensitive fluorescent dye from the group of chemicals known as arylsulfonates. Pyranine is soluble in water and has applications as a coloring agent, biological stain, optical detecting reagent, and a pH indicator. Pyranine is also used in yellow highlighters to provide their characteristic fluorescence and bright yellow-green colour. It is also found in some types of soap.

Synthesis
It is synthesized from pyrenetetrasulfonic acid and a solution of sodium hydroxide in water under reflux. The trisodium salt crystallizes as yellow needles when adding an aqueous solution of sodium chloride.