The Eight Precious Metals Considered Noble Metals

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Most metallic chemical elements are known to easily oxidize and corrode, especially at high temperatures. Metals that have excellent resistance to oxidation and corrosion, even at high temperatures, are called noble metals. While there is no strict definition for this group of metals, it usually includes those that are extremely rare.

Thus, metals that are labeled “precious” are also considered noble metals (note, however, that noble metals are not necessarily precious metals). There are nine known precious metals – gold, platinum, iridium, palladium, osmium, silver, rhodium, ruthenium, and rhenium. With the exception of the latter, all are considered noble metals.

Using the “aqua regia” test (aqua regia is a mixture of concentrated nitric acid and concentrated hydrochloric acid, which is highly corrosive), here’s how the eight precious-noble metals react:

• Gold, platinum, palladium, and osmium dissolve.
• Ruthenium dissolves only in the presence of oxygen.
• Rhodium dissolves only when it is in a fine, pulverized form.
• Iridium and silver do not dissolve under any condition.

In another definition, noble metals may refer to electrically conductive elements. Here, the term “noble” serves as a modifying word, such that the electrical conductivity of materials is graded from noble to active. Using this definition for noble metals then, we will find that silver is less noble than, say, graphite (although graphite is an elemental form of carbon and, therefore, is not considered a metal).

Below is a comparison of the chemical nobility of the precious metals (again, excluding rhenium), with details of their specific electronic configurations, simplified reactions (as per the pH diagram), and electrode potentials. The list is presented in order of increasing atomic number.

1. Ruthenium – VIIIb/5; Ru -> Ru2+ + 2 e-; 0.455 V
2. Rhodium – VIIIb/5; Rh -> Rh2+ + 2 e-; 0.600 V
3. Palladium – VIIIb/5; Pd -> Pd2+ + 2 e-; 0.987 V
4. Silver – Ib/5; Ag -> Ag+ + e-; 0.7996 V
5. Osmium – VIIIb/6; Os + 4 H2O -> OsO4 + 8 H+ + 8 e-; 0.838 V
6. Iridium – VIIIb/6; Ir -> Ir3+ + 3 e-; 1.156 V
7. Platinum – VIIIb/6; Pt -> Pt2+ + 2 e-; 1.18 V
8. Gold – Ib/6; Au -> Au3+ + 3 e-; 1.498 V

The second item appearing after the name of the metal (the three items are separated by semi-colons) represents the metal’s reaction in water. In the pH diagram, the pH symbol is labeled on the horizontal axis to signify the -log function of the concentration of H+ ion. The lines, which represent equilibrium for the concentration, are drawn for ions at unit activity. Other concentrations may be represented by additional lines. The voltage potential is represented by a vertical axis, which is labeled Eh, where “h” stands for hydrogen.

Physics has an even more strict definition for noble metals. Here, it is required that the electronic structure’s d-bands are filled. If this definition is followed, only gold and silver (among the precious metals) qualify as noble metals. Also note that the varying reactivity of the precious metals can readily be observed while preparing their surfaces in the vacuum regime called ultra high vacuum.

Safety Issues Concerning Precious Metals

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The metallic chemical elements collectively known as precious metals are called as such because of their extreme rarity and high economic value. Precious metals occur naturally or are by-products of the processing of other less rare metals.

In order of increasing mass abundance (parts per billion), the precious metals are rhenium, rhodium, iridium, ruthenium, osmium, gold, platinum, palladium, and silver. These metals are not radioactive and are mostly used for industrial purposes and for jewelry.

Still, precious metals have some safety issues attached to them:

Rhenium:
Since rhenium is used in very small amounts, its toxicity is virtually unknown. The hazardous property of rhenium halide, for example, may be attributed either to rhenium itself or to the other elements that make up the compound. Another rhenium compound – potassium perrhenate – is known to have a median lethal dose much like that of sodium chloride (commonly known as table salt).

