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Author: Subject: Iodine clock reaction variations
j_sum1
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[*] posted on 12-9-2019 at 04:15
Iodine clock reaction variations


I have been playing around with developing a version of the iodine clock using household products.

My initial approach was to use vitamin C tablets, povidine iodine and cornflour for one solution and bleach for the other. I could not get it to work. There was no delay visible and the iodine was quickly overoxidised to iodate.
NileRed has a similar version using peroxide and (a lot of) iodine tincture. His solutions seemed rather concentrated. I could not get that one to work either although I was still using the povidine, which may have made a difference.


My standard recipe for the reaction is to use a solution of potassium iodate and another which is a very dilute solution of acidified bisulfite and starch. This recipe is very reliable and has a very sudden colour change. So I switched tack and tried this alternative approach.

To a solution of the povidine iodine I added bleach dropwise until the colour cleared and stayed clear. The other solution was a combination of cornflour starch, sodium bisulfate and a tiny amount of metabisulfite. This worked but I need to tweak concentrations to get the timing right.

My problem is that I do not understand how this second reaction works. Presumably, the iodate reacts slowly with the metabisulfite. And when that reaction is complete then... Not sure. The thing is that there is a large excess of the iodate. So what reduces it back to iodine?

Can someone enlighten me. I feel like I amissing somethOKng obvious.




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woelen
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[*] posted on 12-9-2019 at 04:44


You have in fairly acidic solution:
- excess iodate
- bisulfite (metabisulfite does not exist in solution: S2O5(2-) + H2O --> 2 HSO3(-))
- starch

The reaction between iodate and bisulfite is slow at moderately low pH. It produces I(-): 3 HSO3(-) + IO3(-) --> 3 HSO4(-) + I(-)
Iodate and I(-) react to form I2. This reaction is moderately fast (timescale of milliseconds to seconds, depending on pH) and consumes acid: IO3(-) + 6 H(+) + 5 I(-) --> 3 I2 + 3 H2O
I2 reacts with bisulfite. This reaction is extremely fast (IIRC timescale of nanoseconds): I2 + HSO3(-) + H2O --> 2 I(-) + 2H(+) + HSO4(-)

Initially, the iodate reacts with bisulfite. This produces iodide ions. These iodide ions may react with more iodate to produce I2. As long as there is bisulfite in solution, that I2 is immediately captured and converted to iodide and never gets the time to make a visible complex with starch. However, as soon as the metabisulfite is used up, the iodine formed in the second reaction is not captured anymore and it produces the dark blue complex with starch.

If you initially have a little too much bleach, then that first quickly consumes some bisulfite and then the reaction proceeds as described above with the remaining compounds in solution. In this experiment, hence it is important to have excess iodate. If you have excess bisulfite, then the solution remains colorless. Then you end up with I(-), HSO4(-) and some excess HSO3(-).

[Edited on 12-9-19 by woelen]




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[*] posted on 12-9-2019 at 05:11


Thanks woelen. That makes sense.
So, things to watch for:
1. Don't have excess bleach as that will consume the bisulfite.
2. Don't have too much bisulfite as that will lengthen the time and will prevent any colour change at all if in excess.
3. Watch the pH. Particularly important since bleach contains NaOH. It probably means vinegar is off limits as the acid.

I think I can do this.
I am aiming to make an educational resource that can be done by home-schoolers in remote regions and will allow them to investigate reaction rates. Hence my targeting items that are very readily available.




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[*] posted on 12-9-2019 at 06:32


I have attached a JChemEd paper describing an iodine clock reaction designed to work with consumer chemicals. I have had good luck with this as described. This uses 3% H2O2 as the oxidant, tincture of iodine instead of povidone, and ascorbic acid as reductant. (I always have used powdered ascorbic acid instead of tablets, the powder is available OTC.)


Attachment: AscorbicAcidClockB.pdf (79kB)
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Attachment: AscorbicAcidClockA.pdf (167kB)
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j_sum1
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[*] posted on 12-9-2019 at 06:56


These won't open on my phone. I will look at them later. It sounds very similar to Nile's. However, there are reasons for leaning away from peroxide and tincture. Bleach and povidine are ubiquitous around here. Peroxide and iodine tincture take a bit of digging. If I can get away with cleaning vinegar instead of pool acid I will be even happier.



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