An interesting paper in J Chem Ed* describes the analysis of solutions using a photograph of a range of solutions and free PC imaging software (I suggest an alternative mobile phone option below). The authors explicitly aim to make the experiment school friendly, and they outline how to conduct an experiment so as to build a calibration plot and determine the concentration of an unknown (they use something like Lucozade, diluted in a series). The protocol is simple:
- Prepare a series (4 – 5) of known concentrations of a coloured solution in test-tubes or cuvettes.
- Line them up beside each other, along with the blank (e.g. water) and the unknown concentration sample. The paper lines them up in front of a lightbox, but it should be fine to line them up on a windowsill with a piece of white paper against the windowpane.
- Take a photograph, holding the camera about 50 cm away.
- On the computer, use software (ImageJ is free), to determine the colour composition of each.
- Plot a Beer-Lambert plot to form a calibration curve and use this to estimate the concentration of the unknown. For the Beer-Lambert plot, the absorbance is calculated as –log (R/R0) if using red colours, where R is the red value for each sample and R0 is the red value in the blank.
Mobile Phone Only
In fact an even easier protocol if you are taking the photo with an iPhone or iPad is to use the app “RGB Camera“. Open the photo in the app, and choose the “Pixel Inspector” by pressing the (i) button, tap the screen returns the RGB values for that point. Using the photograph from the abstract, I got a pretty good linear trend in this manner.
When plotting the data, you can choose either the R or G or B values. You’ll note that for blue solutions, the B values don’t change very much, or are irregular, so you can use R or G. Similarly for red and green solutions. I think this is a really clever way to demonstrate colorimetry in a relatively easy manner.
*Eric Kehoe and R. Lee Penn, Journal of Chemical Education, 2013, 90 (9), pp 1191–1195.