|N / A||Mg||Al||Si||P||S.||Cl||Ar|
Precious metals are metals that are corrosion-resistant, i.e. that are permanently chemically stable in a natural environment under the influence of air and water . Because of this stability, gold and silver have been used to make jewelry and coins since ancient times. In the last four centuries the platinum metals were also discovered, which are similarly corrosion-resistant as gold. Gold, silver, platinum and palladium in particular play a role on the world markets today . All precious and semi-precious metals are heavy metals .
Precious metals in the classic sense
The precious metals in the classic sense include gold, silver and the platinum metals. In some cases, mercury is also counted among the precious metals, although it is more reactive than the other precious metals in many respects. Noble metals either do not corrode at all in air at room temperature or only corrode extremely slowly and to a very small extent, like silver when it comes into contact with (traces of) hydrogen sulfide . A silver object is not damaged, only an extremely thin layer of black silver sulfide is formed . The precious metals are also not attacked by hydrochloric acid. They are also characterized by the fact that many of their compounds are not thermally stable. Silver oxide and mercury oxide are broken down into their elements when heated. Like other elements that are heavier than hydrogen, precious metals are created through nucleosynthesis .
The theory of redox reactions was refined in the 19th and 20th centuries. New reaction pathways have been discovered. Furthermore, the electrochemical method of potentiometry was developed , with which the strength of reducing agents and oxidizing agents could be precisely measured and compared. This also allowed a more refined classification of metals according to their noble or ignoble character. The semiprecious metals therefore include those that do not react to form hydrogen with aqueous solutions of non-oxidizing acids such as hydrochloric acid or dilute sulfuric acid . This is due to their standard potential , which is higher than that of hydrogen. These metals are also largely inert to atmospheric oxygen. For this reason, they are occasionally dignified in nature .
Metals such as bismuth and copper with their standard potential are much closer to hydrogen than the classic precious metals. They corrode faster in air and dissolve quickly in oxidizing acids such as concentrated sulfuric acid or semi-concentrated (30 percent) nitric acid . In the chemical sense, semiprecious metals are all metals that have a positive standard potential compared to hydrogen in the electrochemical series , but are otherwise not as corrosion-resistant as classic noble metals. According to this definition, the artificial and radioactive technetium can also be described as semi-precious. These semi-precious metals thus occupy an intermediate position between the classic precious and base metals. Some authors even include nickel and tin, although their standard potential is slightly below that of hydrogen.
Short-lived radioactive precious metals
Theoretical considerations based on quantum mechanical calculations suggest that the artificial elements Bohrium , Hassium , Meitnerium , Darmstadtium , Roentgenium and Copernicium are also precious metals. These metals are of no practical importance, however, since their known isotopes are extremely unstable and quickly (with typical half-lives of a few seconds, at most a few minutes) radioactively decay.
The base metals such as aluminum , iron and lead must be clearly distinguished . Since their standard potential is less than that of hydrogen , they are attacked by non-oxidizing acids . As with lead, this can be done very slowly. Non-oxidizing means here that there is no stronger oxidizing agent than the hydrogen ion in the solution.
Other corrosion-resistant metals
In addition to the precious metals, there are also some metals that, due to their passivation, sometimes have a high level of corrosion resistance, which, depending on the chemical environment, sometimes exceeds some precious metals. These are the elements of the 4th subgroup ( titanium , zirconium and hafnium ), those of the 5th subgroup ( vanadium , niobium and tantalum ) and those of the 6th subgroup ( chromium , molybdenum and tungsten ). Other technically important metals that form passive layers are zinc ( 12th subgroup ), aluminum ( 3rd main group ) and silicon and lead ( 4th main group ).
