Palladium (Pd)

• Element Symbol: Pd
• Atomic Number: 46
• Atomic Mass: 106.42
• Group # in Periodic Table: 10
• Group Name: Transition metals
• Period in Periodic Table: 5
• Block of Periodic Table: d-block
• Discovered by: William Hyde Wollaston (1803)

Palladium has an atomic number of 46, and its symbol is Pd. Found in Group 10 of the periodic table, it is a lustrous silvery-white metal. Palladium was discovered by British scientist William Hyde Wollaston in 1803. Wollaston named the element after the asteroid Pallas. Palladium is part of the platinum group of metals that also includes platinum, rhodium, ruthenium, iridium, and osmium. However, palladium is the least dense and has the lowest melting point of all the other platinum metals.

William Hyde Wollaston first discovered palladium when studying some crude platinum ore from South America. The ore was dissolved by Wollaston in a mixture of hydrochloric and nitric acids known as aqua regia. He then neutralized this mixture with sodium hydroxide (NaOH). The platinum that precipitated out of this solution was treated with ammonium chloride to get ammonium chloroplatinate. Next, the ammonium chloroplatinate was treated with mercuric cyanide to get palladium cyanide. Finally, the element palladium was obtained by heating the palladium cyanide.

Wollaston shared his discovery in an unconventional manner because he didn’t want to reveal the process he used to isolate the element. After leaving the metal for sale with a mineral dealer in London’s Gerrard Street, Wollaston anonymously distributed handbills across London that described the properties of this new metal. Some people, such as the decorated chemist Richard Chenevix, were suspicious of the properties that Wollaston claimed for palladium; instead, Chenevix proclaimed palladium to be an alloy of platinum. To back up his own claim, Wollaston anonymously offered twenty guineas to anyone who could artificially produce the element. There were a number of attempts to meet this challenge, but none were successful, so the reward remained unclaimed. Then in 1805, Wollaston went before the Royal Society in London, where he spoke about the properties of palladium and how its isolation from platinum could be successfully carried out. At the conclusion of his talk, he revealed that he himself was the discoverer of palladium. Wollaston said that remaining anonymous was necessary because it gave him the time he needed to understand and explain the metal’s properties.

Physical Properties

Palladium resembles platinum because it is likewise a soft, silver-white color. As an alternative to platinum, it has been used in jewelry since 1939 as a precious metal for making white gold. Because palladium has a naturally white color, it requires no rhodium plating. Palladium’s hardness rating on the Mohs scale is 4.75. Palladium is 10% harder than platinum. Palladium’s malleability is similar to that of gold, and it can be beaten into a thin leaf. Palladium is a solid at 298 K, the temperature that measures an element’s standard state. The density of palladium near room temperature is 12.023 grams per cubic centimeter. Its melting point is 1554.9°C, and its boiling point is 2963°C. The specific gravity of palladium is 12.02 grams per cubic centimeter, and its specific heat is 240 joules per kilogram. The thermal conductivity of palladium is 71.8 watts per meter kelvin. The electrical conductivity of palladium is 1 × 10⁷ siemens per meter. The resistivity of palladium is 1 × 10^-7 per ohm meters. Bulk palladium is nonmagnetic.

Chemical Properties

Palladium has a face-centered cubic crystal structure. Because it is the most reactive element of the platinum group, palladium is called the least noble of the noble metals. Ground into a powder, palladium will catch fire, although under normal conditions, it combines poorly with oxygen. Palladium reacts when mixed with hot acids, but it does not react with acids at room temperature. When very hot, it will also combine with chlorine and fluorine. The six stable isotopes of naturally occurring palladium are palladium-102, palladium-104, palladium-105, palladium-106, palladium-108, and palladium-110. With a half-life of 6.5 million years, 17 days, and 3.63 days, respectively, palladium-107, palladium-103, and palladium-100 are the most stable radioisotopes. Electron capture is the primary decay mode for those isotopes below the most abundant stable isotope (namely, palladium-106); beta decay is the primary mode for those isotopes above palladium-106. Rhodium is the primary decay product below palladium-106, and silver is the primary product above palladium-106.

Applications

Palladium is found in nature as a free metal. It also shows up as an alloy combined with other metals, such as gold, platinum, and other platinum-group metals. Palladium’s abundance in Earth’s crust is about 0.015 parts per million. The world’s demand for palladium is satisfied by a dual process of extraction and refining of the palladium-bearing ores. Furthermore, recycled materials that contain recoverable quantities of palladium, such as the palladium used in catalytic converters, also satisfy the demand for this metal. Palladium has been discovered in placer deposits in Russia’s Ural Mountains and also in certain parts of South and North America, Ethiopia, and Australia. Palladium is produced on a commercial scale using the ore coming from the nickel-copper deposits found in Ontario, Canada, in South Africa, and in Siberia. Even though there is little obtainable palladium in nickel-copper ore, large volumes of this ore are available that can be processed in a way that makes the extraction profitable.

Palladium can absorb up to nine hundred times its own weight in hydrogen gas. For this reason, the element serves as a good catalyst for various reactions, such as hydrogenation, dehydrogenation, and petroleum cracking. Finely ground palladium is used in hydrogenation, which involves the addition of hydrogen atoms, and dehydrogenation, which involves removal of hydrogen atoms. Today, palladium is most often used in catalytic converters for automobiles. Palladium alloys are also used in the jewelry industry. Apart from nickel, palladium is the other metal that, when alloyed with gold, produces white gold. Often, palladium is used as a replacement in jewelry for platinum.

Palladium is used to make surgical instruments, dental fillings and crowns, springs for watches, and electrical contacts. Palladium is used in health care. Palladium-103, for example, has been used in a procedure that allows a patient to avoid a prostatectomy; in this procedure, the palladium-103 is used in a technique referred to as brachytherapy, also known as seeding. This isotope also has applications in radiation therapy for uveal melanoma. In the field of pharmacy, palladium plays a significant role in blood glucose testing, a procedure in which palladium electrodes are used in meters to test blood glucose. Palladium-110 and palladium-108 have been employed in physical experiments such as the study of the decay of neodymium-137 and nuclear fusion.

Bibliography

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Krebs, Robert E. The History and Use of Our Earth’s Chemical Elements: A Reference Guide. Westport: Greenwood, 2006. Print.

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"Palladium: The Essentials." WebElements. Mark Winter and The University of Sheffield, n.d. Web. 25 Nov. 2015. <http://www.webelements.com/palladium/>.

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