Perxenate

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In chemistry, perxenates are salts of the yellow[1] xenon-containing anion XeO4−
6
.[2] This anion has octahedral molecular geometry, as determined by Raman spectroscopy, having O–Xe–O bond angles varying between 87° and 93°.[3] The Xe–O bond length was determined by X-ray crystallography to be 1.875 Å.[4]

Synthesis[edit]

Perxenates are synthesized by the disproportionation of xenon trioxide when dissolved in strong alkali:[5]

2 XeO
3
(s) + 4 OH
(aq) → Xe (g) + XeO4−
6
(aq) + O
2
(g) + 2 H
2
O
(l)

When Ba(OH)
2
is used as the alkali, barium perxenate can be crystallized from the resulting solution.[5]

Perxenic acid[edit]

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:[6][7]

2 HXeO3−
6
+ 6 H+
→ 2 XeO
3
+ 4 H
2
O
+ O
2

Its extrapolated formula, H
4
XeO
6
, is inferred from the octahedral geometry of the perxenate ion (XeO4−
6
) in its alkali metal salts.[6][4]

The pH of aqueous perxenic acid, (pKH
4
XeO
6
), has been indirectly calculated to be < 0, making it a very strong acid. Its first ionization yields H
3
XeO
6
, which has a pK value of 4.29, still relatively acidic. The twice deprotonated species H
2
XeO2−
6
has a pK value of 10.81.[8] Due to its rapid decomposition under acidic conditions as described above, however, it is most commonly encountered as perxenate salts, bearing the anion XeO4−
6
.[6][2]

Properties[edit]

Perxenic acid and the anion XeO4−
6
are both strong oxidizing agents,[9] capable of oxidising silver(I) to silver(III), copper(II) to copper(III),[10] and Mn2+
to MnO
4
.[11] The perxenate anion is unstable in acidic solutions,[10] being almost instantaneously reduced to HXeO
4
.[1]

The sodium, potassium, and barium salts are soluble.[12] Barium perxenate solution is used as the starting material for the synthesis of xenon tetroxide (XeO4) by mixing it with concentrated sulfuric acid:[13]

Ba2XeO6 (s) + 2 H2SO4 (l) → XeO4 (g) + 2 BaSO4 (s) + 2 H2O (l)

Most metal perxenates are stable, except silver perxenate, which decomposes violently.[10]

Applications[edit]

Sodium perxenate, Na
4
XeO
6
, 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.[9]

References[edit]

  1. ^ a b Cotton (2007). Advanced Inorganic Chemistry (6th ed.). Wiley-India. p. 593. ISBN 81-265-1338-1. 
  2. ^ a b Egon Wiberg; Nils Wiberg; Arnold Frederick Holleman (2001). Inorganic chemistry. Academic Press. p. 400. ISBN 0-12-352651-5. 
  3. ^ Peterson, J. L.; Claassen, H. H.; Appelman, E. H. (March 1970). "Vibrational spectra and structures of xenate(VI) and perxenate(VIII) ions in aqueous solution". Inorganic Chemistry 9 (3): 619–621. doi:10.1021/ic50085a037.  edit
  4. ^ a b Hamilton; Ibers, J.; MacKenzie, D. (Aug 1963). "Geometry of the Perxenate Ion". Science 141 (3580): 532–534. Bibcode:1963Sci...141..532H. doi:10.1126/science.141.3580.532. ISSN 0036-8075. PMID 17738629.  edit
  5. ^ a b Charlie Harding; David Arthur Johnson; Rob Janes (2002). Elements of the p block (volume 9 of Molecular world). Royal Society of Chemistry. p. 93. ISBN 0-85404-690-9. 
  6. ^ a b c Klaening, U. K.; Appelman, E. H. (October 1988). "Protolytic properties of perxenic acid". Inorganic Chemistry 27 (21): 3760–3762. doi:10.1021/ic00294a018.  edit
  7. ^ Arnold F. Holleman; Egon Wiberg (2001). Nils Wiberg, ed. Inorganic chemistry. translated by Mary Eagleson, William Brewer. Academic Press. p. 400. ISBN 0-12-352651-5. 
  8. ^ John H. Holloway; Eric G. Hope (1998). A. G. Sykes, ed. Advances in Inorganic Chemistry 46. Academic Press. p. 67. ISBN 0-12-023646-X. 
  9. ^ a b Holcomb, H. P. (March 1965). "Analytical Oxidation of Americium with Sodium Perxenate". Analytical Chemistry 37 (3): 415. doi:10.1021/ac60222a002.  edit
  10. ^ a b c Allen J. Bard; Roger Parsons; Joseph Jordan; International Union of Pure and Applied Chemistry (1985). Standard Potentials in Aqueous Solution. CRC Press. p. 778. ISBN 0-8247-7291-1. 
  11. ^ Linus Pauling (1988). General chemistry (3rd ed.). Courier Dover Publications. p. 251. ISBN 0-486-65622-5. 
  12. ^ Thomas Scott; Mary Eagleson (1994). Concise encyclopedia chemistry. Walter de Gruyter. p. 1183. ISBN 3-11-011451-8. 
  13. ^ Charlie Harding; David Arthur Johnson; Rob Janes (2002). Elements of the p block. Great Britain: Royal Society of Chemistry. pp. 92–93. ISBN 0-85404-690-9.