Electrochemical Cell Conventions

Table 2.2 Summary of electrochemical cell conventions, terminology, and relationships...

A special example of electrical work occurs when work is done on an electrochemical cell or by such a cell on the surroundings -w in the convention of this article). Themiodynamics applies to such a cell when it is at equilibrium with its surroundings, i.e. when the electrical potential (electromotive force emi) of the cell is... 

In order to describe any electrochemical cell a convention is required for writing down the cells, such as the concentration cell described above. This convention should establish clearly where the boundaries between the different phases exist and, also, what the overall cell reaction is. It is now standard to use vertical lines to delineate phase boundaries, such as those between a solid and a liquid or between two innniscible liquids. The junction between two miscible liquids, which might be maintained by the use of a porous glass frit, is represented by a single vertical dashed line, j, and two dashed lines, jj, are used to indicate two liquid phases... 

Potentiometric measurements are made using a potentiometer to determine the difference in potential between a working or, indicator, electrode and a counter electrode (see Figure 11.2). Since no significant current flows in potentiometry, the role of the counter electrode is reduced to that of supplying a reference potential thus, the counter electrode is usually called the reference electrode. In this section we introduce the conventions used in describing potentiometric electrochemical cells and the relationship between the measured potential and concentration. 

Also, by convention, potentiometric electrochemical cells are defined such that the indicator electrode is the cathode (right half-cell) and the reference electrode is the anode (left half-cell). 

Potentiometric electrochemical cells are constructed such that one of the half-cells provides a known reference potential, and the potential of the other half-cell indicates the analyte s concentration. By convention, the reference electrode is taken to be the anode thus, the shorthand notation for a potentiometric electrochemical cell is... 

Scale of Operation Coulometric methods of analysis can be used to analyze small absolute amounts of analyte. In controlled-current coulometry, for example, the moles of analyte consumed during an exhaustive electrolysis is given by equation 11.32. An electrolysis carried out with a constant current of 100 pA for 100 s, therefore, consumes only 1 X 10 mol of analyte if = 1. For an analyte with a molecular weight of 100 g/mol, 1 X 10 mol corresponds to only 10 pg. The concentration of analyte in the electrochemical cell, however, must be sufficient to allow an accurate determination of the end point. When using visual end points, coulometric titrations require solution concentrations greater than 10 M and, as with conventional titrations, are limited to major and minor analytes. A coulometric titration to a preset potentiometric end point is feasible even with solution concentrations of 10 M, making possible the analysis of trace analytes. 

Scale of Operation Voltammetry is routinely used to analyze samples at the parts-per-million level and, in some cases, can be used to detect analytes at the parts-per-billion or parts-per-trillion level. Most analyses are carried out in conventional electrochemical cells using macro samples however, microcells are available that require as little as 50 pL of sample. Microelectrodes, with diameters as small as 2 pm, allow voltammetric measurements to be made on even smaller samples. For example, the concentration of glucose in 200-pm pond snail neurons has been successfully monitored using a 2-pm amperometric glucose electrode. ... 

Figure 1. Sketch of an electrochemical cell whose equilibrium (open circuit) potential difference is AE. (a) Conventional configuration and (b) short-circuited configuration with an air gap. M and R are the electrodes, S is the solvent (electrolyte solution). Cu indicates the cables connecting the two electrodes to a measuring instrument (or to each other).

In this method the creation of defects is achieved by the application of ultrashort (10 ns) voltage pulses to the tip of an electrochemical STM arrangement. The electrochemical cell composed of the tip and the sample within a nanometer distance is small enough that the double layers may be polarized within nanoseconds. On applying positive pulses to the tip, the electrochemical oxidation reaction of the surface is driven far from equilibrium. This leads to local confinement of the reactions and to the formation of nanostructures. For every pufse applied, just one hole is created directly under the tip. This overcomes the restrictions of conventional electrochemistry (without the ultrashort pulses), where the formation of nanostructures is not possible. The holes generated in this way can then be filled with a metal such as Cu by... 

It is important to discuss cell notation and conventions. Instead of drawing a complete diagram to present electrochemical cells, it is convenient to specify a cell in line formula form or, as may be said, shorthand form. The Cu-Zn cell is thus presented as ... 

