Failed kiddie chemistry experiment

June 21, 2015 6:06 PM

We tried to do the Iodine Clock Reaction today for the kids, using 1) HSO3 mixed with starch and 2) KIO3. Unfortunately, instead of changing from colourless to dark blue (cue impressed sounds from the kids) there was no colour change at all. No precipitate or apparent temperature change, either, so I'm not sure anything at all happened chemically. Any ideas what might have gone wrong?
posted by thelaze to Science & Nature (7 answers total)

Did you have little enough HSO3 to be fully consumed? If there was too much relative to the KIO3, you wouldn't get the color change. What amounts of each did you use?
posted by ocherdraco at 7:39 PM on June 21, 2015 [1 favorite]

Response by poster: It was pretty much a free pour of roughly equivalent amounts. At first perhaps 75-100 mL each, and then we kept adding a little bit more of both solutions up to maybe 200 mL each before finally giving up.
posted by thelaze at 7:54 PM on June 21, 2015

The Wikipedia article mentions that the HSO3 needs to be acidified with sulfuric acid (H2SO4). Did that happen?
posted by jamjam at 9:11 PM on June 21, 2015

Response by poster: Wouldn't HSO3 in solution become H2SO4 or is my chemistry that bad?
posted by thelaze at 4:27 AM on June 22, 2015

HSO3- shouldn't be converted directly into H2SO4 since they're not a conjugate acid/base pair. What should happen is that H2SO4, a strong acid, will completely dissociate in solution into H+ and HSO4- (some of the HSO4- will then be converted into H+ and (SO4)2-). Some of the HSO3- I assume would be converted into H2SO3 and (SO3)2- but that won't consume all of the HSO3-, and since the goal is to have HSO3- be the limiting reagent, I don't think this should be an issue. (This happens because HSO4- <> H+ + SO4- is an equilibrium reaction, which means that you'll end up with a mix of all 3 species based on the equilibrium constant (Ka for acidic reactions). The same thing will happen with HSO3-, only this will happen in two directions. You will have an acidic reaction: HSO3- <> H+ + SO32-m and then the reaction with HSO3- acting as a base: HSO3- + H2O <> OH- + (SO3)2-. Everything except H2SO4 is a weak acid (meaning partial as opposed to complete dissociation), so you should have some of each species. The equilibrium constants will dictate the ratios of everything. Based on the Kas for H2SO3 and HSO3-, it looks like you should have plenty of HSO3- remaining. Of course, this will be affected by other potential reactions if you don't have pure H2SO3/HSO3-/SO32- solutions, but this should be fine for the experiment.

More relevant to this experiment, do you know the approximate starting concentration of HSO3- and IO3-, respectively? The actual volume isn't really key to this, it's the concentrations (molarity; mole/L). The number of moles will determine what is the limiting reagent. If you know grams of IO3- and grams of HSO3- that you have prior to adding in water, it's easy to calculate moles form there.

However, if you don't know concentrations, I would give it another go, but cut way back on the HSO3-. You could even do a couple of trials with different volumes for extra science fun! At the very least I would cut back to let's say 25 mL of HSO3- and 75 ml of IO3- (so roughly 1/3 HSO3-, and 3/4 IO3-.). Definitely add in some H2SO4 since it looks like acidity is key based on the wiki page. If you still get nowhere, then I would try maybe 10 mL of HSO3- and 80 mL of IO3- (a 1:8 ratio). Feel free to increase volumes as necessary. What's important is the relative amount of HSO3- and IO3-.

These are just very rough guesses. If this doesn't work the next time you try it, let us know. I could probably crunch some numbers to get a better idea, although temperature and other factors effect this as well. Still, I think cutting back on HSO3- and adding H2SO4 should do the trick.

And be sure to update us with results!

(Also, apologies in advance if I screwed up any of the explanations/numbers here. Mornings are not my favorite time of the day, and I'm rushing off to work.)
posted by litera scripta manet at 5:06 AM on June 22, 2015

Response by poster: Hmm... the solutions were brought over by a friend who had apparently tried the exact same way previously (eyeballing equal amounts of the two reagents mixed together) with total success. Is there some way one of the reagents could have degraded into a different solution?
posted by thelaze at 8:15 AM on June 22, 2015

Yes, bisulfite degrades by reacting with oxygen. If the bottle was left open, it will have gone off (or have reduced concentration, so you need to use more)
posted by firesine at 2:34 PM on June 22, 2015