Chemistry Week: Chemical traffic light
By Georgia Mann
Happy National Chemistry Week to you all! This week is a great opportunity for us to support the importance and value of Chemistry in our lives (plus it is a lot of fun!).
Georgian Chemist Samuel Parkes once said “Nothing tends to imprint chemical facts upon the mind so much as the exhibition of interesting experiments”, 1 which remains equally as true today. The “Chemical Traffic Light” experiment can definitely qualify and is perfectly in keeping with this week’s “Chemistry Colors Our World” theme!
This particular reaction is classified as a Chemical Clock reaction__. It takes place when the concentrations of the products and reactants (within a particular reaction mixture) can change periodically. This periodic change can occur following rapid changes in the property of the components or over a specific time frame. Unlike most chemical reactions, the products spontaneously change back into reactants and those reactants into products like weight shifting on a see-saw. The concentration of one component rises to a maximum level and then decreases to a minimum level and continues to fluctuate and oscillate over time, until finally, an equilibrium is reached.
There is a small collection of well-known oscillating chemical reactions (the Belousov–Zhabotinsky reaction, the Iodine clock reaction and the Briggs–Rauscher reaction); however, it is also important to note that these types of reactions occur naturally as well. As the great mathematician Alan Turing showed, from the very beginning with the patterning of the embryo, oscillating reactions apply to biochemical systems. One such reaction underlines all human life; glycolysis2. This metabolic pathway converts glucose into pyruvate and results in the simultaneous formation of Adenosine Triophosphate, or ATP, of which 100 molecules are made every second!
What is happening in this reaction?
This traffic light experiment involves an oscillating oxidation and reduction reaction (for the more attentive of you this may be bringing back memories of the old high school mnemonic OIL RIG – oxidation is loss of electrons, reduction is gain). Alkaline glucose solution acts as a reducing agent and, when added to the solution containing the indicator (coincidentally named indigo carmine!), a colour change of green to red to yellow is observed. However, due to the content of atmospheric oxygen (21%), when the reaction flask is shaken and the reduced indigo carmine is re-oxidised, the reverse is seen. The colour of the solution changes to red upon commencement of oxidation and, if vigorous shaking takes place, a fully oxidised green solution is re-generated.
Control over the change from red to green – now there`s something to think about the next time you`re in a traffic jam!
1.The Chemical Catechism Third Edition, London 1808, p557