Reactions of precious metals
All precious metals can be brought into solution with suitable aggressive chemicals. Gold and some platinum metals dissolve quickly in aqua regia . Silver and the semi-precious metals react vigorously with nitric acid. In mining, cyanide solutions are used in conjunction with atmospheric oxygen to loosen gold and silver from rocks. Attack by atmospheric oxygen is only possible because stable cyanido complexes with gold and silver are formed as products. In aqua regia, too, the formation of stable complex compounds (chloro complexes) is one of the decisive factors for the oxidizing effect of the milieu. Incidentally, precious metals often do not behave “nobly” at all towards very electropositive metals, but here often willingly and with the release of energy they form intermetallic phases .
Physical understanding of the precious metal character
In the physical sense, the amount of precious metals is significantly smaller; it's just copper, silver and gold. The criterion for classification is the electronic band structure . The three metals listed all have completely filled d-bands, which therefore do not contribute to conductivity and practically no reactivity. For platinum this applies e.g. B. not. Two D-type bands cross the Fermi level . This leads to a different chemical behavior, which is why platinum is used much more often than gold as a catalyst . The difference in the production of pure metal surfaces in an ultra-high vacuum is particularly striking . While gold is comparatively easy to prepare and remains pure for a long time after preparation, carbon monoxide binds to platinum or palladium very quickly .
Chemical understanding of precious metals
As already indicated for the base metals , precious and semi-precious metals are metallic elements (and possibly certain alloys , such as corrosion-resistant steels ) whose normal potential is positive compared to the hydrogen electrode , which means that they are not attacked by dilute acids . The elements that come into consideration are sorted according to their normal potential compared to the H-electrode in an aqueous solution at pH 7:
|gold||Ib / 6||Au → Au 3+||1.498 V|
|platinum||VIIIb / 6||Pt → Pt 2+||1.18V|
|iridium||VIIIb / 6||Ir → Ir 3+||1.156 V|
|palladium||VIIIb / 5||Pd → Pd 2+||0.987 V|
|osmium||VIIIb / 6||Os → Os 2+||0.85V|
|silver||Ib / 5||Ag → Ag +||0.799 V|
|mercury||IIb / 6||2 Hg → Hg 2 2+||0.7973 V|
|polonium||VIa / 6||Po → Po 2+||0.65V|
|Rhodium||VIIIb / 5||Rh → Rh 2+||0.60V|
|Ruthenium||VIIIb / 5||Ru → Ru 2+||0.45V|
|copper||Ib / 4||Cu → Cu 2+||0.337 V|
|Bismuth||Va / 6||Bi → Bi 3+||0.32V|
|Technetium||VIIb / 5||Tc → Tc 4+||0.272V|
|rhenium||VIIb / 6||Re → Re 4+||0.259 V|
|antimony||Va / 5||Sb → Sb 3+||0.152 V|
As a semi-metal, antimony is not one of them, and with polonium it is possibly its high radioactivity and macroscopic inavailability (before the construction of nuclear reactors), because of which it was classically not regarded as a noble metal - but nowadays it is available in gram quantities. The subdivision, i.e. potential limit, of these elements into precious metals and semiprecious metals is quite arbitrary and is not handled uniformly. However, it is mostly drawn between copper and ruthenium, since the latter can in principle be attacked by moist air due to the redox reaction O 2 + 2 H 2 O + 4 e - ⇄ 4 OH - (aq) with a normal potential of +0.4 V.
Bronze is not a precious metal, but typically a copper-tin alloy . Because gold, silver and bronze medals are awarded at the Olympic Games and other competitions, bronze is sometimes incorrectly referred to as precious metal in the parlance of sports reports.
- Which bands cross the Fermi level can be seen on The Fermi Surface Database .
- Mining and industry of the individual continents in the Alexander World Atlas 2000, Klett-Perthes, Gotha 2000.
- AF Holleman , E. Wiberg , N. Wiberg : Textbook of Inorganic Chemistry . 102nd edition. Walter de Gruyter, Berlin 2007, ISBN 978-3-11-017770-1 , p. 2009.
- AF Holleman , E. Wiberg , N. Wiberg : Textbook of Inorganic Chemistry . 91st – 100th, improved and greatly expanded edition. Walter de Gruyter, Berlin 1985, ISBN 3-11-007511-3 .