Chemistry runs through it
A yellow boat navigates a bright green river with a city skyline and blue sky in the background.
Credit: Shutterstock
Green chemistry, literally: The dye that makes the Chicago River flow emerald is probably some kind of fluorescein salt.
If you’ve even been to the Saint Patrick’s Day parade in downtown Chicago, you’ve witnessed to a colorful tradition, bigger even than the whole green beer thing. On the morning of the parade, members of the Chicago Journeymen Plumbers Local 130 of the United Association union make the Chicago River go green.
The tradition of dyeing the river for Saint Patrick’s Day goes back to 1962. According to union lore, the bright-green hue was inspired by an accident: Plumbers have historically used dyes to detect water leaks. A plumber working with a dye spilled it on his white overalls, causing them to turn “Irish green.” The union’s business manager, Stephen Bailey, saw the results and asked about the color. When the plumber explained, Bailey made the logical next step—he called the mayor of Chicago and proposed dyeing the Chicago River green for Saint Patrick’s Day.
Since then, the Local 130 has refused to spill what the dye is. It’ll say only that it’s environmentally friendly and fades after a few hours. According to the Chicago Saint Patrick’s Day Parade Committee, the dye goes into the river as an orange powder and quickly turns to that antifreeze shade that we know and love. The Local 130 attributes this transformation to the magic of the leprechaun. Not to rain on anyone’s parade (especially the supposed second-largest Saint Patrick’s Day parade in the US), but this Newscriptster has a feeling that the color change is really down to chemistry.
Chucking dyes into water isn’t just for plumbers anymore. You too can buy some and color the water source of your choice. These types of dyes are xanthene-based orange solids when dry. They’re also certified as safe for use in potable water, as required by the US Environmental Protection Agency, and will biodegrade.
One such xanthene-based dye that’s used for water and sometimes even medical tracing is fluorescein. This orange powder is also known to turn green in alkaline conditions. According to Chicago River Checkup, the pH of the Chicago River at the spot closest to where the plumbers dump the dye was 7.7 in December. Is that alkaline enough to coax a good “Irish green” out of the dye? Perhaps.
This Newscriptster guesses the dye is some kind of fluorescein salt, which is more soluble in water than hydrocarbon-heavy fluorescein. In water, scientists sometimes use sodium fluorescein to pinpoint where groundwater feeds into rivers, and it turns the same “Irish green” that the leprechauns (ahem) magically change the Chicago River into.
Will the world ever know for sure? This Newscriptster tried to learn the dye’s identity from the Local 130, but the spokesperson stayed mum. Perhaps if you can catch a leprechaun, you could extract that information as well as score a pot of gold.
Sweet rainbows
A gradient rainbow solution fills a plastic test tube.
Credit: Leigh Krietsch Boerner/C&EN
Taste the rainbow: Food dye and various concentrations of sugar water turn a test tube into a dazzling display of density.
If you want to create your own chemistry-based Saint Patrick’s Day magic but you don’t have a river to dye, you can make a mini density rainbow using food coloring, sugar, and water.
This Newscriptster achieved a gradient-type rainbow in a large plastic test tube by mixing various concentrations of sugar in water with food coloring.
Experimentation revealed that the best rainbow was achieved with 40 g of sugar in 180 mL water for purple, 32 g sugar in the same amount of water for blue, 24 g sugar for green, 16 g for yellow, 8 g for orange, and just plain water for red. The results were lovely, if not super distinct.
There was no pot of gold at the end of this rainbow in a tube, but this Newscriptster’s 10-year-old did ask about density at dinner that night. To a science-nerd parent, that is pretty golden.
Please send comments and suggestions to newscripts@acs.org.
Chemical & Engineering News
ISSN 0009-2347
Copyright © 2025 American Chemical Society
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