In chemistry, perxenates are

salt Salt is a mineral composed primarily of sodium chloride (NaCl), a chemical compound belonging to the larger class of salts; salt in the form of a natural crystalline mineral is known as rock salt or halite. Salt is present in vast quanti ...

s of the yellow

xenon Xenon is a chemical element with the symbol Xe and atomic number 54. It is a dense, colorless, odorless noble gas found in Earth's atmosphere in trace amounts. Although generally unreactive, it can undergo a few chemical reactions such as the ...

-containing anion . This anion has

octahedral molecular geometry In chemistry, octahedral molecular geometry, also called square bipyramidal, describes the shape of compounds with six atoms or groups of atoms or ligands symmetrically arranged around a central atom, defining the vertices of an octahedron. The ...

, as determined by

Raman spectroscopy Raman spectroscopy () (named after Indian physicist C. V. Raman) is a spectroscopic technique typically used to determine vibrational modes of molecules, although rotational and other low-frequency modes of systems may also be observed. Raman s ...

, having O–Xe–O bond angles varying between 87° and 93°. The Xe–O bond length was determined by X-ray crystallography to be 1.875 Å.

Synthesis

Perxenates are synthesized by the

disproportionation In chemistry, disproportionation, sometimes called dismutation, is a redox reaction in which one compound of intermediate oxidation state converts to two compounds, one of higher and one of lower oxidation states. More generally, the term ca ...

of xenon trioxide when dissolved in strong

alkali In chemistry, an alkali (; from ar, القلوي, al-qaly, lit=ashes of the saltwort) is a basic, ionic salt of an alkali metal or an alkaline earth metal. An alkali can also be defined as a base that dissolves in water. A solution of ...

: :2 XeO₃ () + 4 OH⁻ () → Xe () + () + O₂ () + 2 H₂O () When Ba(OH)₂ is used as the alkali, barium perxenate can be crystallized from the resulting solution.

Perxenic acid

Perxenic acid is the unstable conjugate acid of the perxenate anion, formed by the solution of

xenon tetroxide Xenon tetroxide is a chemical compound of xenon and oxygen with molecular formula XeO4, remarkable for being a relatively stable compound of a noble gas. It is a yellow crystalline solid that is stable below −35.9 ° C; above that temper ...

water Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as ...

. It has not been isolated as a free acid, because under acidic conditions it rapidly decomposes into xenon trioxide and

oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as we ...

gas: : Its extrapolated formula, H₄XeO₆, is inferred from the octahedral geometry of the perxenate ion () in its alkali metal salts. The p''K''_a of aqueous perxenic acid has been indirectly calculated to be below 0, making it an extremely strong acid. Its first ionization yields the anion , which has a p''K''_a value of 4.29, still relatively acidic. The twice deprotonated species has a p''K''_a value of 10.81. Due to its rapid decomposition under acidic conditions as described above, however, it is most commonly known as perxenate salts, bearing the anion .

Properties

Perxenic acid and the anion are both strong

oxidizing agent An oxidizing agent (also known as an oxidant, oxidizer, electron recipient, or electron acceptor) is a substance in a redox chemical reaction that gains or " accepts"/"receives" an electron from a (called the , , or ). In other words, an oxid ...

s, capable of oxidising silver(I) to silver(III), copper(II) to copper(III), and manganese(II) to

permanganate A permanganate () is a chemical compound containing the manganate(VII) ion, , the conjugate base of permanganic acid. Because the manganese atom is in the +7 oxidation state, the permanganate(VII) ion is a strong oxidizing agent. The ion is a tr ...

. The perxenate anion is unstable in acidic solutions, being almost instantaneously reduced to . The

sodium Sodium is a chemical element with the symbol Na (from Latin ''natrium'') and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 of the periodic table. Its only stable ...

potassium Potassium is the chemical element with the symbol K (from Neo-Latin '' kalium'') and atomic number19. Potassium is a silvery-white metal that is soft enough to be cut with a knife with little force. Potassium metal reacts rapidly with atmosp ...

, and

barium Barium is a chemical element with the symbol Ba and atomic number 56. It is the fifth element in group 2 and is a soft, silvery alkaline earth metal. Because of its high chemical reactivity, barium is never found in nature as a free element. ...

salts are soluble. Barium perxenate solution is used as the starting material for the synthesis of

(XeO₄) by mixing it with concentrated sulfuric acid: : Ba₂XeO₆ (s) + 2 H₂SO₄ (l) → XeO₄ (g) + 2 BaSO₄ (s) + 2 H₂O (l) Most metal perxenates are stable, except silver perxenate, which decomposes violently.

Applications

Sodium perxenate, Na₄XeO₆, can be used for the analytic separation of trace amounts of

americium Americium is a synthetic radioactive chemical element with the symbol Am and atomic number 95. It is a transuranic member of the actinide series, in the periodic table located under the lanthanide element europium, and thus by analogy was na ...

from

curium Curium is a transuranium element, transuranic, radioactive decay, radioactive chemical element with the chemical symbol, symbol Cm and atomic number 96. This actinide element was named after eminent scientists Marie Curie, Marie and Pierre Curie ...

. The separation involves the oxidation of Am³⁺ to Am⁴⁺ by sodium perxenate in acidic solution in the presence of La³⁺, followed by treatment with

calcium fluoride Calcium fluoride is the inorganic compound of the elements calcium and fluorine with the formula CaF2. It is a white insoluble solid. It occurs as the mineral fluorite (also called fluorspar), which is often deeply coloured owing to impurities. ...

, which forms insoluble fluorides with Cm³⁺ and La³⁺, but retains Am⁴⁺ and Pu⁴⁺ in solution as soluble fluorides.

References

Oxyanions Salts Xenon(VIII) compounds Octahedral compounds {{inorganic-compound-stub