Four-electrode cell

In the first step, when the electrodes are in the upper position, the current I is applied through the electrodes a and b, while the voltage V across the electrodes c and d is measured. Then the electrode function is switched by 90 thus the current J2 flows through electrodes b and c and voltage V2 is read from electrodes d and a. From the values 

SO obtained the first and second electrode orientation apparent resistances R = V1//1 and R2 = V2//2, respectively, are calculated. From these two resistances, the average resistance in the upper position R y = R + R i)l l is calculated. In the second step, the electrodes are immersed deeper by A IF and a similar process as in the first step is repeated, resulting in the value /fu- By this arrangement actually only the resistance of the disk thickness A IF is measured. An extensive mathematical analysis of the method was given by Ohta et al. (1981), which results in the following conductivity equation 

As it follows from Eq. (8.70), only the distance in the electrodes depth of immersion must be known. When the temperature of the measured liquid is changed, the change in the surface level (new depths of immersions) need not be measured. Ohta et al. (1981) also discussed the role of different parameters, i.e. the ratio between the electrode diameter and their distances, the material of the cmcible, electrode displacement from the center of the cmcible, the depth of measured liquid, immersion depth, the frequency of the measuring current, etc. 

This method is rather cumbersome and time consuming when performed manually, as it is necessary to measure eight current/voltage data and change precisely the electrode immersion, to obtain a single conductivity value. In order to eliminate this disadvantage, a computerized device is needed. 

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In order to determine the net current flowing, i, as a function of q (and in some cases time) it is generally necessary to work with three or four electrode cells where the electrode of interest (the working electrode) carries current into the cell and a second electrode (the auxiliary or subsidiary electrode) carries the current out of the cell. The third electrode is the reference electrode although in cases where one of the phases is not a metal a second reference electrode is required (four electrode cell). 

The overpotential is directly determined by measuring the potential difference between the working and reference electrodes in an arrangement such as that of Fig. 10.9. (In a four electrode cell the potential difference must be measured between the two reference electrodes.) Frequently a potentiostat is used to impose a known overpotential and the current flowing in the cell is measured as a function of ij. 

In general it will be necessary to measure via impedance measurements using a four electrode cell. A schematic diagram of the cell which would be used for such measurements is shown in Fig. 10.15. The expected behaviour will be as described in Eqn (10.3) except that Warburg impedances can arise from either or both phases. An example of an impedance spectrum of the H2O/PVC interface is shown in Fig. 10.16. The application of a constant overpotential will, in general, lead to a slowly decaying current with time due to the concentration changes which occur in both phases, so that steady state current potential measurements will be of limited use. 

The conductivity of a solution is measured using an AC bridge with a two-elec-trode conductance cell on one arm (Fig. 5.40(a)) a balance is sought, manually or automatically, by adjusting the variable resistance and capacitance in another arm of the bridge. Usually AC voltage of a few volts and 1 kHz is applied to the cell. The impedance caused by the double-layer capacity at the electrodes does not affect the measured values of conductivity. In some cases, the conductance is measured with a four-electrode cell, as shown in Fig. 5.40(b). For practical methods of measurement, see the reviews in Ref. [25],... 

Fig. 5.40 Circuits for conductivity measurements with two-electrode cell (a) and four-electrode cell (b). In (a), S AC voltage source D detector I, II, III bridge elements. In (b), S constant-current source POT potentiometer Rs variable resistor C and C electrodes for current flow P and P electrodes for voltage measurement.

Figure 6.4. Detailed diagram of hardware configuration for post-column addition of SPR. (1 = Conductivity detector Waters 431 detector, four electrode cell design 2 = waste line 4 x 0.009 in. stainless connected to 431 + 24 X 1/16 X 0.060 in PTFE tubing 3 = tee to 431 15 x 1/16 x 0.010 in PTFE to 431 inlet 4 = column to lee shortest 1/16 x 0.010 in PTFE from column to tee 5 = tee Unmount tee from check valve block for shortest path length 6 = analytical colunm Waters 1C PAK A or 1C PAK A HR 7 = check valve to tee 2 x 1/8 in o.d. PTFE 8 = check valve 9 = polisher column to check valve 3 x 1/8 in o.d. PTFE 10 = polisher column 8 x 25 mm containing AGI x 8, 200 mesh 11 = reservoir to polisher column 12 x 1/8 in. o.d. PTFE 12 = air supply minimum of 90 p.s.i. compressed air supply 13 = reservoir for SPR reconfigure with outlet on left side. From Ret [9] with permission.)...

In an experimental setup as described above, there is usually a choice between a two-, three-, or four-electrode cell. These options determine which part of the electrochemical cell is characterized by the impedance measurements. 

The SECM is capable of quantitative determination of ET rates at the ITIES in a straightforward manner as long as appropriate account of possible coupled ion transfer processes is taken [82]. The study of ET kinetics at the liquid-liquid interface is an area of topical interest [134-138]. Conventional electrochemical measurements employ a four-electrode cell to drive the ET reaction, however, in the SECM studies, the electrodes are all in one phase and the problems associated with double layer capacitance, iR drop, and restricted potential window are therefore avoided. In particular, the study of ET rates at high driving force is possible. 

Conductivity detector Waters 431 detector, four electrode cell design 2 = waste line ... 

In order for STM to work with electrochemical interfaces, the instrument is equipped with a bipotentiostat for independent potential control of both the tip and surface with respect to a chosen reference electrode in a four-electrode cell, so that both electrodes are under well-defined electrochemical conditions. Furthermore, since Faradaic current could also flow through the tip electrode, this would be superimposed on the tunneling current and interfere severely with the detection of tunneling current, and even destabilize the geometry of the tip apex. It is therefore essential to insulate the side wall of the metallic tip electrode to suppress the Faradaic current while leaving a small tip apex for tunneling [7,8]. 

Figure 17.3.8 Schematic diagram of the four-electrode cell.

Fig. 4. Scheme of the four-electrode cells with two platinum counter electrodes (CE 1 and CE 2) and two reference electrodes (RE 1 and RE 2) A - connection to a microsyringe for the adjustment of the interface, B - glass barrier with a round hole, C - liquid/liquid interface, D - insulated copper wire [31]... 

FIGURE 8-1 Example of impedance measurement setup with a frequency-response analyzer and a four-electrode cell... 

FIGURE 8-13 Total impedance load measurement error in four-electrode cells using shielded and no-shielded cables... 

Four-electrode cell impedance measurements are reliable if the sample impedance is sufficiently small with respect to the input impedance... 

Conductivity sensors may be either the contacting tsrpe or the induction type. The contacting type may utilize either 2 or 4 electrodes. The four electrode cell permits measurement of higher ranges (up to 200 mS), and the correction of polarization effects due to deposits forming on the electrodes. Two electrode types are suitable for lower conductivity ranges and are easier to insert into a flow through cell but cannot compensate for polarization. Four electrode types are incorporated into the system in a similar manner to pH probes. 

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