Zero-current measurement

To note that potentiometric titrations are always zero-current measurements. 

Although electron-transfer reactions occur, the electrode in no way supplies or conducts away the electrons - it is merely a probe of the potentials of the constituent redox couples in solution. Accordingly, potentiometric titrations are always zero-current measurements. 

In the case of zero-current measurements the electrical potential difference between the two end phases of the cell is measured under open circuit conditions. Information about thermodynamic quantities of reaction systems, for example, about chemical potentials, activities, or partial pressures, is obtained from such measurements. This was already described. 

Almost all studies regarding direct hydrocarbon SOFCs show comparatively poor performance (lower OCP and higher polarization resistance) with hydrocarbon fuels when compared to H2 fuel, Fig. 3.2. Since most of these tests are performed by switching fuel on the same cell, the drop in performance must be linked to the anode. It is possible that the increased polarization resistance may be due to lower diffusivity of the hydrocarbon fuels, but the electrodes are typically highly porous and the current density per unit area is relatively low. In addition, the oxidation of 1 mole of hydrocarbon fuel yields a significantly greater number of electrons than 1 mole of H2 fuel (H2, CH4, and C4H10 total oxidation yield 2, 8, and 26 moles of electrons, respectively). Furthermore, the cell OCP is an equilibrium, zero current, measurement and is therefore not directly influenced by gas diffusivity. Therefore, it is unlikely that gas diffusivity limits the performance for pure fuels at low conversion. The conclusion must then be that the anode electrocatalytic activity toward hydrocarbon oxidation is the primary factor in reduced SOFC performance. 

Figure 2.14. Passive bridge zero-current measuring system.

Zero-Current Measurements for the Determination of Heavy Metals... 

Table 6.1 includes also methods or their sequences that cannot be classified according to the schemes presented above. There are two examples on zero-current measurements by (equilibrium) potentiometry the use of classic F -ISE, when the... 

Potentiometers Measuring the potential of an electrochemical cell under conditions of zero current is accomplished using a potentiometer. A schematic diagram of a manual potentiometer is shown in Figure 11.2. The current in the upper half of the circuit is... 

According to the definition, a passive technique is one for which no appHed signal is required to measure a response that is analytically usehil. Only the potential (the equiHbrium potential) corresponding to zero current is measured. Because no current flows, the auxiHary electrode is no longer needed. The two-electrode system, where the working electrode may or not be an ion-selective electrode, suffices. 

After measuring the zero profile, AU measurements are carried out with the injection of a cathodic protection current. In contrast to the zero profile measurements, the distance between the individual measurements is 25 to 50 m. Shorter distances between the measuring points are used only at depths where there are unusual AU profiles. Current should be injected at at least three different levels. The protection current density of about 12 mA m obtained from experience should be the basis for determining the maximum required protection current. As shown by the results in Fig. 18-3, the AU profiles are greater with increasing protection current. The action of local cells is suppressed when the AU values no longer decrease in the direction of the well head. This is the case in Fig. 18-3 with a protection current I = 4A. 

The algebraic sum of the three currents measured from all three sections have to be theoretically always zero however, minute differences have been found in the separate tests. 

In making measurements of current flowing within a structure, it is extremely important that additional resistance, as for example a shunt, is not introduced into the circuit, as otherwise erroneous results will be obtained. One method is to use a tong test meter. Such instruments are, however, not particularly accurate, especially at low currents, and are obviously jmpracticablein thecaseof, say, a 750 mm diameter pipeline. A far moreaccurate method and onethat can beapplied to ail structures, isthe zero-resistance ammeter or, as it is sometimes called, the zero-current ammeter method. The basic circuit of such an instrument is shown in Fig. 10.47. 

Galvanic current Measurement of the galvanic current between two different metals can be easily measured using a zero resistant ammeter ". This method can have specific application, e.g. to provide a signal indicating failure of a protective coating in a process vessel. Commercial probes are available for industrial monitoring. 

Almost all kinetic investigations on azo coupling reactions have been made using spectrophotometric methods in very dilute solutions. Uelich et al. (1990) introduced the method of direct injective enthalpimetry for such kinetic measurements. This method is based on the analysis of the zero-current potential-time curves obtained by the use of a gold indicator electrode with a surface which is periodically restored (Dlask, 1984). The method can be used for reactions in high (industrial) concentrations. 

