Alternating current potential drop

Alternatively, one may control the electrode potential and monitor the current. This potentiodynamic approach is relatively easy to accomplish by use of a constant-voltage source if the counterelectrode also functions as the reference electrode. As indicated in the previous section, this may lead to various undesirable effects if a sizable ohmic potential drop exists between the electrodes, or if the overpotential of the counterelectrode is strongly dependent on current. The potential of the working electrode can be controlled instead with respect to a separate reference electrode by using a potentiostat. The electrode potential may be varied in small increments or continuously. It is also possible to impose the limiting-current condition instantaneously by applying a potential step. 

The emf of a cell is best determined by measurements with a potentiometer, since this method gives a very close approach to reversible operation of the cell. The cell potential is opposed by a potential drop across the slide wire of the potentiometer, and at balance only very small currents are drawn from the cell (depending on the sensitivity of the nnU detector used and the fineness of control possible in adjusting the slide wire). Alternatively a high-resolution digital voltmeter with a large internal impedance can be used. Potentiometers and digital voltmeters are described and compared in Chapter XVI. 

In agreement with the accepted molecular orbital treatment (42), which predicts that the total electron count around the two metals wUl not exceed 34, none of the complexes 19 and 20 can be reduced beyond the 34-electron state (in the potential range available at the dropping mercury electrode in nonaqueous solvents) [20] (MM = Ni2) has 34 valence electrons. The heterogeneous charge-transfer rates, measured by alternating current polarography, confirm that the electron-transfer reactions of these compounds are very rapid (41). 

The mean surface concentrations enforced by depend on many factors (a) the way in which is varied (b) whether or not there is periodic renewal of the diffusion layer (c) the applicable current-potential characteristic and (d) homogeneous or heterogeneous chemical complications associated with the overall electrode reaction. For example, one could vary sequential potentiostatic manner with periodic renewal of the diffusion layer, as in sampled-current voltammetry. This is the technique that is actually used in ac polarography, which features a DME and effectively constant during the lifetime of each drop. Alternatively one could use a stationary electrode and a fairly fast sweep without renewal of the diffusion layer. Both techniques have been developed and are considered below. The effects of different kinds of charge-transfer kinetics will also be examined here, but the effects of homogeneous complications are deferred to Chapter... 

Sacrificial anodes are normally electrically connected to the structure to be protected through welding or bolting. One way to monitor the current output from selected anodes is to connect the anode through a resistor with known value to the structure. By measuring the potential drop across the known resistor, the current from the anode can be calculated. This principle is shown schematically in Figure 19.5. The use of a Swain meter is an alternative if all the anodes are electrically connected without any resistor. 

Fig. 30. Potential-time profiles (A) of the alternating currents, (B) of the square-wave polarographic techniques, both on a negative ramp. Parameters the drop time, A and dEsw amplitudes of the potential waves. The relation z)Esw = dEAc holds.

The AC polarographic wave can be distorted, if the resistance of the electrolytic cell is not minimized. The product of the solution resistance, Rs and the alternating current, results in the potential drop Iac s which causes the decrease of the amplitude of the applied AC potential and affects the phase angle between E c and Iac- The consequences are the broadening and the lowering of the AC peaks. 

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