Noble Metals - Electrochemistry

Electrochemistry

Metallic elements, including several non-noble metals, sorted by their chemical "nobility" (noble metals bolded):

element group period reaction potential
Gold 11 6 Au3+ + 3 e− → Au 1.42 V
Platinum 10 6 Pt2+ + 2 e−→ Pt 1.18 V
Iridium 9 6 Ir3+ + 3 e− → Ir 1.156 V
Palladium 10 5 Pd2+ + 2 e− → Pd 0.987 V
Osmium 8 6 OsO4 + 8 H+ + 8 e− → Os + 4 H2O 0.838 V
Silver 11 5 Ag+ + e−→ Ag 0.7996 V
Mercury 12 6 Hg2+
2 + 2 e−→ 2 Hg		 0.7973 V
Polonium 16 6 Po2+ + 2 e−→ Po 0.65 V
Rhodium 9 5 Rh2+ + 2 e− → Rh 0.600 V
Ruthenium 8 5 Ru2+ + 2 e− → Ru 0.455 V
Copper 11 4 Cu2+ + 2 e− → Cu 0.337 V
Bismuth 15 6 Bi3+ + 3 e− → Bi 0.308 V
Technetium 7 5 TcO2 + 4 H+ + 4 e− →Tc + 2 H2O 0.272 V
Rhenium 7 6 ReO2 + 4 H+ + 4 e−→ Re + 2 H2O 0.259 V
Antimony 15 5 Sb2O3 + 6 H+ + 6 e−→ 2 Sb + 3 H2O 0.152 V

The columns group and period denote its position in the periodic table, hence electronic configuration. The simplified reactions, listed in the next column, can also be read in detail from the Pourbaix diagrams of the considered element in water. Finally the column potential indicates the electric potential of the element measured against a Standard hydrogen electrode. All missing elements in this table are either not metals or have a negative standard potential.

Antimony is considered to be a metalloid and thus cannot be a noble metal. Also chemists and metallurgists consider copper and bismuth not noble metals because they easily oxidize due to the reaction O2 + 2 H2O + 4 e
- ⇄ 4 OH−(aq) + 0.40 V which is possible in moist air.

The film of silver is due to its high sensitivity to hydrogen sulfide. Chemically patina is caused by an attack of oxygen in wet air and by CO2 afterward. On the other hand, rhenium coated mirrors are said to be very durable, despite the fact that rhenium and technetium are said to tarnish slowly in moist atmosphere.

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