E.m.f. series of metals

Because of the many variables which can influence the corrosion reaction, the use of the e.m.f. series of metals to predict the behaviour of galvanic couples in a given service environment can be hazardous and misleading. Numerous examples of coatings expected to act cathodically which have, in fact, been anodic have been reported in the literature and specialised lists of galvanic couples in different environments have been compiled. ... 

These considerations show the essentially thermodynamic nature of and it follows that only those metals that form reversible -i-ze = A/systems, and that are immersed in solutions containing their cations, take up potentials that conform to the thermodynamic Nernst equation. It is evident, therefore, that the e.m.f. series of metals has little relevance in relation to the actual potential of a metal in a practical environment, and although metals such as silver, mercury, copper, tin, cadmium, zinc, etc. when immersed in solutions of their cations do form reversible systems, they are unlikely to be in contact with environments containing unit activities of their cations. Furthermore, although silver when immersed in a solution of Ag ions will take up the reversible potential of the Ag /Ag equilibrium, similar considerations do not apply to the NaVNa equilibrium since in this case the sodium will react with the water with the evolution of hydrogen gas, i.e. two exchange processes will occur, resulting in an extreme case of a corrosion reaction. 

MacCarthy, P., and Perdue, E. M. (1991). Complexation of metal ions by humic substances fundamental considerations. In Interactions at the Soil Colloid-Soil Solution Interface. NATO ASI Series, Vol. 190, ed. Bolt, G. H., De Boodt, M. F., Hayes, M. H. B., and McBride, M. B., Kluwer, Dordrecht, The Netherlands, 469-492. 

As the E.M.F. applied to the cell is increased, the potential rises rapidly at first until a cathodic process, e.g., deposition of a metal or electrolytic reduction, that is, reduction in its widest sense, can occur as with a stationary electrode (cf. Fig. 113), the current can now increase with relatively little increase of potential, until the limiting current for the particular species undergoing reduction is attained. The steady increase of the applied e.m.f. causes the cathode potential to increase until another reduction process can take place, when, once again, the current increases to the limiting value. It is seen, therefore, that as the contact G is moved from I to ff, thus increasing the e.m.f. between the electrodes C and Ey the current increases in a series of waves, each wave repre-... 

The circuit consisted of a stabilized voltage supply, KFKI type P-13-1RK, and the conductivity cell that was connected with the electrometer in series. The lower limit of the SEA type 6-ATCC-5 electrometer sensitivity was 10"13 amp. The variations in the output current were displayed on a Graphispot, type GR4VAD recorder. The sample was warmed by two different methods, both essentially spontaneous. The sample was either exposed to the surrounding air or left to warm slowly inside of the metal block that was cooled to liquid nitrogen temperature before the measurement. The sample temperature was measured by recording the thermo e.m.f. vs. time of an iron constantan thermocouple put in the middle of the opened sample. The temperature was measured to a 5° accuracy. 

All voltaic cells consist of a series of conducting phases in contact the electrodes are generally metallic and there are one or more liquid electrolytes. At any phase boundary, where two or more phases of different composition meet, there is a difference of potential. The e.m.f. of the cell is the algebraic sum of all these phase-boundary potentials, including any metal contact potentials that may be present. The e.m.f. of a cell is the potential difference between two pieces of metal of identical composition, the ends of the chain of conducting phases. 

The data refer to various temperatures between 18 and 25°C, and were compiled from values cited by Bjerrum, Schwarzenbach, and Sillen, Stability Constants of Metal Complexes, part II, Chemical Society, London, 1958, and values taken from publications of the lUPAC Solubility Data Project Solubility Data Series, International Union of Pure and Applied Chemistry, Pergamon Press, Oxford, 1979-1992 H. L. Clever, and F. J. Johnston, J. Phys. Chem. Ref Data, 9 751 (1980) Y. Marcus, Ibid. 9 1307 (1980) H. L. Clever, S. A. Johnson, and M. E. Derrick, Ibid. 14 631 (1985), and 21 941 (1992). 

McBride M.B. Processes of heavy and transition metal sorption by soil mineral. In Interactions at the Soil Colloid-Soil Solution Interface, G.H. Bolt, M.F. De Boodt, M.H.B.Hayes, M.B. McBride, eds. NATO ASI Series (Series E Applied Sciences-Vol 190). Dordrecht, Netherlands Kluwer Academic Publishers, 1991. 

Although POMs have been known for about 200 years [6d,e,f] a large number of novel polyoxoanions with unexpected shapes and sizes are still being discovered. However, despite this diversity that precludes from complete rational and systematic design of synthesis for all POMs, a few guidelines can be retained. For specific details, the reader is referred to books, reviews, and original papers [3-7, 27]. Primarily, the evolution of aquated metal cations in aqueous solution depends on the pH of the medium, going from the hydrated cation [M(H20)x] , viable in very acidic medium, to the oxoanion [MOx] - obtained in very basic solution (see Sch. lb). The route between these two extremes is populated by a series of more or less stable oxocations and hydroxocations. 

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