“Enchanting fluorescein” experiment

This col­or­ful ex­per­i­ment demon­strates the abil­i­ty of some sub­stances to ra­di­ate light un­der the im­pact of an ex­ter­nal en­er­gy source (usu­al­ly an ul­tra­vi­o­let one). From this ex­per­i­ment, you will find out why flu­o­res­cent (ul­tra­vi­o­let) paints glow which are used in ban­knotes, and dec­o­rat­ing rooms and night clubs. Flu­o­res­cein is also used to turn rivers green on St. Patrick’s Day.

Safe­ty pre­cau­tions

Care must be tak­en when work­ing with an al­ka­li and its so­lu­tions.

Reagents and equip­ment:

• flu­o­res­cein 2 g;
• sodi­um hy­drox­ide 2 g;
• dis­tilled wa­ter;
• beaker 5 l.

Step-by-step in­struc­tions

We pre­pare the al­ka­line so­lu­tion: we sprin­kle 2 g of NaOH into the beaker, and add 50 ml of dis­tilled wa­ter and stir. Then we add 2 g of flu­o­res­cein to the so­lu­tion. We get a sat­u­rat­ed so­lu­tion of flu­o­res­cein salt. We pour wa­ter into the beaker, turn off the light and turn on the UV lamp. With a pipette we drip the so­lu­tion of flu­o­res­cein salt into the wa­ter. En­chant­ing green rings and “clouds” form.

Pro­cess­es de­scrip­tion

Flu­o­res­cence is the ra­di­a­tion of light by a sub­stance, the atoms of which have re­ceived an abun­dant amount of en­er­gy (for ex­am­ple un­der the im­pact of ul­tra­vi­o­let light). With­out an en­er­gy source, flu­o­res­cence stops. Many sub­stances have this prop­er­ty, for ex­am­ple: flu­o­res­cein, qui­nine, the chloro­phyll of plants and many oth­ers When flu­o­res­cein is dis­solved in an al­ka­line so­lu­tion of NaOH, a wa­ter-sol­u­ble salt forms. De­pend­ing on the light­ing and con­cen­tra­tion of the so­lu­tion, the flu­o­res­cein changes col­or. When sev­er­al drops of a sat­u­rat­ed so­lu­tion of flu­o­res­cein are added to wa­ter, it glows green in ul­tra­vi­o­let light.