Series of Standard Potentials

In the same way as we found the standard electrode potential of the reaction 2e = Zn to be -0.76 V, we can determine potentials for any other redox process. A series of such standard potentials is shown in Table 3.1. For example, the standard potential of the reaction Cu + 2e = Cu is 0.34 V. Thus, the reversible cell consisting of a standard Cu electrode and a standard Zn electrode has a cell voltage of E = 0.34-(-0.76)= 1.1 V. 

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For transmetallations with a metal (metallo-de-metallations, Scheme 10-95) arylmercury compounds are particularly suitable due to the position of mercury as a noble metal in the electrochemical series of standard potentials (for examples see Makarova, 1970). 

It can be seen from this equation that the greater the difference between the standard potentials of the two metals Mi and M2, the larger will be the equilibrium ratio of activities (or concentrations) of the respective ions. The greater the difference between the standard potentials, therefore, the more completely will one metal displace another from a solution of its ions. The metal with the more positive (oxidation) potential, as recorded in Table XLIX, will, in general, pass into solution and displace the metal with the less positive potential. The series of standard potentials, or electromotive series, as it is sometimes called, thus gives the order in which metals are able to displace each other from solution the further apart the metals are in the series the more completely will the higher one displace the lower one. It is not true, however, to say that a metal lower in the series will not displace one higher in the series some displacement must always occur until the required equilibrium is established, and the equilibrium amounts of both ions are present in the solution. 

Since we know the values for the silver-silver halide electrodes versus the standard hydrogen electrode (see above), we can easily set up a series of standard potentials vs. H2 assuming (H+ H2) = 0.0 V. Recently Scrosati et made measurements on double amalgam cells in pure methanol. Their results for the alkali metal chlorides and bromides are in good agreement with others but the iodide systems appear to involve some error since a value of --0.2993 V is obtained for of cell (Vc). Table 2.7.2 lists the standard potentials of the alkali-metal electrodes. 

A list of standard potentials of electrode reactions, organized by numerical value (Table 2.8), is referred to as series of standard potentials of eleelrode reactions or electromotive force series. A more complete set of such data can be found in reference [6],... 

Galvanic series are fundamentally different from the series of standard potentials of electrode reactions (Chapter 2), which is based entirely on thermodynamic quantities. In a galvanic series, the value of the potential and the relative positions of the metals in the series can vary depending on the environment. Furthermore, individual metals may appear twice in the same galvanic series, once each for the active and the passive states. For example, according to Table 7.7, stainless steel in the active state is less noble than copper, while for the same steel in the passive slate... 

Generally, the available cell voltage of electrochemical cells depends on the thermodynamics of the two electrode reactions in the prevailing electrolyte, hence the difference in the electrode potentials, and is confined, according to the Electrochemical Series of Standard Potentials, to a few volts. Cells with an aqueous electrolyte exhibit a limitation given by the stability window of water, namely 1.23 V at standard conditions. As stated above, the H2/O2 fuel cell allows practical open circuit voltages of around 1.0 V. At cell voltages above 1.23 V, typically around 1.5 V, decomposition of water into H2 and O2 occurs. 

From the measured electron-transfer equilibrium constant and the known standard potential for the reference D /D- couple it has been possible to determine E for the PhO /PhO- couple. The method, however, is non-trivial and does not lend itself to the rapid determination of standard potentials for a large series of related compounds. 

Calibration of ion-selective electrodes for use in quantitative analysis is usually done by preparing a series of standards as in most other instrumental analysis methods (see Chapter 7), since the measured potential is proportional to the logarithm of the concentration. The relationship is... 

From the values of standard potentials in electrochemical series... 

For dilute solutions, the Debeye-Huckel law (log 7 = —0.5zf/°5) indicates that 7 will be a constant for a given ionic strength /. Therefore, the same quantity of inert electrolyte, called the support electrolyte, must be added to the sample and to the series of standards to increase the concentration of external ions and stabilise the ionic strength. This addition of ISAB (Ionic Strength Adjustment Buffer) is intended to limit variation in 7. Under these conditions, the measured difference in potential only depends on the concentration of the ion to be analysed and is given by equation (18.3). 

The experimental protocol used to conduct measurements is based on the following principle a series of standard solutions is prepared by successive dilution of a stock solution and an excess but constant volume of buffer (ISAB or TISAB) is added at each step. Sample solutions are prepared in the same fashion. For each of the standards, the potential across the electrodes is measured and a semi-logarithmic calibration curve E — f(q) is obtained (Fig. 18.6). Using this curve and the potential difference obtained for each of the sample solutions, the concentration of species i can be obtained. 

