Element, Ion.E.P.El.,Ion.KIon.
K, K+ [B](−2.92)  6E50
Na, Na+ [C]−2.44   2.5E42
Ba, Ba2+(−2.54)  2.1E88
Sr, Sr2+(−2.49)  4.0E86
Ca, Ca2+(−2.28)  2.0E79
Mg, Mg2+(−2.26)  4.1E78
Al, Al3+ [D]−0.999 ? 1.3E52
Mn, Mn2+−0.798  5.7E27
Zn, Zn2+−0.493  1.4E17
Cd, Cd2+−0.143  9.5E4
Fe, Fe2+ [E]−0.122 ? 1.8E4
Co, Co2+ [F]+0.0138 ?0.3314
Ni, Ni2+ [G]+0.108 ? 1.8E−4
Sn, Sn2+<+0.085  <1.1E−3
Pb, Pb2++0.129  3.3E−5
H2, H+ [H]+0.277  1.52E−5
Cu, Cu2++0.606  8.3E−22
As, As+++<+0.570  <2.7E−30
Bi, Bi3+<+0.668  <1.4E−35
Sb, Sb3+<+0.743  <1.7E−39
Hg, Hg++1.027  1.38E−18
Ag, Ag++1.048  6E−19
Pt, Pt4+<+1.140  5E−80
Au, Au3+<+1.356  <1.8E−71
F3, F− [H](+2.24)  9.0E88
Cl2, Cl− [H]+1.694  3.16E29
Br2, Br−+1.270  1.23E22
I2, I−+0.797 ? 7.26E13
O2, HO− [I]+0.698  1.36E12

[A] The table is based on Wilsmore's compilation of solution-tension potentials, Z. phys. Chem., 36, 91 (1901).

[B] Values in parentheses have been estimated by indirect measurements.

[C] G. N. Lewis, J. Am. Chem. Soc., 32, 1467 (1910).

[D] Values marked with? are uncertain.

[E] Calculated from the data of Richards and Behr (Z. phys. Chem., 58, 301 (1907)), who found the potential of iron against 0.5 molar FeSO4 to be −0.15 volt. The degree of ionization of 0.5 molar FeSO4 is taken as 22%. [Λ = 25.8 (Kohlrausch and Holborn, loc. cit., p. 152) and Λ∞ is taken as 117, as for ZnSO4 (ibid., p. [200]).] On account of the doubtful value for the degree of ionization, the values in the table are marked?, but the value found by Richards and Behr appears to be quite accurate.

[F] Calculated from the data of Schildbach (Z. für Elektroch., 16, 967 (1910)). The same uncertainty as to the degree of ionization exists as that discussed in the previous footnote.

[G] Calculated from the data of E. P. Schoch (Am. Chem. J., 41, 208 (1909)). The same uncertainty as to the degree of ionization exists as that discussed in footnote 5, p. [294].

[H] The values for gaseous elements refer to the gases under one atmosphere pressure.

[I] The potential of oxygen at 18°, 760 mm., against an alkaline solution in which [HO−] = 1. KIon refers to the concentration of HO−, with which oxygen under atmospheric pressure would be directly in equilibrium, at 18°.