Recently my students had the opportunity to see how plant pigment can convert light energy into chemical energy using the Flinn activity entitled Fantastic Fluorescing Chlorophyll.1,2 Students extract chlorophyll from spinach using a solvent and a centrifuge. When a flashlight with a UV black light bulb is shone directly into the collected green solution, a red fluorescence can be observed around the edges of the flask. This is due to a small amount of light from the flashlight generated in a range that excites chlorophyll molecules. The effect is quite spectacular due to the contrast between the green, where light doesn’t penetrate, and the red, where it does.
After receiving this activity in an email from Flinn2 I decided it would fit nicely into my explanation of atomic spectra. Beforehand, I introduce electromagnetic spectrum to my students via a kinesthetic activity. I had students jump from the seats of their desks to the ground while flickering the classroom lights on and off. This demonstrates the electron falling from an excited state to the ground state. These types of activities tend to prove worthwhile, especially for student recall.
Fresh spinach leaves are ground using a mortar and pestle.
Ground spinach in the solvent is placed in a centrifuge.
Black light shone directly at test tubes with chlorophyll extract — look carefully to observe the surface of the solution as green.
Upon gathering the lab materials, I realized I did not have ethyl acetate. However, as a tip Flinn lists acetone and 95% ethyl alcohol as potential substitutes. This turned out to be a great opportunity to have students investigate acetone, isopropyl alcohol and ethyl alcohol as substitutes for the ethyl acetate. We found the solvent didn't matter as much as the amount of solvent used. I recommend using 20-25 mL of solvent as suggested by Flinn. Using their mortars and pestles my students ground approximately 10 fresh spinach leaves. Interestingly, chlorophyll extracted from fresh or dry spinach has this effect, but frozen does not.
Students enjoyed the new lab techniques and equipment. Besides grinding with the mortar and pestle, many had never used a centrifuge and were unaware what it meant to “decant a liquid”. On a side note, we spun our samples for 3 minutes and not the suggested 5-10 minutes. We are fortunate to have a centrifuge that spins at 3200 rpm. This is when students witnessed the importance of safety precautions: wearing safety goggles and using a centrifuge in the fume hood — or behind a safety shield. In our initial run we used cheap test tubes and after spinning, their tops were shattered — thankfully without incident because the appropriate safety precautions were taken.
Flinn also suggested making observations while holding a flashlight at a 90⁰ angle to the flask. This, along with three black lights, set up in a triangle pattern around a 100-mL beaker with the extracted chlorophyll, resulted in capturing the magazine cover image. The UV light penetrates into the sides of the beaker but not enough to penetrate through the entire solution. A green color is visible on the surface.
References
- www.flinnsci.com/glowing-chlorophyll/vfm0246/
- Fantastic Fluorescing Chlorophyll, Flinn Scientific BioFax https://www.flinnsci.com/api/library/Download/c3dd9e20b9394ce3a6e17db10f84de66 ☘