Rhodium:
Although rhodium is inert (being a noble metal, as almost all the other precious metals are), it can be reactive, especially if used as compounds. In its basic form, however, rhodium is not known to cause any harm.

Iridium:
Iridium, when finely divided, can pose some hazards – it can ignite in air. Apart from this, accidental exposure to a radioisotope of iridium may cause poisoning by radiation, burns, and even death.

Ruthenium:
Three conditions may be associated with exposure to ruthenium: it can stain the skin; it may accumulate in bones; and it may increase the risk of cancer. Ruthenium tetroxide, a yellow, diamagnetic tetrahedral ruthenium compound, is highly toxic and volatile; it may explode if it comes into contact with combustible materials.

Osmium:
Osmium, like iridium, can ignite spontaneously in air when in finely divided form. The compound osmium tetroxide, in particular, is highly volatile and is extremely toxic if accidentally inhaled, ingested, or comes into contact with the skin.

Gold:
Gold, in its elemental form, does not cause irritation and is not toxic even when ingested. In fact, it is used as a component in some alcoholic drinks and as a food additive. However, ionic chemical compounds of gold (example, gold chloride) can be extremely harmful to the kidneys and liver.

Platinum:
Findings by the U.S. federal agency CDC reveal that exposure to platinum salts, on the short term, may cause nose, throat, and eye irritation. Long-term exposure to these compounds, on the other hand, may cause skin and respiratory allergies.

Palladium:
Palladium in bulk metallic form is completely inert. The same can’t be said though of the metal in finely divided form, which can readily ignite in air.

Silver:
Silver compounds (example, colloidal silver), when absorbed into the body, may cause argyria, a condition characterized by the bluish-gray pigmentation of the skin, mucous tissues, and the eyes. While the condition is not really harmful to one’s health, it is often permanent. Otherwise, silver per se is not at all toxic.

It certainly is important to know that each of the precious metals has possible health hazards so that first-time handlers can take the necessary precautions.

Ruthenium: Most Versatile Of The Platinum Group Metals

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Arranged by name alphabetically, ruthenium is the last of six metallic elements in the platinum group. This precious metal is characterized as being both polyvalent and versatile. In fact, it is known to be more versatile than any of the other five precious metals in the platinum group.

A hard, white metal, ruthenium is known to have four crystal modifications. While it does not tarnish under normal temperature, it does oxidize readily when exposed to air. It can be plated by means of two methods: thermal decomposition and electrodeposition.

Following is a list of some of the properties of ruthenium:

General:
• Chemical Symbol: Ru
• Atomic Number: 44
• Category (as an element): Transition Metal
• Group/ Period/ Block (in the Periodic Table): 8/ 5/ d
• Atomic Weight: 101.07 g.mol-1
• Electron Configuration: [Kr] 4d7 5s1

Physical:
• Density (near room temperature): 12.45 g.cm-3
• Liquid Density (at melting point): 10.65 g.cm-3
• Melting Point: 2334°C, 4233°F, 2607.4°K
• Boiling Point: 4150°C, 7502°F, 4423°K
• Heat of Fusion: 38.59 kJ.mol-1
• Heat of Vaporization: 591.6 kJ.mol-1

Atomic:
• Oxidation States: 8, 7, 6, 4, 3, 2, 1, -2
• Electronegativity: 2.3 (Pauling scale)
• Atomic Radius: 134 picometre
• Covalent Radius: 146±7 picometre
• Ionization Energies: 710.2 kJ.mol-1 (first), 1620 kJ.mol-1 (second), 2747 kJ.mol-1 (third)

Ruthenium-palladium and ruthenium-platinum alloys are used in making electrical contacts for wear resistance. As a matter of fact, ruthenium is a very effective hardener for both palladium and platinum. When a small amount of ruthenium is added to titanium, the latter’s corrosion resistance is improved significantly.

