The impedance concept

3 ALTERNATING CURRENT PERTURBATION. FIRST-ORDER RESPONSE 2.3.1 The impedance concept 

The use of sinusoidal a.c. perturbation, either potential or current, is probably the most popular and penetrating method of studying processes occurring at the electrochemical interface. Its methodology has been developed to an outstanding level, accounting for a variety of phenomena [12, 25, 49, 53]. 

A potentiostatic performance, e.g. with the circuit of Fig. 9, is preferable, because then it is possible to superimpose a small-amplitude a.c. voltage on the mean d.c. potential fed to the potentiostat. Consequently, an a.c. current will flow through the cell. In the first-order approximation its current density, Ay will be of the form 

The network analyzer described above can measure both the impedance and its reciprocal, the admittance, Y = V, 6, or in the complex notation, Y = Y + iY  

A phase-selective a.c. voltmeter will produce data in terms of either F and 6 or Y and Y . Using the relationships above, it is always possible to derive the values of Y and Y , or Z and Z , that are the more suitable data to be subjected to further analysis [49, 53]. 

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The impedance concept is developed here following the work of Tribollet and Newman [30], which means that the system response is linear and corresponds to low amplitude modulation (AQ-4Q). 

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