Perxenate
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
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
2O (l)
When Ba(OH)
2 is used as the alkali, barium perxenate can be crystallized from the resulting solution.[5]
Perxenic acid
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
2O + O
2
Its extrapolated formula, H
4XeO
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
4XeO
6), has been indirectly calculated to be < 0, making it a very strong acid. Its first ionization yields H
3XeO−
6, which has a pK value of 4.29, still relatively acidic. The twice deprotonated species H
2XeO2−
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
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
Sodium perxenate, Na
4XeO
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
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