The important question arises of the actual precision of pH measurement in analytical control. In this connection, it has become common practice to standardize pH determinations, on standard buffer solutions with pH regions where the pH of the solution under test is to be expected. As currently commercially available pH meters, pH electrodes and buffer solutions are of outstanding quality, the reliability of the pH measurement becomes shifted to the performance of the measuring electrochemical cell here as first principle the same cell should be used for the test solution and the standard solution, so that according to the Bates-Guggenheim convention... 

Morita and co-workers (60) have constructed a STM that employs a unique 3D scanner and 3D positioner that is constructed from several piezoelectric cubes. This microscope was subsequently equipped with an electrochemical cell that allows disconnection of the tip and conventional 3 electrode voltammetry to be performed (61). Itaya et. al. have gained similar capabilities by modifying their aforementioned STM (Itaya, K. Higaki, K. Sugawara, S. Chem. Lett.. in press). 

Electropolymerization was carried out in acetonitrile/0.1M TBAP solution in a conventional three compartment electrochemical cell according to previously described procedures. (29-31)... 

The in situ Mossbauer experiments were conducted with 90% - Fe enriched 9 1 Ni/Fe oxyhydroxide films which were deposited in the fashion described above onto a gold on Melinex support(12) in a conventional electrochemical cell. Prior to their transfer into the in situ Mossbauer cell, the electrodes were cycled twice between 0 and 0.6 V vs. Hg/HgO,OH" in 1 M KOH. Two such films were used in the actual Mossbauer measurements in order to reduce the counting time. The in situ Mossbauer cell involved in these experiments was previously described. 

To measure the electrode potential of a test electrodes, M, we usually use an electrochemical cell consisting of test electrode M and reference electrode both of which are coimected by a metal lead of A and A" of the same metallic conductor to a potentiometer outside the cell as shown in Fig. 4-23. The difference in the electrode potential, E, measured between the test electrode and the reference electrode, conventionally called the electromotive force, equals the difference in the Fermi level of electrons between the two electrodes E = - 8j(M) - EjtM )... 

As shown in Fig. 6-3, it is also in the same TUPAC convention that a positive electric charge flows from the left hand electrode through the electrolyte to the right hand electrode, as the cell reaction proceeds in the direction as written in Eqn. 6-3. This defines the sign of the electromotive force of electrochemical cells. 

The electromotive force of an electrochemical cell is the difference in electrode potential between the two electrodes in the cell. According to the TUPAC convention, the electromotive force is the potential of the right hand electrode referred to the potential of the left hand electrode. We consider, for example, a hydrogen-oxygen cell shown in Fig. 6—4 the cell reaction is given by Eqn. 6-1 and the cell diagram is given by Eqn. 6-5 ... 

Interestingly, electrochemical processes are also evident in certain two-electrode STM experiments performed in air. It is well known that water is absorbed on surfaces exposed to humid environments [48,49]. When such circumstances arise in combination with certain bias conditions, me conventional two-electrode STM exhibits some of the characteristics of a two-electrode electrochemical cell as shown in Fig. 4 [50-53]. This scheme has been used for modifying surfaces and building devices, as will be described in me last section of mis chapter. In a similar vein, it has been suggested mat a two-electrode STM may be used to perform high-resolution SECM for certain systems mat include insulating substrates such as mica [50]. 

Cells. An electrochemical cell comprises two (or more) redox couples, with the energy of each being monitored by an electrode. (As we have seen already, the electrode may itself be one part of the redox couple.) By convention, we say that the more positive electrode is the right-hand electrode, while the left-hand electrode is the more negative. The difference in potential between the right-hand and left-hand electrodes is called the cell emf ... 

SHE, standard hydrogen electrode The electrode used as a standard against which aU other half-cell potentials are measured. The following reaction occurs at the platinum electrode when immersed in an acidic solution and cormected to the other half of an electrochemical cell 2H (aq) -H 2e —> H2(g). The half- cell potential of this reaction at 25°C, 1 atm and 1 m concentrations of aU solutes is agreed, by convention, to be OV... 

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