Category Archives: Rhodium

The Six Precious Metals Of The Platinum Group

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In the periodic table of elements, six metallic elements are bunched together in the d-block, specifically in groups 8 to 9, periods 5 and 6. All transition metals, these six elements are collectively referred to as the “platinum group metals”. These precious metals tend to occur with one another in mineral deposits. Likewise, they are alike in both chemical and physical properties.

In the order of their arrangement in the periodic table, the six metallic elements of the platinum group are ruthenium, rhodium, palladium, osmium, iridium, and platinum. A brief description of each of these precious metals is provided below.

1. Ruthenium – This element is represented by the symbol Ru. Its atomic number is 44. It is mostly found in platinum ores and often used in platinum alloys as a catalyst. Its two main physical characteristics refer to its hardness as a metal and to its silvery-white color. The former characteristic makes ruthenium ideal for use in making wear-resistance electrical contacts. The price of this precious metal as of January 2010 is estimated to be about USD173 per troy ounce (USD5,562 per kilogram).

2. Rhodium – This element is represented by the symbol Rh. Its atomic number is 45. Its occurrence is similar to that of ruthenium, and its primary use is as a catalytic converter. Rhodium is considered perhaps the rarest element. It is also known to be the most expensive precious metal, with a price estimated to be about USD2,750 per troy ounce (USD88,415 per kilogram) as of January 2010.

3. Palladium – This element is represented by the symbol Pd. Its atomic number is 46. It is also considered one of the rarest precious metals. Palladium closely resembles its co-member in the platinum group – platinum. It is soft and is silvery-white in color. Like rhodium, palladium is largely used as a catalytic converter. As of January 2010, the price of this metal is estimated to be approximately USD424 per troy ounce (USD13,632 per kilogram).

4. Osmium – This element is represented by the symbol Os. Its atomic number is 76. It is found in nature as an alloy in platinum ores. Osmium is considered the densest natural element. It is brittle and is blue-gray in color. Because of its hardness, osmium is alloyed with the other metals in its group and used in electrical contacts and high-quality fountain pen tips. The price of osmium, as of January 2010, is about USD32.15 per troy ounce (USD12,217 per kilogram).

5. Iridium – This element is represented by the symbol Ir. Its atomic number is 77. Like osmium, iridium is very hard and brittle; it has a different color though – silvery-white. Its principal use is for electrical purposes, mainly because of its density and its high resistance to corrosion even at extremely high temperatures. Iridium is considered the fourth least abundant element in the Earth’s crust, after rhenium, ruthenium, and rhodium. Its price is estimated to be about USD408 per troy ounce (USD13,117 per kilogram) as of January 2010.

6. Platinum – This element, after which this group of precious metals is named, is represented by the symbol Pt. Its atomic number is 78. Platinum is dense, ductile, and malleable; it is gray-white in color. Known to be highly resistant to corrosion, this precious metal is used in jewelry, electrical contacts, and laboratory equipment. Its price, as of January 2010, is USD1,555 per troy ounce (USD49,995 per kilogram), making it the second most expensive precious metal, after rhodium.

Evident from the description of each of these six precious metals is that all of them have outstanding catalytic properties and high resistance to tarnish and wear. These characteristics make any of them well suited for fine jewelry. Additionally, their excellent resistance to extremely high temperatures makes them ideal for many different industrial uses.

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.

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.