Dummy cell

Figure 6.16 Dummy-cell test circuits for three-electrode instrumentation.

A few comments are in order on the probable validity of conclusions based on this equivalent circuit to real cells. Quite simply stated, real cells that are properly designed will have the same properties as dummy cells of the same values of Rs, Ru, and Cdl. Important design features of a cell are (1) equal resistance between all points on the surface of the working electrode and the auxiliary electrode (2) low-impedance reference electrode and (3) low stray capacitance between electrodes, between leads, and to shields. Spherical symmetry is a good, but somewhat inconvenient, method of meeting the first requirement a parallel arrangement also works with planar electrodes. At the very... 

It is important to realize that a potentiostat s rise time using a dummy cell cannot be used as evidence for accepting a measurement made on a real sample. 

Make sure the cell enable switch is turned off. Connect the cell cables to the cell green lead to the working electrode, red lead to the counter electrode, and the reference to the white pin jack. If a dummy cell is being connected, attach the leads as indicated in step 3 below. 

There are four polarization resistance experiments that need to be run. Two are experiments on dummy cells, and two are experiments on real cells. For the dummy cell experiments, use the dummy cell labeled Polarization Resistance. For the actual cell experiments, use the 303SS immersed in the acidified chloride solution. Repeat steps 2a through 2f for each experiment. 

Figure 16 Wiring diagram for dummy cell with reference electrode (RE) at point B.

Figure 23 Linear polarization curves from the dummy cell showing the influence of scan rate.

Figure 25 EIS results of dummy cell simulating two different solution resistances. At point A no solution resistance is simulated. At point B a solution resistance of 4700 2 is simulated. 

Figure 15.6.1 Simple dummy cells, a) For a nonfaradaic system, where is the double-layer capacitance and / u + Rn is the solution resistance, with / u being uncompensated, (b) For a system passing faradaic currents through Rf, as well as nonfaradaic ones through C. ...

With the electrochemical instrument turned off, disconnect the cell and replace it with a 10 kfl resistor (dummy cell), with the reference and counter electrode leads connected together on one side of the resistor and the working electrode lead connected to the other side. Scan from +0.5 to -0.5 V with the sensitivity of the instrument set for currents of about 100 /xA, The resulting scan should be a straight line that intersects the origin with maximum currents of 50 fxA. 

Derive a formula describing current flow in the dummy cell shown in Figure 15.6.1a on application of a step in gj-ef from 0 V to an arbitrary value j-ef Derive equation 15.6.1 from your result. 

The next step, after all experimental parameters have been given their correct values, is usually a calibration. A dummy cell is used, consisting of electronic components that imitate the behaviour of the real cell as closely as possible. The simplest one, which also is in many cases a completely adequate one, is shown in Fig.5. It consists of a capacitance (double layer capacitance) in parallel with a resistance (charge transfer resistance), and then, in series with this circuit, another resistance (solution resistance). The admittance of the dummy cell is recorded in an ordinary experiment and the transfer function, T(u), of the instrument is set equal to the ratio of the calculated, 0( )5 to the measured, ym( )) admittance of the dummy cell i.e. 

Fig.5. Dummy cell with three components Solution resistance Rn, double layer capacitance Cdi, and the faradaic impedance Z, which in many cases is represented by a resistance, the charge transfer resistance Ret ...

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