|MadSci Network: Chemistry|
The clock which you have observed is based on a reaction which belongs, funnily enough to a class of reactions discussed in a recently posted question about fractal reactions. Starch and iodine are used only to visualize the effect, since the complex of the iodine with starch is intensely colored.
I have once received a very nice explaination of the mechanism of this reaction (Which is by the way called the Briggs-Raucher reaction) from Dr. Lev A. Gorenstein at Purdue University. Hereby I reproduce his explanation.
The following piece is adopted from the Sept. 1984 issue of Russian "Chemistry and Life"
Solution 1. Dilute 102.5ml of 30% H2O2 by water to the total volume 250ml.
Solution 2. Dissolve 1.1 ml of concentrated H2SO4 (HClO4 works as well) in 10 ml of water, add water to make the volume about 50-60ml, dissolve 4.28g KIO3 (slightly heating it) and add the water till the total is 100ml of solution.
Solution 3. Make a solution of 0.08 g of starch in a small amount of water (heating needed), dissolve there 3.9 g of malonic acid and 0.85 g of MnSO4, and add water to form 250ml of solution.
Add 100ml of solution 2 in the big beaker, and 100ml of solutions 1 and 3 in smaller ones. Then simultaneously add solutions 1 and 3 to the big beaker (stirring). Then sit and start staring at it ;-). First it'll be colourless, but in several seconds starts changing.
The summary equation is : CH2(COOH)2 + 4H2O2 ---> 3CO2 + 6 H2O
But the reaction's stoichiometric concentration ratios are different from ones that can be found from this equation. The conditions when the color changes are well seen by human eye were found experimentally. The magazine says that they were found to be: 0.067M KIO3, 1.2M H2O2, 0.053M H2SO4, 0.05M malonic acid, 0.0067M MnSO4. Starch is added in minor amounts - 0.01%, and Iodide-starch complex forms only when the I- concenration reaches 10E-4 M. And after full oxidation of malonic acid the reaction stops.
Mn2+ Mn2+ -----------\ Mn2+ \ / H2O2 \ / H2O2 \ / \ / Mn3+ Mn3+ CH2(COOH)2 + HIO3 ----> ICH2-COOH + CO2 + H2O ----> HI + CO2 (colorless) ^ (colorless) | (colorless) | | | This is the main cycle. | | | H2O2 H2O2 | | | starch, I- v I-*I2*starch complex <---------------------- I2 (blue) (yellow)A good reference for this and other reactions of the same sort is :
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