The Landolt iodine clock reaction has been, and continues to be one of my favorite demonstrations. I perform this demonstration every semester for the General Chemistry classes at Duke. It has also been performed at numerous outreach events for audiences of all ages and backgrounds, and used as a hands-on activity for gifted and talented elementary aged children. Haley Barrier and I presented this demonstration at the 2012 BCCE, Penn State University, as part of the Demonstration Extravaganza. No matter who the demonstration is performed for, it always elicits “oohs, aahs and wows” from the audience when the solution suddenly, unexpectedly changes from colorless to deep blue-black.
There are four reactions involved in this demonstration. The first is the reduction of potassium iodate by sodium bisulfite, which has been determined to be a fast reaction. Both the reactants and products of this reaction are colorless.
IO31-(aq) + 3 HSO31-(aq) → I1-(aq) + 3 SO42-(aq) + 3 H1+(aq)
Over time, a second, slower reaction occurs that forms tri-iodide, which is amber in color.
6 H1+(aq) + IO31-(aq) + 8 I1-(aq) → 3 I31-(aq) + 3 H2O(l)
The tri-iodide formed is rapidly converted into iodide by the bisulfite, thus the amber color due to the presence of tri-iodide is not observed.
H2O(l) + I31-(aq) + HSO31-(aq) → 3 I1-(aq) + SO42-(aq) + 3 H1+(aq)
However, if the bisulfite is the limiting reagent, when it is consumed neither the iodate nor the tri-iodide can be reduced. Therefore, only the second reaction can occur, forming tri-iodide. The tri-iodide binds with the starch forming a complex that is blue/blue-black in color.
2 I31-(aq) + Starch → Starch-I51-(complex-blue) + I1-(aq)
This demonstration can be used to depict a chemical change, oxidation-reduction reactions, illustrate how concentration affects the rate of a reaction, limiting and excess reagents and reaction order (first-order with respect to the iodate concentration) depending on the background of the audience.
The tri-iodide, starch-I51- complex, and any remaining IO31-can be reduced into I1- by the addition of a small amount of solid sodium thiosulfate before disposing of the solution. This can be done in front of the audience, which is a nice way not only to show how the blue-black color of the solution can be made to disappear but also to talk about how the solution, before the addition of the sodium thiosulfate, is harmful to bacteria involved in the treatment of wastewater.
Editor’s note: The Flinn Fax version of this demonstration shows I2 as the first intermediate in the reaction of iodide with iodate, which then reacts with iodide ion to give the fleeting tri-iodide species.
The chemical reactions given in this article agree with:
- B. Shakhashiri, Chemical Demonstrations: A Handbook for Teachers of Chemistry (Vol. 4), Madison, WI: The University of Wisconsin Press, 3-25, 1992.