General Rules of Electrochemistry

Rule 1. The first rule is the requirement of the closed electrical circuit. This means that at least two electrodes must be present in the electrochemical cell. From a purely electrical point of view, it means that we have a sensor electrode (the working electrode) and a signal return electrode (often called the auxiliary electrode). This requirement does not necessarily mean that a DC electrical current will flow in a closed circuit. Obviously, if we consider an ideal capacitor C in series with a resistor R (Appendix C), a DC voltage will appear across the capacitor, but only as a transient DC current will not flow through it. On the other hand, if an AC voltage is applied to the cell, a continuous displacement charging current will flow. 

An open electrical circuit (such as an electrochemical cell consisting of only one electrode ) simply means that a small and undefined capacitor (the missing electrode ) has been placed in series in the circuit. The result is predictable it will readily respond to high-frequency electrical fluctuations (e.g., noise), but no information that depends on DC behavior (i.e., on the composition of the sample) can be obtained. 

Rule 2 The second rule is the condition of electroneutrality. It means that in an electrochemical cell, the sum of positive charges must equal the sum of negative charges. Thus, separation of positive and negative charges occurs at every interface, but their sum is always zero. 

Rule 3 This is not so much a rule as it is an important general point regarding the nature of the interfacial reaction in all electrochemical sensors. Charge transport within the transducer part of the sensor, and/or inside the supporting instrumentation, is electronic. On the other hand, the charge transport in the sample can be electronic, ionic, or mixed (electronic/ionic). In the latter two cases, an electron 

Here O represents the oxidized species, R the reduced species, and n is the number of electrons. Species O and R differ only by n electrons they are called a redox couple. The rate constants kc and ka describe the dynamic nature of the reduction (at the cathode) and oxidation process (at the anode). When the passage of current through the interface results in measurable changes of bulk concentration, it is convenient to write Faraday s law in terms of molar concentration C and volume FCeii of the cell. 

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