JCE Pick: Improving Student Results in the Crystal Violet Chemical Kinetics Experiment

Every year, I have asked my AP students to determine the reaction order of both crystal violet (CrV) and sodium hydroxide (NaOH) using spectrometers. The lab I gave them is a modification of the kinetics lab from Pasco’s Advanced Chemistry Through Inquiry (I wrote about this manual here) and Flinn’s classic write-up. My students found the reaction order of crystal violet to be 0,1, and 2. They found the same with sodium hydroxide. Then I have had to tell them that the reaction order of each is actually 1. Explaining the error analysis is difficult for my students and it has been difficult for me to grade. Have you been there, too?

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Fig. 1. Abstract Figure from the article used with permission.

I recently stumbled upon this article and it is a super handy resource for kinetics labs (and a nice review of Analytical courses from my college days).

A few potential culprits:

1. Spectrometer Limitations - deemed to have the most significant effect on data - Remember that time in Analytical Chemistry when you discussed target absorbances to prevent deviations from the linearity we see in Beer’s Law? Right. Absorbances of CrV should be between about 0.1 to 0.7. So, suppose you are like me and don’t make solutions particularly carefully because you don’t care what the numbers are. You just care what your students can do with the numbers. This is the “fake it” method of making the CrV solution- add a drop or two of concentrated CrV and call it a day. What I should do as an addendum to the “fake it” method of solution preparation is to double check the absorbance and adjust accordingly. (I now have to apologize for "faking it" to my past students.)

2. Signal Processing - When does a student know when to stop collecting data? The authors tested a few different scenarios - but stopping at a common absorbance seems to be the best option. Additionally, authors discuss the benefits of LoggerPro vs Excel… but I have unfair bias towards Excel so I choose to ignore it (my husband is an engineer...enough said).

3. Interference from Particulates - Have you ever noticed (and ignored) that the final solution is a bit “milky-white” and not perfectly clear? While not deemed to be a major culprit by the authors, it might be a “tangible” source of error to discuss with your students.

The authors conclude with some practical guidelines aligned with the aforementioned culprits. All in all, in my completely biased opinion, this article satiates my desire for an efficient, practical, immediately-applicable article. It’s worth a full read - and if your students are ready, it might be valuable for them to read it too.

Thank you to the authors for publishing this - I hope I represented it well. I hope to see more like this in the future!

PS - If you don’t have access to colorimeters/spectrometers, check out Tom’s post on how to use smartphones as an absorption spectrophotometer.