Bullion is a mass of any one of the known precious metals. By strict definition, precious metals are those metallic elements that are rare. Bullion is commonly made of either gold or silver. Its value is determined by the worth of the metal rather than by its face value as money. To put it another way, bullion is valued based on the mass and purity of the metal used, instead of its artificial currency value.

New sources of ore have been discovered and there also have been improvements in the mining and refining processes. These two factors may cause the values of gold, silver, and the other precious metals to diminish. Also, the "precious" qualification of a metal is determined by the market value or high demand.

Bullion is traded on commodity markets in two forms: bulk ingots or coins, the latter minted by the government of a country. At least ten countries are known to mint gold and silver bullion coins. These are Australia, Austria, Canada, China, Mexico, Poland, South Africa, Switzerland, the United Kingdom, and the United States.

While bullion coins are issued as legal tender, with nominal values assigned to them on minting, such face values are far below the commodity value of the metals themselves. Here's an example: Most of the gold coins issued by national governments, particularly those with currency values of between 10 and 100 U.S. dollars, usually contain no less than 31 grams of gold. On the average (considering the consistent rise in the exchange rate of gold), the value of gold is around USD12 per gram. Here, it is clear that the currency value assigned by the government to a gold bullion coin has no meaning.

Below is a list of some of the government-issued gold and silver bullion coins:

1. Australian Gold Nugget, Lunar Series I, and Lunar Series II

2. Austrian Philharmoniker

3. Canadian Maple Leaf

4. Chinese Gold Panda

5. Mexican Centenario, Libertad, and Onza

6. Polish Orzel bielik

7. South African Krugerrand

8. Swiss Vreneli

9. British Britannia and Sovereign

10. American Buffalo, American Eagle, and Double Eagle

The 10,000-dollar Australian Gold Nugget is one of the world's largest bullion coins. Minted by the Australian government, this bullion coin is made of 1 kilogram of 99.9% pure gold. Some other bullion coins larger than the Australian Gold Nugget have come out. However, these are not produced in mass quantities and are not practical to handle. Two examples are given here: One is the 100,000-euro Vienna Philharmonic, minted in 2004, which contains 31 kilograms of gold; the other is the 1 million-dollar Canadian Maple Leaf, minted in 2007, which contains 100 kilograms of gold.

Three factors - metal, purity, and weight - affect the value of bullion. The overall value of bullion is determined by the metal used. We know, of course, that platinum is worth more than gold, which, in turn, is worth more than silver. It is easy to understand, therefore, that silver bullion coins have become popular with collectors because of their relative affordability.

 

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.