pantone159
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Briggs-Rauscher Oscillating Reaction
I picked up 'The Facinating Oscillating Reaction Kit', made by Universe of Science http://www.universeofscience.com/osc.html) to check out the Briggs-Rauscher oscillating reaction. (As well as to get a small sample of malonic
acid.)
I decided to more-or-less ignore the instructions in the kit (but use the chems), and rather to follow the instructions from Bassam Z. Shakhashiri,
Chemical Demonstrations, Vol 2, Chap 7.1, p. 248-256, University of Wisconsin (1985). I tweaked the concentrations a bit to allow using either of two
H2O2 concentrations.
So, I prepared the following solutions, all using distilled water: ('m' means mol/kg of solution.)
A-1: 2.667 m (about 9%) H2O2
A-2: 0.27 m Sodium iodate + 0.10 m H2SO4 (ACS)
A-3: 0.20 m Malonic acid + 0.026 m MnSO4 + some starch
Also, low strength H2O2, which can substitute for A-1,
B-1: 3% H2O2 (aka drugstore strength.)
To perform the reaction, combine 4.0 g of A-1 (or B-1), 2.0 g of A-2, and 2.0 g of A-3 in a vessel, stirring as constantly as possible. (In my case,
this meant frequent swirling, but a magnetic stirrer would be ideal.) This is intentionally a small scale, as small as practical (much smaller than
the official instructions), allowing me to run more trials while saving as much of the chems as I could.
The kit instructions said to use the 3% H2O2. The Shakhashiri prep said you could use either the strong or weak H2O2 solutions, but the strong was
more dramatic. I tried both.
Trials with 9% H2O2:
After combining the colorless solutions, the mixture turns light yellow/brown, the yellow color gadually becomes darker, then suddenly the mix turns
purple. The purple color (which was not as dark/intense as the usual iodine-starch color) quickly fades to very light yellow, which then gets darker,
then turns purple, then quickly fades again. This repeats c. 15-20 times, over about 8 minutes. At this point, the mix turns darker purple-brown,
and remains this color. Eventually, crystals of I2 form in the mix. To dispose of this, sodium thiosulfate was added until the mix remained
colorless (smaller amounts will make the purple color disappear, but it will return), then this was poured down the drain.
Trials with 3% H2O2:
This ran considerably slower, but still gave good color changes. After combining the solutions, the mix very slowly started to turn yellow/brown.
Hardly any color had formed after 1 minute, and was still not a strong yellow color after 2 minutes. At 2.5 minutes, the mix turned purple very
briefly, then faded to colorless. The mix started to turn brown again, and at about 2.75 min, it turned purple, staying this color for a decent
amount of time, then fading. A total of 6 oscillations were seen, counting the first (very weak) one, and when about 7 min had passed, the mix turned
darker purple-brown and the reaction was done.
So, this didn't show nearly as many oscillations as with stronger H2O2. However, the color changes were at least as dramatic. Maybe even more so,
the purple color got even darker, than it did with the 9% H2O2. (Perhaps with the slower oscillations, there was more time for the purple color to
get fully formed, before it faded away?)
Trial contaminated with tap water:
While my scale was on loan for a day, I instead measured out the solutions by volume, washing my single graduated cylinder in between with TAP water.
Specifically, I measured 4.0 ml of 9% H2O2 in the cylinder, poured this into a test tube, washed the cylinder with tap water, then measured 2.0 ml of
A-2, poured this into a test tube, washed the cylinder again with tap water, then measured 2.0 ml of A-3, and poured this into a test tube. The
contents of the test tubes were combined, and...
Nothing happened. No oscillations, no color changes. After c. 12 hours or so, the mix did turn dark purple (like the other runs), but nothing
interesting happened in the meantime.
This reaction is supposedly messed up by even small amounts of chloride ion. I suspect that enough Cl- was in my tap water to do this. (The kit
instructions say to use tap water, which I think would not work.)
