Electrochemical Behavior
In the following, E° represents the standard potential, in V, relative to the standard hydrogen electrode (NHE) at 25 °C. Activities are referred to the hypothetical ideal state at unit molality for all solutes involved in the electrode reaction. Standard
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ELectrochemicaL Behavior
General References:
Heumann, T., Stolica, N.D., in: Bard, A.J., Encyclopedia of the Electrochemistry of the Elements, Vol. 5, New York 1976, pp. 135/225. Vas'ko, A.T., Elektrokhimiya Molibdena i Vol'frama, Kiev 1977. Vas'ko, A.T., Kosenko, V.A., Electrochemical Behavior of Ghromium, Molybdenum and Tungsten, Dopov. Akad. Nauk Ukr. RSR, B 35 No. 6 [1973] 530/3. Piontelli, R., Gonsiderations sur l'Electrochimie des Metaux, J. Ghim. Phys. 45 [1948]115/22. 6.1
Standard Potentials
In the following, EO represents the standard potential, in V, relative to the standard hydrogen electrode (NHE) at 25 °G. Activities are referred to the hypothetical ideal state at unit molality for all solutes involved in the electrode reaction. Standard potentials in alkali ne solutions are referred to the half-reaction H2 +20H- ~ 2H 20+2e- (Eo= -0.828V at 25°G), and are symbolized by E~. Signs comply with the Stockholm convention: with increasing oxidizing strength of the system the potential becomes more positive [1]. The opposite rule is sometimes used by British and American authors; in these cases the signs have been changed. A survey of calculated standard redox potentials of Mo, Mo02 , and Mo03 in 1 N H2S0 4 and 1 N KOH is given by Pozdeeva et al. [17,52]. In addition to the well-known compounds Mo0 2 and Mo0 3 , the intermediate oxides M0 20 3 and M0 20 5 were dealt with in some papers atthough they are probably not pure compounds but mixtures, see" Molybdän" Erg.-Bd. B 1, 1975, p. 78. If these oxides were homogeneous compounds, their free energies of formation could be ßG~:::l -186 kcal/mol (M0 20 3 ) and -286 kcal/mol (M0 20 5 ) [2]. Standard potentials which were derived from these ßG~ values (see pp. 127, 129) are to be considered as fictitious. The standard redox potentials of crystalline oxygen compounds of Mo and other metals seem to depend linearlyon the valence state of the metal (M) in the compound or on the distance M -0 of the oxygen compound concerned. A comparison of Mo, W, V, and Re showed that a higher degree of distortion of the crystal laUice of the oxygen compounds entaiLed a greater difference in the EO vaLues of corresponding higher and Lower oxidation states of the element concerned [53]. 6.1.1
MOll
I Mo Couples
In Melts
The dependence of the Mo equilibrium potential in alkali metal chloride metts on the Mo concentration (Mo being introduced in the mett by anodic dissolution) shows that Mo ions are present not only in the trivalent state but also, to a lesser extent, in the divalent state. This is a consequence of the equilibrium Mo +2 Mo3+ ~ 3 M0 2+. By a method given by the authors [3,4] the standard potential of Mo ~ M0 2+ +2e- on mole fraction scale can be calculated from the equilibrium potential measured vs. a standard chlorine electrode. The values at 1100 K and the linear relationship between EO and the temperature are given in the Table 6/1 for different alkali metal chloride melts. Like the Mo3+ I Mo standard potential (see p. 127), the M0 2+ I Mo standard potent
