Single-Electrode Measurement

Since it is not possible to measure a single electrode potential, one electrode system must be taken as a standard and all others measured relative to it. By international agreement the hydrogen electrode has been chosen as the reference ... 

Glass membrane pH electrodes are often available in a combination form that includes both the indicator and the reference electrode. The use of a single electrode greatly simplifies the measurement of pH. An example of a typical combination electrode is shown in Figure 11.12. 

In galvanic cells it is only possible to determine the potential difference as a voltage between two half-cells, but not the absolute potential of the single electrode. To measure the potential difference it has to be ensured that an electrochemical equilibrium exists at the phase boundaries, e.g., at the electrode/electrolyte interface. At the least it is required that there is no flux of current in the external and internal circuits. 

A problem with compiling a list of standard potentials is that we know only the overall emf of the cell, not the contribution of a single electrode. A voltmeter placed between the two electrodes of a galvanic cell measures the difference of their potentials, not the individual values. To provide numerical values for individual standard potentials, we arbitrarily set the standard potential of one particular electrode, the hydrogen electrode, equal to zero at all temperatures ... 

Thus, the first chapter touches on an aspect of electrochemistry for which the author has become justly well known application of the Wagner and Traud theorem of 1938 according to which electrochemical systems may function on a single electrode. In the next chapter, the article by Koczorowski treats a seldom-visited but truly fundamental area, that of voltaic measurements at liquid interfaces. 

Figure 12. Schematic representation of the setup for single particle measurements by electrostatic trapping (ET). Pt denotes two freestanding Pt electrodes (dashed region). A ligand-stabilized Pd cluster is polarized by the applied voltage and attracted to the gap between the Pt electrodes. (Reprinted with permission from Ref. [29], 1997, American Institute of Physics.)...

Walker, J. L. Single Cell Measurement with Ion-Selective Electrodes, in Medical and Biological Applications of Electrochemical Devices (Koryta, J., ed.) New York, Wiley, 1980, p. 109... 

If the unknown cell in the Cu-Zn cell is connected to the circuit, the emf measured is the combined potentials of two single electrode potentials for the two metals (zinc and copper) making up the cell, and it is impossible to state from the value of the emf measured what proportion is due either to the zinc, or to the copper. 

In many limiting-current measurements the expected current distribution is only moderately nonuniform, and a single unsegmented electrode will yield well-defined limiting-current plateaus. The various techniques by which the limiting current at a single electrode can be generated are discussed in the next section. 

In the case of a single electrode, however, the decrease of its dimensions requires the measurement of very low currents. To overcome this problem it is convenient to use microelectrode arrays [136, 137], Despite the fact that in such arrays microelectrodes are electronically connected to each other, analytical properties of such assemblies are advantageous over those of a conventional macro-electrode [138, 139],... 

He J, Chen F, Liddell PA, Andreasson J, Straight SD, Gust D, Moore TA, Moore AL, Li J, Sankey OF, Lindsay SM (2005) Switching of a photochromic molecule on gold electrodes single-molecule measurements. Nanotechnology 16(6) 695... 

It is impossible to determine the potential of a single electrode only its potential relative to another electrode can be measured. 

The charge transfer kinetics of azobenzene at the mercury electrode is slower than that of methylene blue, thus the frequency interval provided by modem instra-mentation (10 < //Hz < 2000) allows variation of the electrochemical reversibility of the electrode reaction over a wide range [79]. The quasireversible maxima measured by the reduction of azobenzene in media at different pH ate shown in Fig. 2.47 in the previous Sect. 2.5.1. The position of the quasireversible maximum depends on pH hence the estimated standard rate constant obeys the following dependence A sur = (62-12pH) S- for pH < 4. These results confirm the quasite-versible maximum can be experimentally observed for a single electrode reaction by varying the frequency, as predicted by analysis in Fig. 2.75. 

J. L. Walker, Single cell measurement with the lon-selective electrodes. Chapter 4... 

Surfactant Activity in Micellar Systems. The activities or concentrations of individual surfactant monomers in equilibrium with mixed micelles are the most important quantities predicted by micellar thermodynamic models. These variables often dictate practical performance of surfactant solutions. The monomer concentrations in mixed micellar systems have been measured by ultraf i Itration (I.), dialysis (2), a combination of conductivity and specific ion electrode measurements (3), a method using surface tension of mixtures at and above the CMC <4), gel filtration (5), conductivity (6), specific ion electrode measurements (7), NMR <8), chromatograph c separation of surfactants with a hydrophilic substrate (9> and by application of the Bibbs-Duhem equation to CMC data (iO). Surfactant specific electrodes have been used to measure anionic surfactant activities in single surfactant systems (11.12) and might be useful in mixed systems. ... 

Although the cell potential of a single electrode cannot be measured, two different electrodes can be connected to each other to measure the flow of electrons that occurs between them. Electrode potential is the difference in electrical potential between the metal strip and the solution in a half-cell. The larger the electrode potential, the greater the energy required to move an electron from the metal strip to the solution. 

Combination Electrode. Combining the sensing and reference electrode as one single electrode provides convenience during routine testing. The rugged singlebody construction is beneficial in routine analysis and helps in measurement of the pH of small sample volumes. It also eliminates the potential problem of a... 

What Happens When One Tries to Measure the Potential Difference Across a Single Electrode/Electrolyte Interface ... 

The discussion so far can be summarized as follows The value of the potential difference across a single electrode/electrolyte interface cannot be measured with potential measuring instruments. The sum of the potential differences across at least... 

Figure 15-14 Solid colored circles show the drift in apparent pH of a low-conductivity industrial water supply measured continuously by a single electrode. Individual measurements with a freshly calibrated electrode (black circles) demonstrate that the pH is not drifting. Drift is attributed to slow clogging of the electrode s porous plug with AgCI(s). When a cation-exchange resin was placed inside the reference electrode near the porous plug, Ag(l) was bound by the resin and did not precipitate. This electrode gave the drift-free, continuous reading shown by open diamonds. [From S. Ho, H. Hachlya. K. Baba. Y. Asano. and H. Wada, Improvement of the Ag I AgCt Reference Electrode and Its Application to pH Measurement," talonta 1995,42.1685.]...

A. J. Cunningham and J. B. Justice, Jr., Approaches to Voltammetric and Chromatographic Monitoring of Neurochemicals in Vivo, J. Chem. Ed. 1987, 64, A34. Another Nafion-coated electrode can detect 10 10 mol of the neurotransmitter nitric oxide within a single cell [T. Malinski and Z. Taha, Nitric Oxide Release from a Single Cell Measured in Situ by a Porphyrinic-Based Microsensor, Nature 1992, 358, 676]. 

The final cell design, as in semi-infinite electrochemistry, depends very much on the goals of the experimenter. For example, when one is interested in single-electrode coulometry for n value, concentration, or spectral measurements, the cell requirements are minimal. Potential scan experiments (voltammetry, potential scan coulometry, steady-state voltammetry) or experiments... 

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