jarynth
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Aqua regia analogues
I've been entertaining the idea of substituting HF, HBr or HI for the HCl in the preparation of aqua regia.
Although HF in solution is rather corrosive, it's a 'weak' acid; its hazardous nature stems from its biological activity (small size of the molecule
affords quick penetration etc). 'Hydrofluoric acqua regia' would probably also dissolve glass, but what about Au, Pt? To what extent would nitrosyl
fluoride be produced?
According to wikipedia, the reaction of auric ions with hydrochloric acid is an equilibrium reaction which favors formation of chloraurate anions (AuCl4-). The
question comes down to: would this shift be more or less favorable in the case of hydrofluoric acid?
In contrast, HI is a much stronger acid than HCl, so (intuitively) this substitution should yield the best version of aqua regia.
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kclo4
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HI + HNO3 would contain nitrosyl iodide, Iodine, and some more water. NOI is very unstable so it would decompose into I2 and NO almost instantly,
from what I have read. Seems to me, although I don't really know that this combination wouldn't be all that reactive since it would just be a
solution of Iodine (and some precipitate) and some dilute Nitric acid.
Fluorine is more electronegative then Oxygen so i don't think the HF could get oxidized by Nitric acid.
HBr might do the best, since its a stronger acid, but I believe the NOBr is stable enough for practical purposes.
[Edited on 3-10-2008 by kclo4]
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12AX7
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It's my recollection HF + HNO3 can be used with the more stubborn right-hand-side transition metals (e.g. Nb).
Tim
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not_important
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Tim's correct, but it's not really an Aqua Regia analogue. The HF remains HF, or HnFn or H(+) F(-), the fluoride ion complexes with those metals,
removing the passivation layer as fast as it forms while the nitric acid provides the oxidation drive.
SFAIK gold doesn't have the hot for fluorine, AuBr3 has a slightly lower heat of formation than AuCl3, while AuI3 is much lower and slowly loses
iodine to give AuI.
HI is too good of a reducing agent, it does form I2. HBr works, as does aqueous HBr + Br2, or Br2 + HCl, or Br2 + H2SO4, or even H2O2 + HBr or H2O2 +
Na/KBr + moderately strong H2SO4.
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