Small-amplitude techniques were originally developed in an attempt to isolate the faradaic response of an electrochemical cell from the associated effects of solution resistance and capacitance. To understand the need for these techniques, it will be helpful to briefly consider an electrical model of the typical three-electrode electrochemical cell as represented by Figure 5.1a. The solution between electrodes can be well represented by a resistance whose magnitude is directly proportional to distance and inversely proportional to the concentration and mobility of the ionized species. These are normally predominately electrochemically inert electrolytes added to decrease the solution resistance (the supporting electrolyte). This is represented in Figure 5.1b by the resistors, Rc, Ru, and Rref. The electrochemical charge transfer, or faradaic process, takes place only at the interface between the solution and an electrode. The current at an electrode depends upon the concentration of electrochemically active species and the potential of the electrode relative to the E°s of the available electrochemical couples. The electron flux resulting from each couple will range from zero to an amount limited only by the rate of mass transport to the electrode surface. This is represented by the boxes labeled Zf and Zf in Figure 5.1b. The supporting electrolyte is chosen such that its ions are not reducible or oxidizable at potentials expected for the working electrode. When a potential is applied to an electrode, either the anions or cations of the supporting electrolyte will be attracted to the surface electrostatically, but will not be able to undergo a charge 142transfer reaction because the potential is not sufficiently positive or negative. This leads to a sheet of ions very close (angstroms) to a conductor of opposite charge with no electron transfer between them. This is the classical definition of a capacitor and is termed the double-layer capacitance. This is represented in Figure 5.1b by the capacitors C′dl and Cdl. Schematic representation of an electrochemical cell: (a) three electrodes; (b) equivalent circuit for three-electrode cell; (c) equivalent circuit for the working-electrode interphase; (d) a "solution" impedance in series with two parallel "surface" impedances. https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781315274263/1022c1cb-68c4-4170-89e2-a38cf9102755/content/fig5_1.tif"/>