Not so Green Up Close: a Peek at a dyed Chicago River Sample

Dean J. Campbell*

*Mund-Lagowski Department of Chemistry and Biochemistry, Bradley University, Peoria, Illinois 61625, United States

Since 1962, a portion of the Chicago River in Chicago, IL, has been dyed green to celebrate St. Patrick’s Day.1,2 The exact composition of the dye (which is actually orange as a solid) is kept secret, although it is believed to be a xanthene-type dye that is chemically similar to fluorescein. In 2025, the river was dyed on March 15. The next day, I obtained a sample of green water from the river. Even though the small water sample containing dilute dye was not very green, the large volume of the water containing the dilute dye in the river made the river appear very green. Figure 1 is a still frame capture from Video 1, which shows the sample in comparison with its general sampling area.

Figure 1. Water sample acquired from the green-dyed Chicago River. Screenshot from Video 1.

Video 1. The green Chicago River sample is not as colorful as the whole river.

Chem Demos YouTube channel (accessed 5/1/2025).

The sample was brought back to my lab and studied using a Vernier Fluorescence/UV-VIS Spectrophotometer on March 19.³ Some of the data are shown in Figure 2. Figure 2A shows the absorbance spectra of the green river water sample. The sample absorbs in the ultraviolet light region and also weakly around 490 nm. I did not have a sample of undyed river water for comparison, so I simply checked a sample of hard tap water from my house. This sample also absorbs some UV light, but not noticeably around 490 nm. The spectral data in the rest of the Figure 2 are the results of a dye-partitioning experiment. A sample of the green river water was shaken in a vial with 1-octanol and the liquids were allowed to separate. The less-polar 1-octanol phase rose up above the water phase. The process was repeated with green river water with added hydrochloric acid, and again with green water and added sodium hydroxide. Figure 2B shows fluorescence spectral data for the lower water phase for each experiment. The large signal at ~400 nm is from an excitation LED shining down into the cuvette in the spectrophotometer. Untreated water and base-treated water fluoresced weakly at about 525 nm, but the acid-treated water did not.

In contrast, Figure 2C shows fluorescence spectral data from an excitation LED at ~400 nm for the upper 1-octanol phase for each experiment. 1-Octanol above acid-treated water appeared to fluoresce weakly at a variety of wavelengths, including at about 675 nm. Untreated water and base-treated water did not appear to show a fluorescence signal.

Figure 2. Absorbance and fluorescence spectral data for the green Chicago River water sample. A) Absorbance spectra for river water and hard tap water. B) Fluorescence spectra for river water in contact with 1-octanol. C) Fluorescence spectra for 1-octanol in contact with river water.

The phenomenon of fluorescence localizing in the water phase for untreated and base-treated river water trials, and localizing in the 1-octanol phase over acid-treated water, was visible to the naked eye when irradiated with ~400 nm light. The water fluorescence appeared greenish and the 1-octanol fluorescence might have been orangish. However, I could not get a good picture of the samples using either of the two different digital cameras. It appears that the purple glow from the ~400 nm light source obscured the weak fluorescence in the cameras. The behavior of a green dye moving between water and 1-octanol, depending on the acidity or basicity of the water, is similar to that observed for fluorescein and disodium fluorescein. Figure 3 shows (left to right) fluorescein partitioning into the 1-octanol phase above the water phase, base-treated fluorescein partitioning into the water phase below the 1-octanol phase, disodium fluorescein partitioning into the water phase below the 1-octanol phase, and acid-treated disodium fluorescein partitioning into the 1-octanol phase above the water phase.

Figure 3. Fluorescein and disodium fluorescein partitioning between water and 1-octanol under various conditions.

All of these observations are consistent with the hypothesis that the dye used for the Chicago River is similar to a sodium salt of fluorescein.¹ In neutral and basic conditions, the dye anions partition into the more polar water phase. In acid conditions, the dyes convert into less-charged, more nonpolar forms and partition into the more nonpolar 1-octanol phase.  

            Exploration of the green Chicago River water sample has shown:

1. Faintly colored substances can still appear intensely colored, given enough distance for the light to travel through the substance. This can connect to discussions of path length in the Beer-Lambert-Bouguer Law, A=abc, in spectroscopy.
2. Different forms of dyes can distribute differently between more polar water and less polar 1-octanol. This can connect to discussions of the octanol-water partition coefficient Kow in toxicology and environmental chemistry.

Safety

The dye used to turn the Chicago River green is supposed to be relatively innocuous,¹ but the river water itself could have any number of additional hazards. Exercise caution when sampling an open waterway. The PubChem site for 1-octanol mentions the GHS Hazard Statement H319: Causes serious eye irritation [Warning Serious eye damage/eye irritation].⁴ The PubChem site for fluorescein mentions the GHS Hazard Statements H302: Harmful if swallowed [Warning Acute toxicity, oral] and H319: Causes serious eye irritation [Warning Serious eye damage/eye irritation].⁵ There are other toxicity effects for 1-octanol and fluorescein described on PubChem, so take measures to protect yourself if you work with these compounds. Acids and bases can be corrosive.

Acknowledgements

This work was supported by Bradley University and the Mund-Lagowski Department of Chemistry and Biochemistry with additional support from the Illinois Heartland Section of the American Chemical Society and the Illinois Space Grant Consortium.

References

1.   Boerner, L.K. Chemical & Engineering News. March 14, 2025, Vol. 103. It’s Saint Patrick’s Day. Let’s dye stuff. https://search.app/tkd1gamH7mbHBRxR7 (accessed March 2025).

2. Larsen, L. Sierra. Should Chicago Dye Its River Green for St. Patrick’s Day? https://www.sierraclub.org/sierra/should-chicago-dye-its-river-green-st-... (accessed March 2025).

3. Vernier. Vernier Fluorescence/UV-VIS Spectrophotometer. https://www.vernier.com/product/vernier-fluorescence-uv-vis-spectrophoto... (accessed May 2025).

4. National Library of Medicine. PubChem: 1-Octanol. https://pubchem.ncbi.nlm.nih.gov/compound/957 (accessed May 2025).

5. National Library of Medicine. PubChem: Fluorescein. https://pubchem.ncbi.nlm.nih.gov/compound/16850 (accessed May 2025).