Potentiometry (discussed in Chapter 5), which is of great practical importance, is a static (zero current) technique in which the information about the sample composition is obtained from measurement of the potential established across a membrane. Different types of membrane materials, possessing different ion-recognition processes, have been developed to impart high selectivity. The resulting potentiometric probes have thus been widely used for several decades for direct monitoring of ionic species such as protons or calcium, fluoride, and potassium ions in complex samples. 

Controlled-potential (potentiostatic) techniques deal with the study of charge-transfer processes at the electrode-solution interface, and are based on dynamic (no zero current) situations. Here, the electrode potential is being used to derive an electron-transfer reaction and the resultant current is measured. The role of the potential is analogous to that of the wavelength in optical measurements. Such a controllable parameter can be viewed as electron pressure, which forces the chemical species to gain or lose an electron (reduction or oxidation, respectively). 

However, in this case the EMF measured will be distorted by another effect [i.e., the variation of electrostatic potential within a given conductor, which is caused by a temperature gradient in the conductor (the Thomson effect, 1856)]. Potential gradients will arise even at zero current, in both the electrolyte (between points and Aj)... 

For this reason and following a suggestion of M. I. Temkin (1948), another conventional parameter is used in electrochemistry [i.e., the real activation energy described by Eq. (14.2)], not at constant potential but at constant polarization of the electrode. These conditions are readily realized in the measurements (an electrode at zero current and the working electrode can be kept at the same temperature), and the real activation energy can be measured. 

The procedure is as follows. In switch position 1 and while repeatedly depressing tap key K, the variable resistor R, is adjusted once for each measurement to zero current through galvanometer G, so that the emf of the standard cell C8t (Weston 1.01832 V) becomes accurately compensated over the constant resistor Rj. Next, in switch position 2 the unknown emf of cell Cx is... 

The H2S removal was also verified by effluent gas analysis as a function of applied current. Fig. 35 shows representative H2S outlet concentration as a function of current density. It is seen that the cathode outlet can be brought to near zero H2S content with sufficient applied current. Measurements at the anode outlet showed very little... 

The thin-layer configuration and its associated diffusion problems means that it is possible to oxidise (or reduce) all of the electroactive species in the thin layer before they can be replenished to any marked degree. Consider, for example, the 0"+/0 couple, with a standard redox potential well within the "electrochemical window of the solvent, so that the current in the absence of the couple is small and can easily be accounted for. With the electrode pushed against the window the potential is stepped cathodic enough to ensure the rapid reduction of the 0" + and the current measured as a function of time, the concentration such that the time for the current to reach zero, or a steady residual value, is small. If the area under the I ft curve is A ampere seconds, then the charge passed Q = A coulombs. Thus, the number of moles of 0"+ reduced, N0, is given by ... 

Potentiometric measurements are usually performed under zero-current conditions in a galvanic cell of the type ... 

The electrochemical detection of pH can be carried out by voltammetry (amper-ometry) or potentiometry. Voltammetry is the measurement of the current potential relationship in an electrochemical cell. In voltammetry, the potential is applied to the electrochemical cell to force electrochemical reactions at the electrode-electrolyte interface. In potentiometry, the potential is measured between a pH electrode and a reference electrode of an electrochemical cell in response to the activity of an electrolyte in a solution under the condition of zero current. Since no current passes through the cell while the potential is measured, potentiometry is an equilibrium method. 

Measurement of the potential of a galvanic cell, usually at zero current cell potential governed by the potential of an indicator electrode which responds to changes in the activity of the species of interest. 

Potentiometry is the most widely used electroanalytical technique. It involves the measurement of the potential of a galvanic cell, usually under conditions of zero current, for which purpose potentiometers are used. Measurements may be direct whereby the response of samples and standards are compared, or the change in cell potential during a titration can be monitored. 

Electrochemical measurements at equilibrium are made at zero current. 

We have already seen from Faraday s laws how a zero current implies that no redox chemistry occurs. Accordingly, we stipulate that the meter must draw absolutely no current if we want to measure the battery s emf at equilibrium. Henceforth, we will assume that all values of emf were determined at zero current. 

Care The output voltage of a battery is only an emf when measured at zero current, i.e. when not operating the watch. 

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