I know that noble metals such as gold/platinum are unaffected by conc. mineral acids such as HCl or H2SO4 or HNO3 but Can be dissolved in aqua regia (mix of HCl and HNO3 in 3:1), but never heard about HF .Can conc. HF (100% pure) dissolved noble metals.If yes, what will be the products NOTE:-I only mean 100% HF (superacid) not aqueous HF.
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2$\begingroup$ In short, no. Besides, HF is not much of a superacid. $\endgroup$– Ivan NeretinNov 1, 2018 at 8:57
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2$\begingroup$ Pure HF is a superacid due to H2F+ and have a H_0 of -15.1 . Refer Wikipedia $\endgroup$– Harsh jainNov 1, 2018 at 9:07
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$\begingroup$ Yeah, sure, but it is not all that much when we talk about superacids. $\endgroup$– Ivan NeretinNov 1, 2018 at 9:12
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2$\begingroup$ It isn't the acidity of aqua regis that enables it to dissolve precious metals: it is its oxidising ability combined with the solubilising nature of the chloride in solution. HF doesn't oxidise and doesn't solubilise. Which is why it doesn't work here. $\endgroup$– matt_blackNov 2, 2018 at 17:56
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2 Answers
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A look at the Nernst equation shows that increasing proton activity is of only limited impact in dissolving noble metals via hydrogen displacement. Say you have a metal $M$ for which the standard reduction potential of $M^{n+}$ is $-1\text{ V}$ and an acid whose proton activity is $15$ orders of magnitude greater than $1\text{ M }\ce{HCl(aq)}$. Then the equilibrium potential for generating one atmosphere hydrogen gas at $25°\text{C}$ with the metals in the standard states is given by:
$E=-1+\frac{(2.303)(15)(8.314)(298.15)}{96487}=-0.11\text{ V}$
and the reaction remains unfavorable even with a fairly good superacid. And the most noble metals go well below $-1\text{ V}$ in that first term.
So we use a stronger oxidizing agent like nitric acid to help things along.
Hydrofluoric acid can be used to break passive oxide films on some substrates. For example, a combination of hydrofluoric and nitric acid is one way to dissolve silicon, which would otherwise form a passive oxide. See here for the reaction, and a Google search on "silicon etching" gives many references (most of the good ones, of course, behind paywalls).
answered Nov 1, 2018 at 10:20
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Gold or platinum is more or less unaffected by hydrofluoric acid even at elevated temperatures. From the handbook of corrosion data:
The noble metals including gold, silver or platinum is more or less unaffected by anhydrous hydrogen fluoride or aqueous hydrofluoric acid of any concentration at ambient temperature.[...] The resistance of silver to hydrofluoric acid can only be affected in presence of sulfides and oxygen. Silver is not passivated by hydrofluoric acid as it is by other halogen acids. Platinum and gold are unaffected at high temperature.
For more information, see this yahoo answer or this reddit post.
answered Nov 1, 2018 at 14:31