Thomas S. Briggr and Warren C. Rauscher Galileo High School Lux Laboratory 1150 Francisco Street San Francisco, California 94109
Previously reported oscillating reactions either involve relatively faint color changes or require phenanthroline, an expensive indicat0r.l We have found an oscillating iodine clock reaction that gives striking cyclic changes from colorless to gold to blue using simple reagents. This reaction provides an excellent lecture demonstration and minht be adapted to a variety of student investipations. It resembles the iodate-hydrogen peroxide oscillating reaction of Brav.2 .. but it operates at room temperature with greater intensity. The followinn mixture has been found to give the best results: 0.067 M potassium iodate, 1.2 M hydrogen peroxide, 0.053 M perchloric acid, 0.050 M malonic acid, 0.0067 M manganese(I1) sulfate, and 0.01% starch. These concentrations may he about 30% more dilute without degrading the reaction (see figure) and an equivalent amount of sulfuric acid may replace the perchloric acid. Constant stirking improves the cycling but is not essential. Demonstration solutions are stored for extended periods in three containers so that mixing equal volume$ from each gives the ahove composition. One holds the hydrogen peroxide, another the iodate with mineral acid and the third the malonic acid, starch, and manganese. In -~~addition to the visible fluctuations in iodine concentration this reaction exhibits large fluctuations in the concentration of iodide ion. The blue starch complex does not appear until the iodide concentration approaches its peak value of 10-4 M during each cycle. This is in accord with the observations of Thoma and French3 that iodide is necessary for the formation of the blue complex. Starch indicator fails in the Bray reaction, presumably due to a lower concentration of iodide in the absence of malonic acid. Iodide oscillation is easily observed by the student by means of a silver-silver iodide electrode immersed in the oscillating mixture. This electrode is readily made from a clean silver wire by briefly making it the anode in a dilute iodide solution. For the reference electrode we recommend the use of a calomel electrode in conjunction with a salt bridge. A standard pH meter can he connected to these electrodesand calibrated at one point with a M iodide solution. Permanent records are obtained with a chart recorder. Many variations of this reaction exist. We have observed short-lived oscillations by substituting 2,4-pentan~
~~~~~
~
~~
~
~
'Reviews of these reactions appear in J. CHEM. EDUC., 49, No. 50972). 2Brav. - , W. C.. J.Amer. Chem. Soc.. 43.1262(1921). 3Thoma, J. A,, and French, D., J. Amer. Chem. Soe., 82, 4144(1960). Zhabotinskii, A. M., Dokl. Akod. Nouk. SSSR, 157,392(1964).
496
/ Journal of Chemical Education
An Oscillating Iodine Clock
edione for malonic acid. Cerium mav he used in place of manganese and gives higher frequency oscillations of variable amplitude. In fact. invisible oscillations in iodide occur in the absence of metal catalyst in a narrow region of higher concentration, e.g., 0.092 M potassium iodate, 0.069 M malonic acid, 3.24 M hydrogen peroxide, and 0.16 M sulfuric acid. Inasmuch as no oscillating reaction is well understood a t this time, little can he said of the mechanism of the oscillating iodine clock. Its similarity to the Bray reaction was noted ahove. If the malonic acid is not present in the demonstration mixture the manganese or cerium catalyse a rapid initial formation of iodine. Even this simple demonstration of catalysis is not well known in the literature. I t should he noted that manganese and cerium and malonic acid are part of the widely publicized Belousov oscillating reaction4 in which they interact with hromate. The malonic acid is a halogen consumer and halide producer in both the Belousov reaction and the oscillating iodine clock. Obviously, much research is needed to elucidate these oscillating reactions. We hope the spectacular nature of this new reaction will motivate interest in the difficult kinetic problems related to it. The authors patefully ac~ i l l i a mR. knowledge helpful discussions with Jollv and Robert E. Connick of the Universitv of Califor-
minutes Oscillations produced by a solution of 0.050 M potassium iodate. 0.038 M rnalonic acid. 0.0055 M manganese(ll) sulfate. 0.88 M hydrogen peroxide. 0.035 M perchloric acid, and 0.01% starch. Cycling begins when manganese is added. The signal is produced by a silver-silver iodide electrode.
