Chemistry Magic

I haven't done much pure chemistry in university. I did some of the basic physical, organic, and inorganic chemistry in my first year, but that's about it. Second year was more focalized with the likes of biochemistry and systems biology. By third year, there isn't much chemistry involved anymore. This is a shame really, because I, for one, quite enjoy the subject. Such is my enthusiasm for chemistry actually that I just had to write about the Briggs-Rauscher reaction after viewing this demonstration video:

The Briggs-Rauscher reaction is an oscillating chemical reaction. It is one of those reactions which make people of all ages reflect on how cool science exactly is. The reaction basically begins with the mixing of three colorless solutions. The resulting mixture formed then oscillates periodically in color from clear to amber to dark blue then back to clear, amber, and so on. It is a riveting scene. And the chemistry behind it is even more fabulous.

The three solutions initially mixed together contain a source of iodate (IO3-), a strong acid (H+), malonic acid (CH2(COOH)2), manganese cation (Mn2+), hydrogen peroxide (H2O2), and starch indicator. What ultimately happens during this oscillating chemical reaction is that the iodate, malonic acid hydrogen peroxide, and H+ react in a stepwise manner, ensuring a series of inter-dependent reactions. Some of these reactions contribute to the color changes that are seen during the reaction.

Two component reactions can be said to occur upon the initial mixing (triggering of the reaction). One of them does not actually result in the direct production of colored species per se. It merely involves the production of the all-important colorless hypoiodous acid (HOI) from iodate and hydrogen peroxide.  The hypoiodous acid then contributes to the formation of iodine (I2) and eventual accummulation of iodide (I-). These two account for the amber and blue colors respectively.

You might think that it's becoming too agonizingly chemistry-complicated but please bear with me for a little longer. This is where it becomes really interesting. As the reaction starts, a lot of hypoiodous acid is present and it gives out iodide in the presence of hydrogen peroxide. Then, the acid actually acts with the iodide it just helped produce and forms iodine, resulting in the sudden amber color that is seen. However, as more and more of the hypoiodous acid reacts with iodide, the former's concentration decreases. Iodide which is less and less acted upon by the disappearing acid becomes more prominent in the mixture and this offsets the color of the latter to dark blue as it interacts with iodine and the starch indicator. Eventually, iodine is eaten up by malonic acid and its removal turns the solution to colorless once again. From then onwards, it is the same cycle over and over.

Awesome, right? You might want to watch the video again.

Chemistry magic, I tell you.


References:

Scott, E., Schreiner, R., Sharpe, L., Shakhashiri, B., & Dirreen, G. Oscillating Reactions. (Modified by Prof. Wu at Colorado School of Mines)

Shakhashiri, B. Chemical Demonstrations: A Handbook for Teachers of Chemistry, vol. 2 (1985).