My Iodine Clock Reaction Lab

2026-02-19 8:35 PM

#chemistry #teaching #lab #kinetics

For the last two years, I've done an iodine clock reaction with my second year chemistry students as the main teaching lab for our chemical kinetics unit. We use 0.2M sodium iodate and 0.2M sodium bisulfite as our reagents instead of the classical sodium thiosulfate and iodine. I've found this to be reliable and uses materials I already had in the lab. The amounts of reagent are low enough that I can make enough stock for eight lab groups to have a little extra in case they need to re-run a trial.

I introduce the reaction mechanism and main points the day before so students can read prior to starting. The entire wet procedure can be done during one 60 minute class period with analysis happening in the next class or for homework. The lab protocol gives students a detailed table of reagent amounts for each trial. They essentially perform a serial dilution from the 0.2M stock samples keeping the bisulfite concentration constant for the first trial runs and then keeping the iodate ions constant for the second. Students have to retrieve and manage samples, avoiding cross contamination, in order to complete the 10 trials correctly.

Most groups this year (all except for one) got the expected results of a first order rate for iodate and a second order rate for bisulfite. Here's the full procedure if you want to take a look at the student handout along with sample data and preparation notes on the last page.