Fluorescent Dye Experiment: pH of Pyranine - WL

FOV: 4" wide.

This experiment demonstrates the shift in color from yellow-green to blue-green in aqueous solutions of pyranine when made acidic.

The fluid from a yellow hi-liter pen was removed and mixed with water (shown in the center test tube). The left test tube contains that solution with 1 drop of muriatic acid added. The test tube on the right contains the solution with 5 drops of a sodium hydroxide solution.

The pH of the left test tube was ~1 and the right test tube ~14. It was hard to measure the pH of the solutions since it dyed the pH indicator paper yellow.

Upon adding the drop of acid to the pyranine solution, it changed from yellow green to clear. This was because the pyranine had been protonated (provided a hydrogen ion) which reduced the absorption of light at 450nm.

Refer to this article: www.ugcfrp.ac.in/images/userfiles/30241-CPL_399_147.pdf

"At a pH less than 7, the absorption spectrum of pyranine
(HPTS) exhibits a peak at 405 nm which is ascribed to
the protonated form (ROH) while at a pH greater than 7 the
absorption spectrum exhibits an additional band at
450 nm arising from the deprotonated form (RO)."

See also:
nathan.instras.com/documentDB/paper-273.pdf

Contains:
Pyranine sol. pH less than 7 (FL Blue-green >UVabc)
Pyranine sol. pH greater than 7 (FL Yellow-green >BL/UVabc)

Shown under white light.
Note the tinge of blue in the left test tube and blue in the other test tubes was likely due to the small amount of UV from the halogen bulb.

Key:
WL = White light (halogen + LED)
FL = Fluoresces
PHOS = Phosphorescent
Blue = 450nm,
UVa = 368nm (LW), UVb = 311nm (MW), UVc = 254nm (SW)
'>' = "stimulated by:", '!' = "bright", '~' = "dim"

pH of Pyranine
17Nov2015

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