Conclusion:
I thought this was pretty cool. Those who insist on explosions in their experiments won't be satisfied, but for someone like me who likes color
changes, this was interesting.
[Edited on 31-1-2007 by pantone159]
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nitro-genes
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Very cool indeed, wanted to show this reaction to some people in the lab once, I could find all the chemicals needed, except for the malonic acid...
Didn't know there were kits available though for this, the individual sized kit is not to expensive as well...
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pantone159
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I think the kit is a good deal. For around $10, you get 5g of both malonic acid and sodium iodate, which aren't that easy to find, especially cheap.
As a bonus, 5g of sulfamic acid, 1g of MnSO4, 10g of sodium thiosulfate, some starch solution, plus two little scoops that I'll use, plus some other
junk.
That company had a couple of other kits that looked interesting, one was something with a red cabbage based powder that was supposedly stable (and
made stable solutions??) which would be very useful.
I forgot to mention that the kit instructions said to use sulfamic acid, I instead used H2SO4 per Shakhashiri.
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woelen
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These kits are very nice. Malonic acid is amazingly hard to obtain, and i still did not find a reliable and continuous source for that. NaIO3 is
nothing special, I make it myself from NaI. KIO3 can also be used, and can also easily be made from KI. I received some malonic acid from a fiend of
mine, but it was only a few grams and a one-time event.
I did the reaction with potassium bromate (also made that myself from KBr), manganese sulfate, and malonic acid. In that case no starch indicator is
needed, the oscillation is between the red/brown Mn(3+) and the colorless Mn(2+). I also did the experiment with ceric sulfate and then the solution
rapidly oscillates between deep yellow and colorless.
I also tried replacing the malonic acid with citric acid. And indeed, with that I also had oscillations, but the result is much less appealing. With
citric acid, some dirty oily crap is formed during the reaction, with malonic acid, the mix remains perfectly clear.
More surprisingly, I also noticed that I obtained an oscillating reaction with 30% HCl and 30% H2O2. Simply mix the two liquids in a test tube at a
HCl : H2O2 = 2 : 1 volume ratio and observe the oscillating pattern in the evolution of gas. The mix alternatingly produces chlorine gas (low bubbling
activity) and oxygen gas (strong bubbling activity).
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pantone159
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Gas Evolution Oscillating Reactions:
Shakhashiri 7.13 also describes this:
A: Dissolve 26g ammonium sulfate in 100 mL 0.2M H2SO4.
B: Dissolve 28g Na nitrite in 100 mL H2O.
Combine 10 mL of A and B, with stirring. Oscillations in the evolution of gas (N2) occur.
I haven't tried this.
Shak also mentions the Bray-Liebhafsky reaction, which produced oscillations in the evolution of gas (O2) from a mix of H2O2, KIO3, and H2SO4.
Bray, J Am Chem Soc, 43:1262 (1921).
Liebhafsky et al, J Am Chem Soc, 100:87 (1978).
As far as making IO3 and BrO3, I assume you are talking about the electrolysis setup which has been much discussed on this board. I don't have such
apparatus (yet), so these aren't quite so easy for me. (I do like the idea of making these myself, however.)
The Shakhashiri prep for Briggs-Raucher actually specified KIO3, but the kit came with NaIO3 so I used that instead.
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pantone159
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Ceric(IV) sulfate vs ammonium nitrate
What are the advantages/disadvantages of ceric(IV) sulfate vs ceric(IV) ammonium nitrate for the BZ and related reactions?
My books say the ammonium nitrate, the original discovery was with the sulfate.
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woelen
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I don't think there is any relevant difference. I did the experiment with the ceric sulfate, but sulfate, nitrate and also ammonium are just spectator
ions.
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pantone159
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I saw the same kit again here, also US $10:
http://www.teachersource.com/Chemistry/ChemistryKits/Fascina...
I still think this is a good deal, malonic acid is so hard to get, and the other stuff in there is useful as well.
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