Re: What chemicals are in the Princeton experiment

I am a science teacher, and as part of my curriculum, I use Optical Data videodisc lessons. The lesson on Physical and Chemical change shows a dramatic example of chemical change called "The Princeton Experiment." In the experiment, three clear liquids are added to a flask with an automatic stirring device. As the third liquid is added, the solution quickly turns an opaque yellow, and then almost immediately turns jet-black. Since this experiment has a name and is not explained in the teacher materials, I assume it is rather famous. I would like to know what three liquids are used in the experiment.

The demonstration, more commonly known as the "Old Nassau Clock Reaction," from Nassau Hall on the Princeton campus, was created by Hubert Alyea of Princeton University and may be found in his article in J. Chem. Educ. 1977, 54, 167. The colors developed, orange and black, are also the Princeton colors, and Professor Alyea is reputed to have sung the Princeton Fight Song while performing the demo.

The three solutions consist of

1. A starch and sodium hydrogen sulfite solution
2. A mercuric chloride solution
3. A potassium iodate solution.

The orange color is a precipitate of mercuric iodide, and the black color comes from the "iodine clock reaction," in which iodide is oxidized to iodine, giving the familiar blue complex with starch. The black color results from orange + blue = black.

After the addition of the third solution (containing KIO₃), the reaction commences:

1. Sodium hydrogen sulfite reduces iodate to iodide and hypoiodous acid.
2. Iodide forms an orange precipitate with mercury(II) ion.
3. When the mercury(II) is exhausted, iodide is free to combine with hypoiodous acid to form molecular iodine, which in turn combines with more iodide to form I₃^-.
4. The I₃^- reacts with starch to form the familiar blue iodine-starch complex.

The major problem with actually performing this reaction is that the mercuric iodide precipitate formed is not only a hazardous waste, but one which waste haulers are reluctant to take off your hands. YOU MAY NOT JUST DUMP THE REACTION MIXTURE DOWN THE SINK!

Richard G. Strickert, who was kind enough to provide me with the answer to this question, adds:
In recent years I've used various acid-base reactions along with an indicating solution made from red cabbage juice to make multi-color demonstrations. The solutions are non-hazardous and can be disposed of in the sink.

Many thanks to Dr. Strickert, and to Conrad Shiba, Tom Kenney, and Steve Marsden for their help with my answer to your question!