Standard potentials at 25 °C 3.9.3



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3.9.3 Standard potentials at 25 °C

The standard electrode potential of an electrode reaction is the standard potential difference (or electromotive force, EMF) of a cell whose left hand electrode is a hydrogen gas electrode. This (IUPAC) convention differs from that used in Latimer’s (1938, 1952) classic book on oxidation potentials in that the sign is opposite, i.e. it is a reduction potential.

For example, on the IUPAC convention (Bard et al., 1985)

E⁰(Zn²⁺ | Zn)= −0.763 V

The recombination of standard potentials is the basis for their utility. If there are n possible electrodes, the total number of cells that can be made from pairs of them is n(n − 1). The standard potential differences of all these can be calculated from a tabulation of n − 1 cells in which n − 1 electrodes are combined in turn with a chosen electrode. This is called the standard reference electrode and, in aqueous solution, and other protic solvents, is taken as the hydrogen gas electrode. Effectively this means E⁰(H⁺ | H₂) is set equal to zero at all temperatures.

For example, from the standard electrode potential difference of the cell

Pt | H₂ | H⁺ || Zn²⁺ | Zn E⁰ = −0.763V

that of the cell

Pt | H₂ | H⁺ || Ag⁺ | Ag E⁰ = 0.799 V

may be subtracted to obtain the standard potential difference of the cell

Ag | Ag⁺ || Zn²⁺ | Zn E⁰ = −1.562V

In doing this subtraction, the cell being subtracted is reversed and added so that the electrode in common, the hydrogen gas electrode, is cancelled. In these cell schemes a single vertical bar represents a phase boundary and a double vertical bar represents a liquid–liquid junction, the potential difference across which is assumed to have been minimised.

In 1982 IUPAC recommended adopting a new standard state pressure 10⁵ Pa (or 1 bar) instead of 101 325 Pa (or 1 atm). However, all the tabulated standard potential data refer to the previous standard state condition of 1 atm. The effect of the change on most of the data is less than 0.2 mV, which in most cases is less than the experimental uncertainty. Correction to 1 bar standard state can be made using the equation:

(RT/F)Δv In (101.325) = 0.169Δv mV

where Δv is the increase in number of gas molecules in the cell reaction.

References

W. M. Latimer (1952) Oxidation States of the Elements and their Potentials in Aqueous Solutions, Prentice-Hall,
New York.
A. J. Bard, R. Parsons and J. Jordan (eds) (1985) Standard Potentials in Aqueous Solution, Dekker, New York.

Standard potentials at 25 °C

Electrode reaction	Eº /V	Electrode reaction	Eº /V
Li⁺ + e⁻ → Li ^{. . . . .}	− 3.045	AgI + e⁻ → Ag + I^{− . . . . .}	− 0.152 2
K⁺ + e⁻ → K^{. . . . .}	− 2.925	Sn²⁺ + 2e⁻ → Sn^{. . . . .}	− 0.136
Rb⁺ + e⁻ → Rb ^{. . . . .}	− 2.925	Pb²⁺ + 2e⁻ → Pb^{. . . . .}	− 0.125 1
Cs⁺ + e⁻ → Cs^{. . . . .}	− 2.923	2H⁺ + 2e⁻ → H₂^{. . . . .}	0 exactly
Ba²⁺ + 2e⁻ → Ba^{. . . .}	− 2.92	AgBr + e⁻ → Ag + Br⁻^{. . . .}	+ 0.071 1
Sr²⁺ 2e⁻ → Sr^{. . . .}	− 2.89	Sn⁴⁺ + 2e⁻ → Sn²⁺^{. . . . .}	+ 0.15
Ca²⁺ + 2e⁻ → Ca^{. . . .}	− 2.84	Cu²⁺ + e⁻ → Cu⁺^{. . . . . .}	+ 0.159
Na⁺ + e⁻ → Na^{. . . .}	− 2.714	AgCl + e⁻ → Ag + Cl⁻^{. . . .}	+ 0.222 3
La³⁺ + 3e⁻ → La^{. . . .}	− 2.37	Hg₂Cl₂ + 2e⁻ → 2Hg + 2Cl⁻^{. .}	+ 0.268 16
Mg²⁺ + 2e⁻ → Mg^{. . .}	− 2.56	Cu²⁺ + 2e⁻ → Cu^{. . . . . .}	+ 0.340
Sc³⁺ + 3e⁻ → Sc ^{. . . .}	− 2.03	Fe(CN)₆³⁻ + e⁻ → Fe(CN)₆^{4− .}	+ 0.361 0
Be²⁺ + 2e⁻ → Be^{. . . .}	− 1.97	Cu⁺ + e⁻ → Cu^{. . . . . . .}	+ 0.520
Th⁴⁺ + 4e⁻ → Th^{. . . .}	− 1.85	I₂ + 2e⁻ → 2I⁻ . . . . . . .	+ 0.535 5
Al³⁺ + 3e⁻ → Al^{. . . .}	− 1.67	I⁻₃ + 2e⁻ → 3I⁻^{. . . . . . .}	+ 0.536
Ti²⁺ + 2e⁻ → Ti^{. . . .}	− 1.63	Hg₂SO₄ + 2e⁻ → 2Hg + SO₄²⁻ .	+ 0.613
Mn²⁺ + 2e⁻ → Mn^{. . .}	− 1.18	(AuSCN)⁻₄ + 3e⁻ → Au + 4SCN⁻	+ 0.636
Zn²⁺ + 2e⁻ → Zn^{. . . .}	− 0.762 6	Fe³⁺ + e⁻ → Fe²⁺^{. . . . . .}	+ 0.771
Ga³⁺ + 3e⁻ → Ga^{. . . .}	− 0.529	Hg₂²⁺ + 2e⁻ → 2Hg^{. . . . .}	+ 0.796 0
Fe²⁺ + 2e⁻ → Fe ^{. . . .}	− 0.44	Ag⁺ + e⁻ → Ag^{. . . . . . .}	+ 0.799 1
Cr³⁺ + e⁻ → Cr^{2+ . . . .}	− 0.424	Hg²⁺ +2e⁻ → Hg₂^{2+ . . . . .}	+ 0.911 0
Cd²⁺ + 2e⁻ → Cd^{. . . .}	− 0.042 5	Pd²⁺ + 2e⁻ → Pd^{. . . . . .}	+ 0.915
Ti³⁺ + e⁻ → Ti²⁺ . . . .	− 0.37	AuCl⁻₄ + 3e⁻ → Au + 4Cl⁻^{. .}	+ 1.002
PbSO₄ + 2e⁻ → Pb + SO₄²⁻	− 0.350 5	Pu⁴⁺ + e⁻ → Pu³⁺^{. . . . . .}	+ 1.01
In³⁺ + 3e⁻ → In^{. . . .}	− 0.338 2	Br₂(l) + 2e⁻ → 2Br⁻^{. . . . .}	+ 1.065
Tl⁺ + e⁻ → Tl^{. . . . .}	− 0.336 3	O₂ + 4H⁺ + 4e⁻ → 2H₂O^{. . .}	+ 1.229
Co²⁺ + 2e⁻ → Co^{. . . .}	− 0.277	Tl³⁺ + 2e⁻ → Tl⁺^{. . . . . .}	+ 1.25
V³⁺ + e⁻ → V²⁺ ^{. . . .}	− 0.255	Cl₂ + 2e⁻ → 2Cl⁻^{. . . . . .}	+ 1.358 3
Ni²⁺ + e⁻ → Ni ^{. . . . .}	− 0.257	Au³⁺ + 3e⁻ → Au^{. . . . . .}	+ 1.52

A.K. Covington

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