As to its other applications, ruthenium is used in:

1. film chip resistors;
2. jewelry (when alloyed with gold);
3. high-temperature superalloys (used in making the turbine blades in jet engines);
4. fountain pen nibs;
5. removing hydrogen sulfide from various industrial processes;
6. electrolytic cells for various chemical processes (as a component of mixed-metal oxide anodes);
7. optical sensor device;
8. radiotherapy (particularly of eye tumors).

Along with the five other precious metals in the platinum group, ruthenium is generally found in ores in North America, South America, and the Ural Mountains in Russia. Small quantities of this element also occur in pentlandite (an iron-nickel sulfide) obtained from Sudbury in Ontario, Canada, and in South Africa’s pyroxenite (an ultrabasic igneous rock) deposits.

Commercially, ruthenium is obtained as a by-product from copper and nickel processing (as how all the other platinum group metals are obtained), although it is likewise obtainable through direct processing of platinoid ores. It is isolated by means of a complex chemical process. The process involves reduction of ammonium ruthenium chloride with the use of hydrogen. This yields a powder which, in turn, is consolidated by means of a technique called argon-arc welding.

In terms of abundance in the Earth’s crust, ruthenium ranks 74th among all the different known elements or metals and is, therefore, one of the rarest. Estimates place world reserves at 5,000 tonnes, and annual mining output about 12 tonnes. Ruthenium’s price is estimated to be approximately 1,000 U.S. dollars per troy ounce.

Rhodium: Costliest Precious Metal

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In 1803, the English physicist and chemist William Hyde Wollaston discovered a chemical element that is today considered the most expensive of the precious metals. This precious metal, which belongs to the platinum group metals, is rhodium. With its current price of around 1,000 U.S. dollars per troy ounce, rhodium is about five times costlier than platinum.

Wollaston’s discovery of rhodium was made possible with the use of crude platinum ore. He first dissolved the ore in nitro-hydrochloric acid (also called aqua regia) and neutralized the acid with lye (or sodium hydroxide). By adding salmiac (ammonium chloride), he then precipitated the platinum. All other metals, including rhodium, were precipitated with zinc. Rhodium was further precipitated by the addition of sodium chloride. Finally, it was washed with ethanol and reacted with zinc, forming the precious metal that it is known today.

Since rhodium occurs in ores mixed with other metals (examples: silver, gold, and platinum), its extraction is complex. For example, it is obtained as a white inert metal from platinum ores, which is quite difficult to fuse. The main source of this precious metal is in South Africa, which accounts for about 80 percent of the total world exports. Estimates place the annual production of rhodium at only about 25 tons.

Some of the properties of rhodium are listed below.

General:
• Chemical Symbol: Rh
• Atomic Number: 45
• Category (as an element): Transition Metal
• Group/ Period/ Block (in the Periodic Table): 9/ 5/ d
• Atomic Weight: 102.90550 g.mol-1
• Electron Configuration: [Kr] 4d8 5s1

Physical:
• Density (near room temperature): 12.41 g.cm-3
• Liquid Density (at melting point): 10.7 g.cm-3
• Melting Point: 1964°C, 3567°F, 2237°K
• Boiling Point: 3695°C, 6683°F, 3968°K
• Heat of Fusion: 26.59 kJ.mol-1
• Heat of Vaporization: 494 kJ.mol-1

Atomic:
• Oxidation States: 6, 5, 4, 3, 2, 1, -1
• Electronegativity: 2.28 (Pauling scale)
• Atomic Radius: 134 picometre
• Covalent Radius: 142±7 picometre
• Ionization Energies: 719.7 kJ.mol-1 (first), 1740 kJ.mol-1 (second), 2997 kJ.mol-1 (third)

The principal use of rhodium is as a catalytic converter in automotive vehicles, which reduces the toxicity of engine emissions by converting these into less harmful gases. Rhodium is likewise used in jewelry, such as when it is electroplated on platinum, sterling silver, or white gold (called rhodium flashing) to strengthen the metal or give it a reflective surface.

