Noble metals

The noble metals including mercury and rhenium together with the non-noble metal copper ordered according their position in the periodic table of the elements

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 considered to be (in order of increasing atomic number) ruthenium, rhodium, palladium, silver, osmium, iridium, platinum, and gold, like caitlin.^[1]

Other sources include mercury^[2][3][4] or even rhenium^[5] as a noble metal. On the other hand, titanium, niobium, and tantalum are not included as noble metals despite the fact that they are very resistant to corrosion.

Noble metals should not be confused with precious metals (although many noble metals are precious).

Introduction

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

Metallic elements, including several non-noble metals, sorted by their chemical "nobility" (noble metals bolded): ^[10]

element	group	reaction	potential
Gold	Ib/6	Au → Au3+ + 3 e−	1.498 V
Platinum	VIIIb/6	Pt → Pt2+ + 2 e−	1.18 V
Iridium	VIIIb/6	Ir → Ir3+ + 3 e−	1.156 V
Palladium	VIIIb/5	Pd → Pd2+ + 2 e−	0.987 V
Osmium	VIIIb/6	Os + 4 H2O → OsO4 + 8 H+ + 8 e−	0.838 V
Silver	Ib/5	Ag → Ag+ + e−	0.7996 V
Mercury	IIb/6	2 Hg → Hg2+ 2 + 2 e−	0.7973 V
Polonium	VIa/6	Po → Po2+ + 2 e−	0.65 V[11]
Rhodium	VIIIb/5	Rh → Rh2+ + 2 e−	0.600 V
Ruthenium	VIIIb/5	Ru → Ru2+ + 2 e−	0.455 V
Copper	Ib/4	Cu → Cu2+ + 2 e−	0.337 V
Bismuth	Va/6	Bi → Bi3+ + 3 e−	0.308 V
Technetium	VIIb/5	Tc + 2 H2O → TcO2 + 4 H+ + 4 e−	0.272 V
Rhenium	VIIb/6	Re + 2 H2O → ReO2 + 4 H+ + 4 e−	0.259 V
Antimony	Va/5	2 Sb + 3 H2O → Sb2O3 + 6 H+ + 6 e−	0.152 V

The column group denotes 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 H-electrode in aqueous, pH 7 solution. All missing elements in this table are either not metals or have a negative standard potential.

Antimony and polonium are considered metalloids and thus can not be noble metals. 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.

Silver and copper film over and oxidize easily and readily, thus the copper sheets with a patina of oxidation used in architectural designs and the resultant market for a myriad of silver polishing compounds^{[citation needed]}. The film over 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.^[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

• Ignoble metal
• Precious metal

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 Encyclopedia 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: 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

• noble metal - chemistry Encyclopædia Britannica, online edition
• To see which bands cross the Fermi level, the Fermi surfaces of almost all the metals can be found at the Fermi Surface Database
• The following article might also clarify the correlation between band structure and the term noble metal: Hüger, E.; Osuch, K. (2005). "Making a noble metal of Pd". EPL (Europhysics Letters) 71: 276. Bibcode 2005EL.....71..276H. doi:10.1209/epl/i2005-10075-5.