Noble metal

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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.

The noble metals are metals that are resistant to corrosion and oxidation in moist air, unlike most base metals. They tend to be precious, often due to their rarity in the Earth's crust. The noble metals are most commonly considered to be ruthenium, rhodium, palladium, silver, osmium, iridium, platinum, and gold.[1]

Other sources include mercury,[2][3][4] rhenium[5] or copper as a noble metal. On the other hand, titanium, niobium, and tantalum are not included as noble metals, although they especially resist corrosion. Noble metals should not be confused with precious metals (although many noble metals have high value).

Introduction[edit]

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 can dissolve in nitric acid, though.) 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 acids, including aqua regia. [6]

This term can also be used in a relative sense, considering "noble" as an adjective for the word "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 designers to see at a glance 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]

In physics, the definition of a noble metal is even more strict. It is required that the d-bands of the electronic structure are filled. Taking this into account, only copper, silver and gold are noble metals, as all d-like bands are filled and do not cross the Fermi level.[8] For platinum two d-bands cross the Fermi level, changing its chemical behaviour; it is used as a catalyst. The different 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.[9]

Electrochemistry[edit]

Metallic elements, including noble and several non-noble metals (noble metals bolded):[10]

elementgroupperiodreactionpotential
Gold116Au3+
+ 3 e → Au
1.56 V
Platinum106Pt2+
+ 2 e → Pt
1.18 V
Iridium96Ir3+
+ 3 e → Ir
1.156 V
Palladium105Pd2+
+ 2 e → Pd
0.987 V
Osmium86OsO
4
+ 8 H+
+ 8 e → Os + 4 H
2
O
0.838 V
Silver115Ag+
+ e → Ag
0.7996 V
Mercury126Hg2+
2
+ 2 e→ 2 Hg
0.7973 V
Polonium166Po2+
+ 2 e → Po
0.65 V[11]
Rhodium95Rh2+
+ 2 e → Rh
0.600 V
Ruthenium85Ru2+
+ 2 e → Ru
0.455 V
Copper114Cu2+
+ 2 e → Cu
0.337 V
Bismuth156Bi3+
+ 3 e → Bi
0.308 V
Technetium75TcO
2
+ 4 H+
+ 4 e → Tc + 2 H
2
O
0.272 V
Rhenium76ReO
2
+ 4 H+
+ 4 e → Re + 2 H
2
O
0.259 V
Antimony155Sb
2
O
3
+ 6 H+
+ 6 e → 2 Sb + 3 H
2
O
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
2
O
+ 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.[6] On the other hand, rhenium coated mirrors are said to be very durable,[6] despite the fact that rhenium and technetium are said to tarnish slowly in moist atmosphere.[12]

See also[edit]

References[edit]

  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. ^ a b c A. Holleman, N. Wiberg, "Inorganic Chemistry", Academic Press, 2001
  7. ^ Everett Collier, "The Boatowner’s Guide to Corrosion", International Marine Publishing, 2001, p. 21
  8. ^ 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. 
  9. ^ 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 [1]
  10. ^ D. R. Lidle editor, "CRC Handbook of Chemistry and Physics", 86th edition, 2005
  11. ^ A. J. Bard, "Encyclopedia of the Electrochemistry of the Elements", Vol. IV, Marcel Dekker Inc., 1975
  12. ^ 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[edit]