Precious metals are metals that are particularly resistant to corrosion. Some metals, such as gold and silver are so since ancient times for making jewelry and coins in use. Over the last centuries, the four platinum metals have been discovered that show a similar corrosion resistance as gold.
Precious metals in the classical sense
Among the precious metals in the classical sense include the platinum group metals and gold and silver. Partial mercury is also included among the precious metals, although it is more reactive than the other noble metals in many ways. Precious metals corrode ( rust, oxidize ) at room temperature in air either not, or only very slowly and. A very limited extent, as is the case with the silver when it comes in contact with (trace ) of hydrogen sulfide Even this is only an extremely thin layer of black silver sulfide forms. The silver item is not damaged. From hydrochloric acid, the precious metals are not attacked. Precious metals are further distinguished by the fact that many of its compounds are thermally unstable. So silver oxide and mercury oxide are decomposed on heating into its individual elements.
In the 19th and 20th century, the theory of oxidation-reduction reactions has been refined. New pathways have been discovered. Furthermore, we developed the electrochemical method potentiometry with which one could measure and compare the strength of reducing agents and oxidizing agents exactly. This also allowed a more refined classification of metals according to their noble or base character. To the semi-precious metals, therefore, are those which 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 its standard potential which is higher than that of the hydrogen. These metals are largely inert to atmospheric oxygen. For this reason, they are found in nature in elemental form occasionally.
Metals such as bismuth and copper are much closer to the hydrogen than the traditional precious metals with their standard potential. In air, they corrode faster, and in oxidizing acids such as concentrated sulfuric acid or concentrated half ( 30 percent ) nitric acid, they dissolve quickly. In a chemical sense, semi-precious metals are thus all metals that hold in the electrochemical series a positive standard potential relative to hydrogen, but otherwise not as corrosion resistant as traditional precious metals are. According to this definition, the artificial and radioactive technetium can be described as semi precious. Thus, these semi-precious metals occupy an intermediate position between the classical base and precious metals. Even nickel and tin to be accounted for by some authors, although their standard potential is slightly lower than the hydrogen.
Short-lived radioactive metals
Theoretical considerations due to quantum mechanical calculations suggest that the artificial elements Bohrium, Hassium, Meitnerium, Darmstadium, Roentgenium Copernicium and precious metals are. Practical importance of these metals, however, not because its known isotopes are highly unstable and quickly ( with typical half-lives of a few seconds, at most a few minutes ) decay radioactively.
Clearly delineate the base metals such as aluminum, iron and lead. Since its standard potential is smaller than that of hydrogen, they are attacked by non-oxidizing acids. This may, as in the lead, also be quite slow. Non-oxidising here means that there is no stronger oxidizing agent than is the hydrogen ion in the solution.
More corrosion resistant metals
In addition to precious metals, there are also some metals sometimes have high corrosion resistance due to their passivation, which is also some precious metals exceeds partly depending on the chemical environment. These are the elements of group 4 ( titanium, zirconium and hafnium), the 5th sub-group (vanadium, niobium and tantalum ), and the sub-group 6 ( chromium, molybdenum and tungsten). Other industrially important metals which form passive layers are zinc ( 12 subgroups ), aluminum ( main group 3 ) as well as silicon and lead ( main group 4 ).
Reactions of the precious metals
With suitable harsh chemicals can bring all precious metals in solution. Gold and some platinum metals dissolve rapidly in aqua regia. Silver and semi-precious metals react with nitric acid vividly. In mining, cyanide solutions are used in conjunction with atmospheric oxygen to dissolve gold and silver from rocks. The attack by the oxygen in the air is only possible because of the formation as stable products Cyanidokomplexe with gold and silver. Also in aqua regia, the formation of stable complexes ( Chloridokomplexe ) is decisive for the oxidizing effect of the environment. Precious metals behave otherwise often not "noble" to very electropositive metals, but form here often willingly and Intermetallic under energy release phases.
Physical concept of the noble character
In the physical sense the amount of precious metals is still significantly smaller; there are only copper, silver and gold. The criterion for the classification of the electronic band structure. The three listed metals have all completely filled d- bands which do not contribute to the conductivity, and thus practically no reactivity. For platinum, this does not apply, for example. Two d -like bands crossing the Fermi level. This leads to a different chemical behavior, which is why platinum is in contrast to gold, to make use of catalyst. Particularly striking is the difference in the production of pure metal surfaces in ultrahigh vacuum. For example, while gold is relatively easy to prepare and after preparation long remains pure, binds to platinum or palladium carbon monoxide very quickly.
Chemical understanding of precious metals
As already indicated in the base metals, precious metals and semi-precious metals are simple metallic elements (and possibly certain alloys such as stainless steels ), whose standard potential is positive relative to the hydrogen electrode, which are therefore not attacked by dilute acids. The elements that come into consideration are thus sorted according to their normal potential with respect to the H electrode in aqueous solution at pH 7:
Antimony counts as half- metal does not do so, and with polonium, it is probably his strong radioactivity and macroscopic unavailability (before the construction of nuclear reactors), for which he had been classically not considered it as a precious metal - these days, it is available in gram quantities. The subdivision, ie potential limit of these elements in precious metals and semi-precious metals is rather arbitrary and is not handled uniformly. However, it is usually drawn between copper and ruthenium, as the latter 4 OH (aq ) can in principle be attacked with a normal potential of 0.4 V by moist air due to the redox reaction O2 2 H2O 4 e - ⇄.
In the parlance of sports reports, especially concerning medals during the Olympic Games, the bronze is counted among the precious metals incorrectly.