Briggs–Rauscher reaction

The Briggs–Rauscher oscillating reaction is one of a small number of known oscillating chemical reactions. It is especially well suited for demonstration purposes because of its visually striking colour changes: the freshly prepared colourless solution slowly turns an amber colour, suddenly changing to a very dark blue. This slowly fades to colourless and the process repeats, about ten times in the most popular formulation, before ending as a dark blue liquid smelling strongly of iodine.

The first known homogeneous oscillating chemical reaction, reported by W. C. Bray in 1921, was between hydrogen peroxide (H₂O₂) and iodate (IO₃⁻) in acidic solution. Because of experimental difficulty, it attracted little attention and was unsuitable as a demonstration. In 1958 B. P. Belousov in the Soviet Union discovered the Belousov–Zhabotinsky reaction (BZ reaction), which is suitable as a demonstration, but it too met with skepticism (largely because such oscillatory behaviour was unheard of up to that time) until A. M. Zhabotinsky, also in the USSR, learned of it and in 1964 published his research. In May 1972 a pair of articles in the Journal of Chemical Education brought it to the attention of two science instructors at Galileo High School in San Francisco. They discovered the Briggs–Rauscher oscillating reaction by replacing bromate (BrO₃⁻) in the BZ reaction with iodate and adding hydrogen peroxide. They produced the striking visual demonstration by adding starch indicator. Since then, many other investigators have added to the knowledge and uses of this very unusual reaction.

The initial aqueous solution contains hydrogen peroxide, an iodate, divalent manganese (Mn²⁺) as catalyst, a strong chemically unreactive acid (sulphuric acid (H₂SO₄) or perchloric acid (HClO₄) are good), and an organic compound with an active ("enolic") hydrogen atom attached to carbon which will slowly reduce free iodine (I₂) to iodide (I⁻). (Malonic acid (CH₂(COOH)₂) is excellent for that purpose.) Starch is optionally added as an indicator to show the abrupt increase in iodide ion concentration as a sudden change from amber (free iodine) to dark blue (the "iodine-starch complex", which requires both iodine and iodide.)

...
Wikipedia