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Hello,
Does fluoroantimonic acid dissolve noble metals? Nernst equation suggests that platinum would require pH ≈ −20.1 to be dissolved, using Hammett acidity function as an extension of pH scale it seems that it should dissolve in fluoroantimonic acid. On the other hand possiblility of dissolving gold depends on the exact value of acidity function as it would require pH ≈ −25.7 to be dissolved (fluoroantimonic is sometimes stated to have H0 = −31.3). However wikipedia says that Hammett acidity function of fluoroantimonic acid is in range: −21 > H0 > −23.

Calculations:

Reaction                    Standard electrode potential "pH" required to dissolve
$Pt^{2+}+2e^-\rightarrow Pt$ $E^0=1.188V$ $pH=-\frac{1.188}{0.05916}\approx-20.1$
$Au^{3+}+3e^-\rightarrow Au$ $E^0=1.52V$ $pH=-\frac{1.52}{0.05916}\approx-25.7$

Sources:

  1. https://en.wikipedia.org/wiki/Standard_electrode_potential_(data_page)
  2. https://en.wikipedia.org/wiki/Nernst_equation
  3. https://en.wikipedia.org/wiki/Hammett_acidity_function
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    $\begingroup$ I, for one, would not use potentials in aqueous solutions to predict redox thermodynamics in what is clearly a nonaqueous system (presumably HF solvent). $\endgroup$ Nov 21 at 2:12
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    $\begingroup$ One of the components of fluoroantimonic acid, antimony pentafluoride ($\ce{SbF5 }$) is itself quite a strong oxidiser, and may end up being what causes the dissolution to occur, if any. $\endgroup$ Nov 21 at 6:25
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(Partial answer. Maybe someone can post a complete answer)

Strength of an acid does not necessarily indicate its corrosiveness. Very strong acids like carborane acid will barely react with many materials, while weak acids like hydrofluoric acid can easily eat through glass. Theoretically, fluoroantimonic acid should react with metals but they are rather slow and dissolve much lesser amount of metal when compared to other acids (here is a comparison of how much steel will 1kg of fluoroantimonic acid dissolve vs 1kg sulfuric acid). For a metal to dissolve in acid, the acid needs to have some oxidizing power, and fluoroantimonic acid doesn't really have any, aside from the dissociated hydrogen ions themselves. The hydrogen ions in fluoroantimonic acid are not potent enough oxidizers to corrode metals. There can be another reason of not reacting violently because of formation of an oxide layer which will prevent the metal from reaction. Some may even form insoluble fluoride salts which will settle in acids and does not react further.

For comparison, see this video of the reaction of manganese in triflic acid

Also see this video: THE STRONGEST ACID IN THE WORLD Fluoroantimonic acid (shows the reaction of $\ce{HSbF6}$ with magnesium, sodium and potassium)

More links: Quora, Reddit

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  • $\begingroup$ I don't see really any base for your claims, just speculation. In particular this link to some random forum is lame. $\endgroup$
    – Mithoron
    Nov 24 at 16:31
  • $\begingroup$ While passivation with fluoride may be possible here, same can't be told about oxide. You also undermine your point mentioning aggressiveness of HF, which is indeed the main component of fluoroantimonic acid. $\endgroup$
    – Mithoron
    Nov 24 at 16:37

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