A pH scale could be defined in terms of a single standard buffer. Every practical pH reading, however, involves a liquid-junction potential, and the variation in this potential between the readings of Cell (3-9) for the unknown and the standard is tacitly included in Equation (3-10). The liquid-junction potential is essentially constant for solutions of intermediate pH values, say 3 to 11, but beyond these limits it varies considerably because the concentrations of the imusually mobile H" " or OH ions become appreciable. Although this variation does not affect the definition of pH by (3-10), the measured value of pH deviates appreciably from the known best values of —log %+ for solutions of high acidity or alkalinity. For this reason, a series of standards covering a range of values of pH, has been adopted. For an experimental... 

Connect the ion-selective electrode and the second reference electrode to the pH meter as shown in Figure 21F-1. Prepare a series of standard solutions of the ion of interest, measure the cell potential for each concentration, plot a working curve of eii versus log c, and perform a least-squares analysis on the data (see Chapter 8). Compare the slope of the line with the theoretical slope of (0.0592 V)/n. Measure the potential for an unknown solution of the ion and calculate the concentration from the least-squares parameters. 

We can develop a series of standard electrode potentials by measuring the potentials of other standard electrodes versus the SHE in the way we described for the standard Zn-SHE and standard Cu-SHE voltaic cells. Many electrodes involve metals or nonmetals in contact with their ions. We saw (Section 21-12) that the standard Zn electrode behaves as the anode versus the SHE and that the standard oxidation potential for the Zn half-cell is 0.763 volt. 

The driving force for an ET reaction at the ITIES consists of two components, i.e., the difference of standard potentials of redox mediators and the interfacial potential drop [Eq. (14)]. The dependence of kf on AE° was studied for reactions between ZnPor+ in benzene and the series of similar cyanide complexes in water (26) ... 

Risk Factor Questionnaire. Each subject volunteer completed a series of questions designed to assess self-reported symptoms, and exposure to potentially confounding factors, such as alcohol and tobacco. Questions regarding hobbies and work-shift history were also addressed. Subjects also completed a series of standardized questions from a copyrighted questionnaire termed the SF-36. Both questionnaires were administered electronically after completion of the GASH/BARS. 

In addition to the junction potential ], membrane electrodes have asymmetry potentials of unknown magnitude. These potentials cannot be computed theoretically or easily measured individually, so direct pH measurements require that the pH electrode system be calibrated. In fact, the definition of pH used by NIST and lUPAC is an operational definition based on calibration with a series of standard buffer solutions. 

Equation (5) or (11) can be applied directly to half-cell reactions such as (6) and (7) and the resulting potentials obtained will be identical to those obtained from the overall reactions (9) and (10) because of the definition of the SHE as the universal standard. A selection of standard potentials of half-cell reactions is shown in Table 1 [5]. By international convention, electrode reactions in thermodynamic tables are always written as reduction reactions, so the more noble metals have a positive standard potential. Lists such as that in Table 1 are also called electromotive force series or tables of standard reduction potentials. 

The series of standard reversible potentials of the various metals (Section 3.6) are now and then used to explain and estimate the risk of galvanic corrosion. This can be very misleading, because 1) these potentials express thermodynamic properties, which do not tell us anything about the reaction rate (e.g. passivation tendencies are not taken into account), and 2) if the potential difference between the two metals in a galvanic couple is large, the more noble metal does not take part in the corrosion process with its own ions. Thus, under this condition, the reversible potential of the... 

The calculation of electrode potential E) from Eq. (10.9) requires a series of standard electrode potential (Eq) data. It is well accepted that the standard hydrogen potential is standard electrode potential in the world. Its electrode reaction is ... 

The tendency of metals to corrode is expressed most simply in the standard electrochemical series of Nemst potentials, shown in Table 38.10. These potentials are obtained in electrochemical measurements in which one electrode is a standard hydrogen electrode formed by bubbling hydrogen through a layer of finely divided platinum black. The potential of this reference electrode is defined to be zero. Noble metals are those which have a potential higher than that of a standard hydrogen electrode base metals have lower potentials. 

The International Organization for Standardization (ISO) developed a series of standards for evaluating the biocompatibility of a medical device prior to a clinical study. In particular, ISO 10993-1 2009 clearly outlined its primary aim as the protection of humans from potential biological risks arising from the use of medical devices. 

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