## 2 Oscillatory Reaction and Electrochemical Oscillation  
Ordinary chemical reactions gradually reach equilibrium by decreasing the concentration of reactants and increasing the concentration of products as the reaction proceeds. However, in some complex chemical systems, the concentration of certain components or intermediates will show certain periodic changes with time, and this phenomenon is called chemical oscillation reaction. Oscillatory reactions have a wide range of applications in many fields and have, therefore, attracted the interest of physicists and chemists.  
In 1958, chemist [PERSON] discovered the oscillation reaction system [12]. He observed concentration oscillations in a closed system of dilute sulfuric acid solutions of potassium bromate, citric acid and cerium sulfate, which was the most ordered oscillation phenomenon found at that time. In 1964, [PERSON] optimized the reaction system, which is now called the Belousov-Zhabotinsky system (B-Z system) [13; 14]. Later, [PERSON] et al. [15; 16] established the FKN model, which further improved the basic theory of chemical kinetics of oscillating systems. So far, many other oscillation reaction systems were found, such as Bray-Liebhafsky (B-L) oscillation system [17], Peroxidase-Monoxidase biochemical oscillation system [18] and Briggs-Rauscher oscillation system [19]. Oscillation reactions are reactions far from an equilibrium state and are extremely sensitive to reaction conditions. Factors include reactant concentration, catalyst concentration, reaction temperature, stirring speed, light conditions and the effect of oxygen, etc. Therefore, electrochemical methods such as potentiometry, ion selective electrode and conductormetry are often used to study oscillatory systems [20; 21].  
In recent years, the chemical oscillation system was applied to determine the content or concentration of a variety of organic, inorganic and gaseous substances. The mechanism of chemical oscillation reactions for analytical assays lies mainly in the fact that the substance to be measured can interfere with the oscillation reaction. In the last three decades, techniques such as regular oscillation combined with flow injection [22], analyte pulse perturbation technique [23] and continuous-flow stirred tank reactor [24] have been applied to chemical oscillation reactions, making them widely used for the determination of inorganic and organic compounds, such as CO, Vitamin B1, Vitamin C, Vitamin B6, phenol and formaldehyde [25; 26; 27; 28; 29; 30]. Among them, electrochemical oscillation, as an important branch of chemical oscillation, is the phenomenon of order in time and space in an electric field, which is apart from equilibrium. For electrochemical systems, it is generally always a non-equilibrium state where the reaction occurs by using the polarization property to deviate the potential from the equilibrium potential of that electrode [31]. The electrode reaction itself is a complex multiphase reaction process. Each factor is correlated with each other, which is a nonlinear reaction process, and this makes the oscillation phenomenon easily generated in the electrochemical system [32]. It developed quite rapidly after the chemical oscillation theory was established and has the same advantages as chemical oscillation systems. Moreover, electrochemical oscillation fingerprints contain several characteristic indicators such as induction time, highest and lowest potential, oscillation period and maximum amplitude, and it can be applied to analyze the chemical composition of the TCMs with no requirement of separation and purification and other pre-treatments [33; 34]. Electrochemical oscillation also has certain limitations, such as the need to match the sample and the oscillation system and the need to enhance the oscillation signal. Figure 1 shows a schematic diagram of electrochemical oscillation fingerprints.