Palladium is a lustrous silver-white metal with element symbol Pd and atomic number 46. It has important uses, ranging from electronics to catalysis. Common uses include jewelry and catalytic converters.
Discovery, Naming, and Isolation
William Hyde Wollaston discovered palladium in 1902 and named it after the asteroid Pallas, which was discovered two months earlier. Wollaston isolated the element from crude platinum ore from South America. His method involved dissolving the ore in aqua regia, neutralizing the solution with sodium hydroxide, and then precipitating ammonium chloroplatinate using ammonium chloride. Reacting this compound with mercuric cyanide formed palladium(II) cyanide, which yielded palladium metal upon heating.
Appearance and Properties
Palladium is a shiny, silvery-white metal that has a similar appearance to platinum, which sits below it on the periodic table. It is ductile, malleable, and has the unusual ability to absorb hydrogen. Palladium is soft and has a relatively low melting point for a transition metal. It resists corrosion and remains stable under normal conditions.
Element Group
Palladium is a transition metal that belongs to the platinum group metals (PGMs) in group 10 of the periodic table. Members of this group resemble the element platinum in terms of their color and excellent corrosion resistance and catalytic properties.
Natural and Radioactive Isotopes
Natural palladium is a mixture of seven isotopes, of which six are stable and one is radioactive. The stable isotopes are 102Pd, 104Pd, 105Pd, 106Pd, 108Pd, and 110Pd. Among these, 106Pd is the most abundant. 107Pd is a radioisotope with half-life of 6.5 million years. Over 20 additional radioisotopes have been characterized, ranging in atomic weight from 91Pd to 129Pd.
Abundance and Sources
Palladium is relatively rare in the Earth’s crust. It is a primordial element, meaning it existed in the interstellar medium that formed the solar system. Its primary sources include nickel-copper deposits in South Africa and Russia, and it is often produced as a by-product of mining these metals. Other sources occur in Montana and Ontario. Minerals that contain palladium include polarite and cooperite. Another source of the element is from spent nuclear fuel.
Purification
Purification of palladium involves complex processes like solvent extraction, precipitation, and refining. For example, one notable method is the Mond process, which involves converting ore into palladium(II) chloride and then reducing it to purified metal.
Uses of Palladium
Uses of palladium rely on its catalytic properties, corrosion resistance, and hydrogen absorption. The most common use of palladium is in catalytic converters. Some uses include:
- Catalysis: Palladium is widely used in catalytic converters in automobiles and in chemical processes, such as petroleum cracking, hydrogenation, and dehydrogenation.
- Electronics: It is used in electronic components like multilayer ceramic capacitors, watches, and electrical contacts.
- Jewelry: Palladium is an essential component in white gold and is used in making fine jewelry, often as a coating over sterling silver.
- Hydrogen Storage: Its ability to absorb hydrogen makes it useful in fuel cell technology.
- Tools and Equipment: Palladium finds use in surgical instruments and spark plugs.
- Music: Some concert flutes consist of palladium.
- Medicine: Uses include dental amalgam, blood sugar test strips, and medical implants.
- Currency: Like gold, silver, and platinum, palladium bullion also has an ISO currency code.
Oxidation States and Main State
Palladium exhibits oxidation states from 0 to +4. The 0, +2, and +4 states are the most common and stable.
Biological Role, Health Effects, and Toxicity
Palladium plays no known biological role. While palladium compounds are relatively low in toxicity, inhalation or prolonged exposure causes health issues, including respiratory problems, skin and mucous membrane irritation, and skin allergies. People who are allergic to nickel typically have issues with palladium, too.
Key Palladium Facts for Scientists
| Property | Detail |
|---|---|
| Name | Palladium |
| Symbol | Pd |
| Atomic Number | 46 |
| Atomic Weight | 106.42 |
| Group | 10 |
| Period | 5 |
| Block | d-block |
| Electron Configuration | [Kr] 4d10 |
| Electrons per Shell | 2, 8, 18, 18 |
| State of Matter at Room Temp. | Solid |
| Melting Point | 1554.9 °C |
| Boiling Point | 2963 °C |
| Density | 12.023 g/cm³ |
| Heat of Fusion | 16.74 kJ/mol |
| Heat of Vaporization | 358 kJ/mol |
| Molar Heat Capacity | 25.98 J/(mol·K) |
| Oxidation States | 0, +1, +2, +3, +4 |
| Electronegativity | Pauling scale: 2.20 |
| Ionization Energies | 1st: 804.4 kJ/mol |
| Atomic Radius | 137 pm |
| Covalent Radius | 139 pm |
| Van der Waals Radius | 163 pm |
| Crystal Structure | Face-centered cubic (fcc) |
| Thermal Conductivity | 71.8 W/(m·K) |
| Electrical Resistivity | (20 °C) 10.8 nΩ·m |
| Young’s Modulus | 121 GPa |
| Shear Modulus | 44 GPa |
| Bulk Modulus | 180 GPa |
| Mohs Hardness | 4.75 |
| Magnetic Ordering | Paramagnetic |
Interesting Palladium Facts
- Hydrogen Permeability: Palladium has the highest hydrogen permeability of any metal. This unique property selectively allows hydrogen gas to pass through while blocking other gases, making it useful in hydrogen purification processes.
- Cold Fusion Debate: Palladium was at the center of the controversial “cold fusion” claim in 1989. Researchers Martin Fleischmann and Stanley Pons reported that they had achieved nuclear fusion at room temperature using a palladium electrode.
- Color-Changing Properties: When palladium absorbs hydrogen, it changes color. This property finds use in sensors that detect hydrogen gas.
- Space Exploration: The isotope Palladium-103 occurs in some nuclear batteries, which power spacecraft and satellites.
- Photography: In the early days of photography, palladium salts produced high-quality photographic prints, known as palladium prints. These prints are highly stable and have a wide tonal range.
- Historical Monetary Value: For a brief period in the late 20th century, Russia issued palladium coins as legal tender.
- Medical Applications: Palladium finds use in medicine for its radiopaque qualities.
- Radioisotope Dating: Palladium-107 is one of the longest-lived isotopes used in geological dating.
- Rare Meteorite Component: Palladium occurs in higher concentrations in meteorites than in the Earth’s crust.
- Cultural Significance: In some cultures, palladium jewelry is popular in wedding ceremonies. The metal has associations with good fortune and prosperity.
References
- Garrett, Christine E.; Prasad, Kapa (2004). “The Art of Meeting Palladium Specifications in Active Pharmaceutical Ingredients Produced by Pd-Catalyzed Reactions”. Advanced Synthesis & Catalysis. 346 (8): 889–900. doi:10.1002/adsc.200404071
- Kielhorn, Janet; Melber, Christine; Keller, Detlef; Mangelsdorf, Inge (2002). “Palladium – A review of exposure and effects to human health”. International Journal of Hygiene and Environmental Health. 205 (6): 417–32. doi:10.1078/1438-4639-00180
- Powers, David C; Ritter, Tobias (2011). “Palladium(III) in Synthesis and Catalysis”. Higher Oxidation State Organopalladium and Platinum Chemistry. Topics in Organometallic Chemistry. 35: 129–156. ISBN 978-3-642-17428-5. doi:10.1007/978-3-642-17429-2_6
- Weast, Robert (1984). CRC, Handbook of Chemistry and Physics. Boca Raton, Florida: Chemical Rubber Company Publishing. ISBN 0-8493-0464-4.
- Wollaston, W. H. (1804). “On a New Metal, Found in Crude Platina”. Philosophical Transactions of the Royal Society of London. 94: 419–430. doi:10.1098/rstl.1804.0019