A smartphone can be used in a remarkably simple and inexpensive way to teach your students about absorption spectroscopy and Beer’s Law. In short, light reflected off of colored construction paper is passed through a sample and detected by an RGB application on a smartphone. The materials and setup are so simple that students can easily construct several “spectrophotometers” that can be used in a variety of experiments in your classroom. When using the system students learn some basic ideas about the processes involved in a working absorption spectrometer. Excellent results can be achieved; my students routinely obtain absorbance vs. concentration data sets with linear fits of R2 = 0.98 or better.
The video below describes how to set up an iPhone for use as a “spectrophotometer”. Below the video you can find further helpful tips (not covered in the video) on how to set up your own “smartphone spectrophotometer”. For example, the further tips describe how to use an Android instead of an iPhone in the experiment.
Let me know if you use this experiment in your classes. I’d love to hear about any experiments you conduct in your classroom using this set up.
1. This blog post and the video above describe some aspects of the following publication in the Journal of Chemical Education: http://pubs.acs.org/doi/abs/10.1021/acs.jchemed.5b00844. Several other details, as well as a student worksheet, can be found by accessing the publication link.
2. Any application that can detect average R, G, and B values in real time will work. As stated in the video, the application “Colorometer” works well on the iPhone. Further information regarding this app can be found at the following link: https://itunes.apple.com/us/app/colorometer/id509865412?mt=8.
Two applications that work well on an Android are:
a. “Colorimeter”, (comes with a nominal cost). See: https://play.google.com/store/apps/details?id=com.colorimeter
b. “Colormeter Free”. See: https://play.google.com/store/apps/details?id=com.vistechprojects.colormeterfree&hl=en
3. Because the light detector is an RGB analyzer, it is pedagogically most straightforward to limit the color of background construction paper to be red, green or blue. A color wheel (see below) can be used to help determine the background color of construction paper to be used. To do so, choose the color on the color wheel that is opposite the color of the solution to be analyzed. This “opposite color” is called the “complimentary color”. Since red is the compliment of green, if a green colored solution is to be analyzed, then red construction paper should be used as the background. It may be that the appropriate complimentary color is not red, green or blue. In this case, simply choose a color that closely matches the complementary color. For example, even though orange is the compliment of blue, we have successfully used a red background to analyze blue samples, because red closely matches orange.
4. As an alternative to colored construction paper, light from a computer screen can be used as a light source. To do so, access the following link on the computer:
a. For blue light, set the wavelength to 440 nm.
b. For green light, set the wavelength to 510 nm.
c. For red light, set the wavelength to 680 nm.
5. It is useful to use drinks that can be bought at the grocery store for stock solutions from which the standard solutions can be made. As seen in the video, we have used Fruit Punch PowerAde Zero as a stock solution of 95±5 x 10-6 M Red Dye #40. We have also used Blueberry Pommegranate Gatorade as a stock solution of 12±1 x 10-6 M Blue Dye #1. Using these solutions as “stock”, we often find the concentration of Blue Dye #1 in Mountain Berry Blast PowerAde to be 4.3±0.5 x 10-6 M and Red Dye #40 in Strawberry PowerAde Zero to be 4.5±0.5 x 10-6 M.