Protactinium - History

History

In 1871, Dmitri Mendeleev predicted the existence of an element between thorium and uranium. The actinide element group was unknown at the time. Therefore, uranium was positioned below tungsten, and thorium below zirconium, leaving the space below tantalum empty, and until the 1950s periodic tables were published with this structure. For a long time chemists searched for eka-tantalum as an element with similar chemical properties as tantalum, making a discovery of protactinium nearly impossible.

In 1900, William Crookes isolated protactinium as an intensely radioactive material from uranium; however, he could not characterize it as a new chemical element and thus named it uranium-X. Crookes dissolved uranium nitrate in ether, the residual aqueous phase contains most of the 234
90Th and 234
91Pa. His method was still used in the 1950s to isolate 234
90Th and 234
91Pa from uranium compounds. Protactinium was first identified in 1913, when Kasimir Fajans and Oswald Helmuth Göhring encountered the isotope 234Pa during their studies of the decay chains of uranium-238: 238
92U → 234
90Th → 234
91Pa → 234
92U. They named the new element brevium (from the Latin word, brevis, meaning brief or short) because of its short half-life, 6.7 hours for 234
91Pa. In 1917/18, two groups of scientists, Otto Hahn and Lise Meitner of Germany and Frederick Soddy and John Cranston of Great Britain, independently discovered another isotope of protactinium, 231Pa having much longer half-life of about 32,000 years. Thus the name brevium was changed to protoactinium as the new element was part of the decay chain of uranium-235 before the actinium (from Greek: πρῶτος = protos meaning first, before). For ease of pronunciation, the name was shortened to protactinium by the IUPAC in 1949. The discovery of protactinium completed the last gap in the early versions of the periodic table, proposed by Mendeleev in 1869, and it brought to fame the involved scientists.

Aristid von Grosse produced 2 milligrams of Pa₂O₅ in 1927, and in 1934 first isolated elemental protactinium from 0.1 milligrams of Pa₂O₅. He used two different procedures: in the first one, protactinium oxide was irradiated by 35 keV electrons in vacuum. In another method, called the van Arkel–de Boer process, the oxide was chemically converted to a halide (chloride, bromide or iodide) and then reduced in a vacuum with an electrically heated metallic filament:

2 PaI₅ → 2 Pa + 5 I₂

In 1961, the British Atomic Energy Authority (UKAEA) produced 125 grams of 99.9% pure protactinium by processing 60 tonnes of waste material in a 12-stage process, at a cost of about 500,000 USD. For many years, this was the world's only significant supply of protactinium, which was provided to various laboratories for scientific studies. Oak Ridge National Laboratory in the US is currently providing protactinium at a cost of about 280 USD/gram.