and for elements which form negative ions (see footnote 1, p. [261]),
εElem., Ionv = (−0.0575 / v) log([C] / KIon) volts.
In these equations v represents the valence of the ion. It is clear that for the condition of equilibrium, in which [C] = KIon, the potential is 0. Further, for the potential difference between copper and a cupric salt solution in which [Cu2+] = 1, we would have
εCu, Cu2+ = (0.0575 / 2) log(1 / 8.3E−22) =
21.08 × 0.0575 / 2 = +0.606 volts.
4. Meaning of E.P.Element, Ion. Under E.P.Element, Ion the table gives the potential difference in volts, calculated for the element named and an aqueous solution of its ion in unit concentration (one gram-ion per liter). For instance, for zinc and [Zn2+] = 1 (65.4 grams zinc-ion per liter), we have a potential E.P.Zn, Zn2+ = −0.493. The signs used, in accordance with the convention adopted (p. [261]), indicate the character of the charge on the element electrode (which is named first in the subscript to E.P.). For instance, zinc in a solution in which [Zn2+] = 1 would acquire a negative charge (p. [266]), the potential difference E.P.Zn, Zn2+ being −0.493 according to the table; silver, immersed in a solution in which [Ag+] = 1, would acquire a positive charge, the potential difference E.P.Ag, Ag+ = +1.048.
The potentials given for the gaseous elements represent the potentials of the gases under 760 mm. pressure.
5. Potential Differences Calculated with the Aid of E.P.Element, Ion. The potential corresponding to any concentration [C] of a metal ion may be found from the equation[588]
εEl., Ionv = E.P.El., Ion + (0.0575 / v) log[C] volts,
and the potential for any concentration [C] of the ions of elements forming negative ions is found[588] according to