Briggs–Rauscher Oscillating Reaction

A chemical clock that ticks by itself

Most reactions go one way: reactants turn into products, steadily, until they are used up. The Briggs–Rauscher reaction is different. Mix three colourless solutions and the beaker turns amber, then suddenly blue-black, then clears, and does it all again, over and over, for minutes. It is one of the most striking oscillating reactions in chemistry.

The ingredients

One solution contains potassium iodate and acid; one contains malonic acid, a manganese(II) catalyst and starch; the third is hydrogen peroxide. Between them they produce iodine (I₂) and iodide (I⁻) and then consume them again.

Why it oscillates

The heart of the reaction is two competing routes. When there is little iodide, a fast, self-accelerating route takes over: it makes iodine faster and faster, and the solution turns amber. Peroxide then turns some of that iodine back into iodide. Once enough iodide has built up, it switches the fast route off and a slower route takes over. Iodide and iodine are now present together and form triiodide, which the starch locks into a blue-black complex. Meanwhile the malonic acid soaks up iodine, so both fall away, the solution clears, iodide drops low again, and the fast route restarts.

The key ingredients are feedback: a product that speeds up its own production (autocatalysis), and a second product that later shuts that process down. The same pattern of fast positive feedback and slower negative feedback drives heartbeats, predator–prey cycles and many other natural rhythms.

Why it stops

Every cycle uses up a little iodate, peroxide and malonic acid. The oscillations slow down, and once the reagents are low enough the mixture settles to a steady state. Energy keeps flowing downhill the whole time, which is why oscillating reactions are allowed by thermodynamics.

For the curious: the model

The full mechanism involves around ten steps. This simulation uses a much simpler stand-in with the same kind of behaviour, the Brusselator, developed in Brussels by Ilya Prigogine and René Lefever:

dX/dt = A − (B + 1)X + X²Y,   dY/dt = BX − X²Y

Here Y plays the part of iodine and X of iodide. When B is greater than 1 + A², the steady state is unstable and the system falls into a limit cycle: a repeating loop. The equations are integrated with the fourth-order Runge–Kutta method. B is made to decline slowly as “reagents” are used up, so the cycles lengthen and finally die out, and an Arrhenius factor speeds everything up as the temperature rises. The periods are therefore illustrative rather than predictions for a particular recipe.

Safety

The real reaction uses concentrated hydrogen peroxide, sulfuric acid and potassium iodate, and it gives off iodine vapour. It should only be done by trained people in a fume cupboard with goggles and gloves. Please read our disclaimer.