Oscillating Reactions: The Briggs-Rauscher

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by Super User, 4 months ago
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NOTE: The yellow color of solution A was solved almost instantly by Bill Smathers. The iodate was likely contaminated with iodide. In acid, the lead to the production of I2, which gave the solution a yellow color. Upon addition of B, the I2 was sequestered by malonic acid!

The Briggs-Rauscher oscillating reaction is one of the very few oscillating reactions that we know of.

Throughout the reaction, the concentration of iodide (I-) goes up and down, and this causes a swap between 2 processes. The the concentration is high, the process that consumes iodide is favored. When it is low, the process that produces it is favored. This causes a constant fluctuation of iodide.

As the iodide increases, it forms I2 which is yellow, then the I2 eventually combines with I- to form I3. This complexes with starch and forms a blue-black complex. The iodide producing process is then shutdown and the process that consumes it takes over. The iodide concentration drops, the I3-starch complex falls apart and the color dissipates. The I2 is sequestered by malonic acid and the solution reverts to colorless.

The [i-] continues to fall until the other process then takes over. Iodide is produced again and the cycle repeats itself.

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