Perxenate
Chemical compound
From Wikipedia, the free encyclopedia
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 XeO3 (s) + 4 OH− (aq) → Xe (g) + XeO4−
6 (aq) + O2 (g) + 2 H2O (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, water, and oxygen gas:[6][7]
- 2 HXeO3−6 + 6 H+ → 2 XeO3 + 4 H2O + O2
Its extrapolated formula, H4XeO6, is inferred from the octahedral geometry of the perxenate ion (XeO4−
6) in its alkali metal salts.[6][4]
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 H
3XeO−
6, which has a pKa value of 4.29, still relatively acidic. The twice deprotonated species H
2XeO2−
6 has a pKa value of 10.81.[8] Due to its rapid decomposition under acidic conditions as described above, however, it is most commonly known 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), copper (II) and manganese(II) to (respectively) silver(III), copper(III),[10] and permanganate.[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, 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.[9]