We continue to review some of the most important materials in heat treatment and metallurgy.
Palladium (chemical symbol: Pd)
Palladium is a rare, lustrous, silvery-white metal (Fig. 1). It is one of a group of six metals that are categorized in the so-called platinum group, which includes palladium, platinum, rhodium, osmium, iridium and ruthenium. These metals have similar properties and are often present in the same mineral ores. Metallic palladium is malleable and ductile, does not tarnish in air and is strongly resistant to corrosion in air as well as the action of most acids at standard temperature.
One remarkable feature of palladium is its capacity to absorb up to 900 times its own volume of hydrogen. Hydrogen will easily pass through heated palladium, a property that allows for the easy purification of hydrogen. As palladium absorbs hydrogen, the metal expands visibly, similar to a sponge swelling up as it takes in water.
Palladium was discovered in 1803 by English chemist William H. Wollaston in London after he examined the residue leftover from dissolving platinum in aqua regia (a concentrated solution of hydrochloric and nitric acids). He later isolated the metal in a series of chemical reactions and eventually heated palladium cyanide to extract the raw palladium element. Wollaston named the element after the recently discovered asteroid at the time, Pallas (in reference to the ancient Greek goddess of wisdom).
The prime use for palladium is in catalytic converters (Fig. 2) for automobiles because it aids in the chemical reaction that converts the pollutants in engine exhaust to less harmful gases as they exit our vehicles. Another common use for palladium is in jewelry. It can be added to gold to create a decolorized variety known as white gold. Furthermore, palladium is used in the dentistry industry to make fillings and crowns; the watchmaking industry to make springs for watches; and in the manufacturing industry, particularly to make surgical instruments and electrical contacts.
Here are a few important facts about palladium.[2,5]
- Atomic number: 46
- Atomic weight: 106.42
- Melting point: 1828.0 K (1554.9°C or 2830.8°F)
- Boiling point: 3236 K (2963°C or 5365°F)
- Density: 12.000 grams per cubic centimeter
- Phase at room temperature: Solid
- Element classification: Metal
- Period number: 5
- Group number: 10
- Group name: None
- Electron configuration: [Kr] 4d10
- Periodic Table of the Elements, Sargent-Welch Catalog No. WLS-18806-10, 2004
- Chemicool (www.chemicool.com)
- Royal Society of Chemistry (rsc.org)