Rhodium has at least five other uses or applications:

1. As an alloying agent, to improve platinum’s resistance to corrosion.
2. For optical instruments.
3. As an electrical contact material (because of its stable resistance to contact and low resistance to electricity).
4. As a filter in X-ray systems (such as mammography).
5. In surfaces of high quality pens (because of its high mechanical and chemical resistance).

Obviously because of its extremely high price, rhodium signifies wealth. In some cases, it has been used in awards or recognitions, such as when the Guinness Book of World Records handed out a rhodium-plated disc to Paul McCartney in 1979, in recognition of his being the all-time best-selling recording artist and songwriter in history.

Rhenium: Last Naturally Occurring Stable Precious Metal Discovered

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In the periodic table of elements, rhenium is found as a third-row transition metal in group 7. Known to be one of the rarest precious metals in the Earth’s crust, rhenium has an average concentration of 1 part per billion. It is obtained mainly as a by-product of the refinement of two other chemical elements – copper and molybdenum.

Rhenium was discovered as a trace element in the mineral columbite and in platinum ores. Three German chemists – Otto Berg and the couple Walter Noddack and Ida Tacke – made the discovery in 1925. This find made rhenium the last identified naturally occurring precious metal with stable isotopes. Actually, naturally occurring rhenium is composed of 2 stable isotopes and 26 unstable ones.

Following is a list of some of the properties of rhenium:

General:
• Chemical Symbol: Re
• Atomic Number: 75
• Category (as an element): Transition Metal
• Group/ Period/ Block (in the Periodic Table): 7/ 6/ d
• Atomic Weight: 186.207 g.mol-1
• Electron Configuration: [Xe] 4f14 5d5 6s2

Physical:
• Density (near room temperature): 21.02 g.cm-3
• Liquid Density (at melting point): 18.9 g.cm-3
• Melting Point: 3186°C, 5767°F, 3459°K
• Boiling Point: 5596°C, 10105°F, 5869°K
• Heat of Fusion: 60.43 kJ.mol-1
• Heat of Vaporization: 704 kJ.mol-1

Atomic:
• Oxidation States: 7, 6, 5, 4, 3, 2, 1, 0, -1
• Electronegativity: 1.9 (Pauling scale)
• Atomic Radius: 137 picometre
• Covalent Radius: 151±7 picometre
• Ionization Energies: 760 kJ.mol-1 (first), 1260 kJ.mol-1 (second), 2510 kJ.mol-1 (third)

Rhenium is silvery-white in appearance. It is the third element (after tungsten and carbon) with the highest melting point and the fourth densest (after platinum, iridium, and osmium). Commercially, rhenium is traded in powder form. Its principal application is in the making of certain parts of jet engines. Here, the metal is added to high-temperature nickel-based superalloys.

Other uses of rhenium are as follows:

1. As catalysts in making lead-free, high-octane gasoline.
2. As filaments in making ion gauges, mass spectrographs, and photoflash lamps.
3. As electrical contact materials, due to its high resistance to arc corrosion and wear.
4. As catalysts for hydrogenation of fine chemicals, because of its high resistance to chemical poisoning from phosphorus, sulfur, and nitrogen.
5. As treatment for liver cancer, because of its radioactive isotopes.

Rhenium resources are identified in several countries, which include Chile, Peru, Armenia, Mexico, Russia, Canada, Kazakhstan, and Uzbekistan. The estimated total resource from these eight countries is 6,000 tonnes (6 million kilograms).

The United States alone, on the other hand, has an estimated total resource of 5,000 tonnes (5 million kilograms). These are identified in the states of Arizona, Miami, and Utah. But in spite of these significant resources, the U.S. continues to import a big part of its total consumption of the precious metal from some of the countries mentioned above.

Since rhenium and its compounds are used in very small amounts, very little is known about their toxicity. So far, only a few rhenium compounds have been tested for toxicity, and these include rhenium trichloride and potassium perrhenate.

The price of rhenium is about 250 U.S. dollars per troy ounce (about 8,300 U.S. dollars per kilogram).