fluorescein is green in rivers

June 20, 2007 at 9:23 am | | science@home

I was perusing Wikipedia the other day, when I read the article on fluorescein. The article claims that the molecule is often used as a dye for rivers. I had heard this, too. It makes sense, because fluorescein is generally considered nontoxic and it’s very fluorescent, so it would be a good marker.


But something puzzled me: why is the Chicago river green in the image? When I think fluorescein, I think “Absorbs at 487 nm and emits in the green (510 nm).” Fluorescein in water should look yellow. In fact, there is an image in the Wikipedia article of an eyedropper with a yellow fluorescein solution. And here’s a pic of my vial of fluorescein in water:


Sure looks yellow to me.

I mentioned my confusion to a friend. We concluded that they must dump a shit-load of fluorescein into the river to make it look green. He decided to test this by putting some fluorescein in water (I’m sure he’s not pouring that stuff down the drain!):


As you can clearly see, water looks green. And he didn’t even use very much dye (probably ~1 mg).

Here’s the point: when you look through the solution in a vial, it looks yellow because you’re seeing the transmission. When you look at the solution in a river (or a ice bucket) from above, you only see the fluorescence excited by sunlight. You can even see that it looks more yellow as he pours it out of the ice bucket (because then you can see some transmission as well as fluorescence).

Well, duh! Now, I feel silly for being confused: that was so obvious. I forgot that the quantum yield of fluorescein is nearly unity. Thanks, Wikipedia.


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  1. Shyeah. Fluorescent solutions can appear to change colour depending on the angle from which you view them sometimes. It can be damn tricky to get a decent picture of them.

    Comment by Ψ*Ψ — June 20, 2007 #

  2. […] 16850[CML] Postsfluorescein is green in rivers  posted to Everyday Scientist on Wed 20th Jun 07I was perusing Wikipedia the other day, when I […]

    Pingback by Chemical blogspace - Molecules — June 21, 2007 #

  3. Dilution is key.

    Comment by charles — June 22, 2007 #

  4. you mean that it’s the solution … to pollution?

    Comment by sam — June 22, 2007 #

  5. actually, i was dumping that down the drain

    Comment by krueger — June 29, 2007 #

  6. i know, but i was trying to avoid incriminating you. and me, as an accomplice. i *am* the safety coordinator of my lab.
    i guess it’s probably OK to have a little fluorescein in the bay, if chicago can stain the entire river with it!
    i just bring all my chemical waste directly to the bay and dump it. you know, to cut out the middleman.

    Comment by sam — June 29, 2007 #

  7. […] famously dyes the Chicago River green on St Patrick’s Day (see also Everyday Scientist’s illuminating discussion of whether it looks green or yellow). It’s also used by ophthalmologists to stain your retina and check for scratches. And […]

    Pingback by Chemistry World blog — August 21, 2007 #

  8. I’m glad you wrote about this. I was wondering the same thing about why it looked green in the river when it looked yellow in the lab (orange if more concentrated)

    Comment by Theresa — October 23, 2007 #

  9. As mentioned, we would use fluorescein-impregnated strips to deliver a proper amount to the eye (cornea principally) as under a proper wavelength ultraviolet light, traces of it will remain in ulcerations or abrasion to the cornea. Funny, I can’t remember, but I seem to recall it fluoresces to a mustard-color. It was handy stuff and we usually carried a few strips of it in our ophthalmoscope case for use when needed. Aha.

    Comment by Doc Scott — October 28, 2007 #

  10. Thought I’d put the actual ID in. Somebody may search for me, eh?

    Comment by Charles Scott PA (Emeritus) — October 28, 2007 #

  11. I use sodium fluorescein daily in an ophthalmology office. We inject 4ml of 10% dye into an arm vein and photograph it with an exciter filter as it travels through the retinal blood vessels. It’s a very common procedure in ophthalmology, and the technique has been around since 1960s. Feel free to drop me an email if you want more info.

    Comment by Darrin Landry, CRA — November 17, 2007 #

  12. […] probably wouldn’t taste good, a nitrogen laser in the eye is unappetizing, and I doubt that fluorescein is food-grade. Theodore Hänsch describes some of these fun laser stories in Optics and Photonics […]

    Pingback by Everyday Scientist » the edible laser — May 12, 2008 #

  13. How do you figure out how much needs to be used to turn a river green? My restaurant is turning a river green for St. Pattie’s and I am trying to figure out a formula for it. Any suggestions? I know I need to know GPH (gallons per hour) but thats all I know.

    Comment by Fresh — February 26, 2010 #

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