Noble metal

A collection of the noble metals, including copper, rhenium and mercury, which are included by some definitions. These are arranged according to their position in the periodic table.

In chemistry, the noble metals are metals that are resistant to corrosion and oxidation in moist air (unlike most base metals). The short list of chemically noble metals (those elements upon which almost all chemists agree) comprises ruthenium, rhodium, palladium, silver, osmium, iridium, platinum, and gold.[1]

More inclusive lists include one or more of mercury,[2][3][4] rhenium[5] or copper as noble metals. On the other hand, titanium, niobium, and tantalum are not included as noble metals although they are very resistant to corrosion.

While the noble metals tend to be valuable – due to both their rarity in the Earth's crust and their usefulness in areas like metallurgy, high technology, and ornamentation (jewelry, art, sacred objects, etc.) – the terms "noble metal" and "precious metal" are not synonymous.

The term noble metal can be traced back to at least the late 14th century[6] and has slightly different meanings in different fields of study and application. Only in atomic physics is there a strict definition. For this reason there are many quite different lists of "noble metals".

In addition to this term's function as a compound noun, there are circumstances where "noble" is used as an adjective for the noun "metal". A "galvanic series" is a hierarchy of metals (or other electrically conductive materials, including composites and semimetals) that runs from noble to active, and allows one to predict how materials will interact in the environment used to generate the series. In this sense of the word, graphite is more noble than silver and the relative nobility of many materials is highly dependent upon context, as for aluminium and stainless steel in conditions of varying pH.[7]

Properties

Palladium, platinum, gold and mercury can be dissolved in aqua regia, a highly concentrated mixture of hydrochloric acid and nitric acid, but iridium and silver cannot. Silver is, however, soluble in pure nitric acid. Ruthenium can be dissolved in aqua regia only when in the presence of oxygen, while rhodium must be in a fine pulverized form. Niobium and tantalum are resistant to all acids, including aqua regia. [8]

Physics

In physics, the definition of a noble metal is most strict. It requires that the d-bands of the electronic structure are filled. From this perspective, only copper, silver and gold are noble metals, as all d-like bands are filled and do not cross the Fermi level.[9] For platinum, two d-bands cross the Fermi level, changing its chemical behaviour such that it can function as a catalyst. The difference in reactivity can easily be seen during the preparation of clean metal surfaces in an ultra-high vacuum: surfaces of "physically defined" noble metals (e.g., gold) are easy to clean and keep clean for a long time, while those of platinum or palladium, for example, are covered by carbon monoxide very quickly.[10]

Electrochemistry

Metallic elements, including noble and several non-noble metals (noble metals bolded):[11]

element Atomic number group period reaction potential
Gold 79 11 6 Au3+
 + 3 e− → Au 		1.56 V
Platinum 78 10 6 Pt2+
 + 2 e− → Pt 		1.18 V
Iridium 77 9 6 Ir3+
 + 3 e− → Ir 		1.156 V
Palladium 46 10 5 Pd2+
 + 2 e− → Pd 		0.987 V
Osmium 76 8 6 OsO
4 + 8 H+
 + 8 e− → Os + 4 H
2O 		0.838 V
Silver 47 11 5 Ag+
 + e− → Ag 		0.7996 V
Mercury 80 12 6 Hg2+
2 + 2 e−→ 2 Hg 		0.7973 V
Polonium 84 16 6 Po2+
 + 2 e− → Po 		0.65 V[12]
Rhodium 45 9 5 Rh2+
 + 2 e− → Rh 		0.600 V
Ruthenium 44 8 5 Ru2+
 + 2 e− → Ru 		0.455 V
Copper 29 11 4 Cu2+
 + 2 e− → Cu 		0.337 V
Bismuth 83 15 6 Bi3+
 + 3 e− → Bi 		0.308 V
Technetium 43 7 5 TcO
2 + 4 H+
 + 4 e− → Tc + 2 H
2O 		0.272 V
Rhenium 75 7 6 ReO
2 + 4 H+
 + 4 e− → Re + 2 H
2O 		0.259 V
Antimony 51 15 5 Sb
2O
3 + 6 H+
 + 6 e− → 2 Sb + 3 H
2O 		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 O
2 + 2 H
2O + 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 CO
2 afterward.[8] On the other hand, rhenium coated mirrors are said to be very durable,[8] although rhenium and technetium are said to tarnish slowly in moist atmosphere.[13]

See also

References

  1. ↑ A. Holleman, N. Wiberg, "Lehrbuch der Anorganischen Chemie", de Gruyter, 1985, 33. edition, p. 1486
  2. ↑ Die Adresse für Ausbildung, Studium und Beruf
  3. ↑ "Dictionary of Mining, Mineral, and Related Terms", Compiled by the American Geological Institute, 2nd edition, 1997
  4. ↑ Scoullos, M.J., Vonkeman, G.H., Thornton, I., Makuch, Z., "Mercury - Cadmium - Lead: Handbook for Sustainable Heavy Metals Policy and Regulation",Series: Environment & Policy, Vol. 31, Springer-Verlag, 2002
  5. ↑ The New Encyclopædia Britannica, 15th edition, Vol. VII, 1976
  6. ↑ http://dictionary.reference.com/browse/noble+metal
  7. ↑ Everett Collier, "The Boatowner’s Guide to Corrosion", International Marine Publishing, 2001, p. 21
  8. ↑ 8.0 8.1 8.2 A. Holleman, N. Wiberg, "Inorganic Chemistry", Academic Press, 2001
  9. ↑ Hüger, E.; Osuch, K. (2005). "Making a noble metal of Pd". EPL (Europhysics Letters) 71 (2): 276. Bibcode:2005EL.....71..276H. doi:10.1209/epl/i2005-10075-5.
  10. ↑ S. Fuchs, T.Hahn, H.G. Lintz, "The oxidation of carbon monoxide by oxygen over platinum, palladium and rhodium catalysts from 10−10 to 1 bar", Chemical engineering and processing, 1994, V 33(5), pp. 363-369
  11. ↑ D. R. Lidle editor, "CRC Handbook of Chemistry and Physics", 86th edition, 2005
  12. ↑ A. J. Bard, "Encyclopedia of the Electrochemistry of the Elements", Vol. IV, Marcel Dekker Inc., 1975
  13. ↑ R. D. Peack, "The Chemistry of Technetium and Rhenium", Elsevier, 1966
Notes
  • R. R. Brooks, "Noble metals and biological systems: their role in Medicine, Mineral Exploration, and the Environment", CRC Press, 1992

External links