Concentration oscillations are ubiquitous in living systems, where they involve a wide range of chemical species. In contrast, early in vitro chemical oscillators were all derived from two accidentally discovered reactions based on oxyhalogen chemistry. Over the past 25 years, the use of a systematic design algorithm, in which a slow feedback reaction periodically drives a bistable system in a flow reactor between its two steady states, has increased the list of oscillating chemical reactions to dozens of systems. But these oscillating reactions are still confined to a handful of elements that possess multiple stable oxidation states: halogens, sulphur and some transition metals. Here we show that linking a 'core' oscillator to a complexation or precipitation equilibrium can induce concentration oscillations in a species participating in the equilibrium. We use this method to design systems that produce periodic pulses of calcium, aluminium or fluoride ions. The ability to generate oscillations in elements possessing only a single stable oxidation state (for example, Na+, F-, Ca2+) may lead to reactions that are useful for coupling to or probing living systems, or that help us to understand new mechanisms by which periodic behaviour may arise.

The hydrogen ion is arguably the most ubiquitous and important species in chemistry. It also plays a key role in nearly every biological process. In this Account, we discuss systems whose behavior is governed by oscillations in the concentration of hydrogen ion. The first chemical oscillators driven by changes in pH were developed a quarter century ago. Since then, about two dozen new pH oscillators, systems in which the periodic variation in pH is not just an indicator but an essential prerequisite of the oscillatory behavior, have been discovered. Mechanistic understanding of their behavior has grown, and new ideas for their practical application have been proposed and, in some cases, tested. Here we present a catalog of the known pH oscillators, divide them into mechanistically based categories based on whether they involve a single oxidant and reductant or an oxidant and a pair of reductants, and describe general mechanisms for these two major classes of systems. We also describe in detail the chemistry of one example from each class, hydrogen peroxide-sulfide and ferricyanide-iodate-sulfite. Finally, we consider actual and potential applications. These include using pH oscillators to induce oscillation in species that would otherwise be nonoscillatory, creating novel spatial patterns, generating periodic transitions between vesicle and micelle states, stimulating switching between folded and random coil states of DNA, building molecular motors, and designing pulsating drug delivery systems. We point out the importance for future applications of finding a batch pH oscillator, one that oscillates in a closed system for an extended period of time, and comment on the progress that has been made toward that goal.

A mechanistic model is presented for the Cu"-catalysed oxidation of thiosulfate by hydrogen peroxide, which gives oscillatory behaviour in a flow reactor. A simple four-step model, in which formation of the intermediate HOS203-and attack on that species by H 2 0 2 and S 2 0 3 2 -play key roles, suffices to give oscillatory behaviour. By extending this core set of reactions with additional steps that describe acid-base equilibria and reactions between H 2 0 2 and sulfur species, we obtain better agreement with the observed oscillatory behaviour as well as the ability to simulate the observed batch behaviour and the bistability and complex multi-peaked oscillations found in flow systems. Our results suggest that mechanistic descriptions of sulfur-containing hydrogen peroxide oscillators should emphasize the capability of the sulfur substrate to be partially oxidized to a relatively stable intermediate prior to its total oxidation to sulfate.

Experimental and numerical studies of the temporal behavior of the acidic bromate-1,4-cyclohexanedione batch reaction in the presence of Ce(SO 4 ) 2 , MnSO 4 , or Ru(bipy) 3 SO 4 catalyst are reported. With increasing concentration of catalyst and at [H 2 SO 4 ] e 1 mol/dm 3 these systems show the following bifurcation sequences: uncatalyzed oscillations f clock reaction (with excitability) f catalyzed oscillations, while at [H 2 SO 4 ] g 1.5 mol/dm 3 the sequence is uncatalyzed oscillations f catalyzed oscillations. A chemical mechanism based on kinetic measurements of some component reactions is suggested and used to simulate the experimentally found bifurcation sequences and the monotonic and nonmonotonic recovery of the system after perturbation of the excitable state. A reduced model for the bromate-CHD-catalyst system is developed and analyzed.