Since my freshman design students did not show much interest in designing their own colorimeters, and my son has just gotten to the colorimetry labs in his online AP chemistry class, I decided to prototype my own colorimeter design. (His AP lab relies on eyeballing the light through two columns of fluid and adjusting the length of the light path through one until the intensities seem to match—that’s a very low tech approach, but it seems rather tedious.)
I had made a prototype colorimeter out of black foamcore,which I mentioned in Seventh day of freshman design seminar. I’d meant to blog about it earlier, but I got a bad virus infection of some sort and was out of action for a while. The prototype I’d made earlier was not very functional—it fit the cuvette tightly, but did not provide an easy way to remove the cuvette from the colorimeter. I went through two more prototypes today that would allow me to remove the cuvette, but they looked like they would have bad light leaks. I finally settled on a 4th design, that uses a separate lid, rather than trying to make a hinged lid. I’ve included a PDF file that has the design for this version: Colorimeter-draft4.
I constructed the colorimeter by spray-gluing the pattern I made to the black foam core, then carefully cutting it out with a razor knife. The dashed lines are cut only through the top layer of paper and part way through the foam—bending the foam core then snaps the rest of the foam and leaves the paper on the back as a hinge. I used black electrical tape to hold the colorimeter together, and to block light from coming through the backs of the phototransistor and the LED.
I connected the LED through a current-limiting resistor to the 3.3v supply on the Freedom KL25Z board, and the phototransistor with a current-to-voltage resistor to E22. I actually ended up doing two different circuits, using different LEDs:
When the KL25Z is reset, the analog-to-digital measurement on E20 is recorded as the intensity for the blank cuvette. Then the transmittance (measurement/blank) and absorbance (–log10(transmittance)) are reported 10 times a second on the USB serial line.
With the 700nm LED, I could measure from A=0 to A=1.8 (with an opaque piece of foam core blocking the light). Light leakage around the cuvette and from the outside prevented me from measuring higher absorbancy.
The first thing we tried measuring was a solution of blue food coloring (blue dye #1). My son made a stock solution of 1 drop in 10 ml, and we measured the absorbance (with the 700nm LED) at about 0.0028, which seemed rather low to us. He then made a very concentrated solution with 5 drops in 5ml, which looked almost black to us, and the colorimeter reported it as having an absorbance of 0.157, which seemed absurd—that’s almost clear! We tried looking at the sky through the solution and noticed that the sky looked deep red through the blue. This lead me to suspect that the dye was transparent in the near IR where much of the light from the LED was concentrated.
When I switched to a 627nm LED, I had to use a larger current-to-voltage conversion resistor, as the phototransistor is less sensitive to those wavelengths. This meant that the noise level from light level was increased, and so an opaque object read as absorbance around 1. The stock solution read as 1, as did a 2-fold and 4-fold dilution. We went to a 40-fold dilution (so equivalent to 1 drop in 40ml) and got a reading of 0.637. From there, we started 2-fold dilutions:
|dilution||Absorbance at 627nm|
I was worried that cuvette was stained by the dye, but putting in distilled water after the 1/640 dilution showed that any residual staining could only be contributing a small error.
Here is a plot of one run of the colorimeter:
I’m now trying to figure out why we did not get a good fit for Beer’s Law. Here are some possibilities:
- The dilutions were not done accurately. That would explain random fluctuations, but seems unlikely to give such a clean, consistent deviation from theory.
- Beer’s Law doesn’t apply to this example. That seems really, really unlikely, since this is the canonical experiment done by 1000s of students a year.
- The colorimeter is not linear. I’m relying on the phototransistor providing a current proportional to light intensity, even though the voltage across the phototransistor varies. I think that this is likely to be the problem. To check it, we’d have to redo the experiment with a different circuit—probably a transimpedance amplifier to do the current-to-voltage conversion.
Unfortunately, my son did not keep all the serial dilutions, so to test a different colorimeter circuit I’d have to make a new series. I might do that this weekend if I have time. At least I know what stock concentration to start with—about one drop of food color in 25ml of distilled water.
I’m also interested in trying the colorimeter design with an RGB LED, so that we could try different wavelengths—perhaps doing a Beer’s Law test with yellow food coloring.