perxenate#Perxenic acid
{{Short description|Chemical compound}}
In chemistry, perxenates are salts of the yellow xenon-containing anion {{chem|XeO|6|4-}}.{{Holleman&Wiberg|page=399}} This anion has octahedral molecular geometry, as determined by Raman spectroscopy, having O–Xe–O bond angles varying between 87° and 93°.{{Cite journal| last1 = Peterson | first1 = J. L.| last2 = Claassen | first2 = H. H.| last3 = Appelman | first3 = E. H.| title = Vibrational spectra and structures of xenate(VI) and perxenate(VIII) ions in aqueous solution| journal = Inorganic Chemistry | volume = 9| issue = 3| pages = 619–621 | date=March 1970 | doi = 10.1021/ic50085a037}} The Xe–O bond length was determined by X-ray crystallography to be 1.875 Å.{{Cite journal | author1 = Hamilton| last3 = MacKenzie| last2 = Ibers | first2 = J. | first3 = D. | title = Geometry of the Perxenate Ion | journal = Science | volume = 141 | issue = 3580 | pages = 532–534 | date=Aug 1963 | issn = 0036-8075 | pmid = 17738629 | doi = 10.1126/science.141.3580.532|bibcode = 1963Sci...141..532H | s2cid = 27297165}}
Synthesis
Perxenates are synthesized by the disproportionation of xenon trioxide when dissolved in strong alkali:{{cite book
| title = Elements of the p Block |volume=9 |series=Molecular World
| first1= Charlie |last1=Harding
| first2= David Arthur |last2=Johnson
| first3= Rob |last3=Janes
| publisher = Royal Society of Chemistry
| year = 2002
| isbn = 0-85404-690-9
| page = 93
}}
:2 XeO3 ({{abbr|s|solid}}) + 4 OH− ({{abbr|aq|aqueous solution}}) → Xe ({{abbr|g|gas}}) + {{chem|XeO|6|4-}} ({{abbr|aq|aqueous solution}}) + O2 ({{abbr|g|gas}}) + 2 H2O ({{abbr|l|liquid}})
When Ba(OH)2 is used as the alkali, barium perxenate can be crystallized from the resulting solution.
Perxenic acid
File:Perxenonsäure Structural Formula V.1.svg
Perxenic acid is the unstable conjugate acid of the perxenate anion, formed by the solution of xenon tetroxide in water. It has not been isolated as a free acid, because under acidic conditions it rapidly decomposes into xenon trioxide and oxygen gas:{{Cite journal | last1 = Klaening | first1 = U. K.|last2 = Appelman | first2 =E. H.| title = Protolytic properties of perxenic acid| journal = Inorganic Chemistry | volume = 27 | issue = 21| pages = 3760–3762 | date=October 1988 | doi = 10.1021/ic00294a018}}{{Holleman&Wiberg|page=400}}
:{{chem2|2 HXeO6(3-) + 6 H+ → 2 XeO3 + 4 H2O + O2}}
Its extrapolated formula, H4XeO6, is inferred from the octahedral geometry of the perxenate ion ({{chem|XeO|6|4-}}) in its alkali metal salts.
The pKa of aqueous perxenic acid has been indirectly calculated to be below 0, making it an extremely strong acid. Its first ionization yields the anion {{chem|H|3|XeO|6|-}}, which has a pKa value of 4.29, still relatively acidic. The twice deprotonated species {{chem|H|2|XeO|6|2-}} has a pKa value of 10.81.{{cite book
| title = Advances in Inorganic Chemistry
| volume = 46
| author1 = John H. Holloway
| author2 = Eric G. Hope
| editor = A. G. Sykes
| publisher = Academic Press
| year = 1998
| isbn = 0-12-023646-X
| page = 67
}} Due to its rapid decomposition under acidic conditions as described above, however, it is most commonly known as perxenate salts, bearing the anion {{chem|XeO|6|4-}}.
Properties
Perxenic acid and the anion {{chem|XeO|6|4-}} are both strong oxidizing agents,{{Cite journal| first1 = H. P. | title = Analytical Oxidation of Americium with Sodium Perxenate | journal = Analytical Chemistry | volume = 37| issue = 3 | page = 415| last1 = Holcomb | date=March 1965 | doi = 10.1021/ac60222a002}} capable of oxidising silver(I), copper (II) and manganese(II) to (respectively) silver(III), copper(III), and permanganate.{{cite book
| title = General Chemistry
| author = Linus Pauling
| edition = 3rd
| publisher = Courier Dover Publications
| year = 1988
| isbn = 0-486-65622-5
| page = [https://archive.org/details/generalchemistry00paul_0/page/251 251]
| url-access = registration
| url = https://archive.org/details/generalchemistry00paul_0/page/251
}} The perxenate anion is unstable in acidic solutions, being almost instantaneously reduced to {{chem|HXeO|4|-}}.{{Cotton&Wilkinson6th|page=593}}
The sodium, potassium, and barium salts are soluble.{{cite book
| title = Concise encyclopedia chemistry
| author = Thomas Scott
| author2 = Mary Eagleson
| publisher = Walter de Gruyter
| year = 1994
| isbn = 3-11-011451-8
| page = 1183
}} Barium perxenate solution is used as the starting material for the synthesis of xenon tetroxide (XeO4) by mixing it with concentrated sulfuric acid:{{cite book
| title = Elements of the p block
| author = Charlie Harding
| author2 = David Arthur Johnson
| author3 = Rob Janes
| location = Great Britain
| publisher = Royal Society of Chemistry
| year = 2002
| isbn = 0-85404-690-9
| pages = 92–93
}}
: Ba2XeO6 (s) + 2 H2SO4 (l) → XeO4 (g) + 2 BaSO4 (s) + 2 H2O (l)
Most metal perxenates are stable, except silver perxenate, which decomposes violently.{{cite book
| title = Standard Potentials in Aqueous Solution
| author = Allen J. Bard
| author2 = Roger Parsons
| author3 = Joseph Jordan
| author4 = International Union of Pure and Applied Chemistry
| publisher = CRC Press
| year = 1985
| isbn = 0-8247-7291-1
| page = [https://archive.org/details/standardpotentia0000unse/page/778 778]
| url = https://archive.org/details/standardpotentia0000unse/page/778
}}
Applications
Sodium perxenate, Na4XeO6, can be used for the analytic separation of trace amounts of americium from curium. 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, which forms insoluble fluorides with Cm3+ and La3+, but retains Am4+ and Pu4+ in solution as soluble fluorides.