bolbol
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Dissolving silicate minerals. HF vs HF/HNO3?
I was curious about how a solution of HF/HNO3 works and how it compares to something like HCl/HNO3 which can dissolve gold by oxidizing it and forming
complex ions. HF by itself can dissolve silicate minerals and in some studies I have read about the use of a HNO3 and HF mixtures.
Any insights?
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fluorescence
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I think there might be several reasons for this. For example etching silicon with this mixture seems to produce dark solutions while treating Niobium
with it makes it turn rather green. I think the compound you are etching has quite a bit of an influence on how the compounds act on it.
I think for the reaction with glass the active compound was the H2F(+) which reacts with the Si-OH endgroups and makes the bridging Oxygen in Si-O-Si
more electron-rich so they take more H(+) and fall apart. NaF for example won't etch glass so H+ needs to be present in solution as well. I remember
there was a statement in a journal once where they said that diluting HF with other acids seems odd at first side as the conc. of H2F(+) is already
quite little in solution but you need the H+ to break the bond and the fluorine only to start it so sometimes the effect can be shifted towards
another compound as soon as the Fluoride has acted.
For silicon + HNO3 + HF there is actually a paper which deals with the various compounds formed...
The found that no Si-F species formed on the silicon but rather, as water was added Si-OH...which seems quite strange, usually I would expect Si-F
bonds to be much more stable....
Also they found the active reagent here were species like NO(+), N2O3, [N4O6](2+) so all with N(III) in them. Especially the last one is more of a
radical which is why the solutions turn dark.
Al these steps have Silicon as the corresponding redox system....
But in your case we already start from oxidized Silicon...so maybe there is another reaction involved or it might be because you can increse the
amount of H(+) in solution as you can use more conc. HNO3 than conc. HF....
One hint I found in a book was that HF works great in dissolving minerals as the Fluorosilicates are soluble in acids. However once you reach
different elements in you minerals like Calcium there is quite a problem in dissolving it any further...like CaF2 is insoluble as well...so you add
other acids to bring them into solution again.
So I'd say in summary it has 3 effects on minerals:
- The other acid might increase the conc of H+ which is required for the Si-O-Si bonds to break
- If the terminated positions can actually react with acids to form different compounds or OH-terminated products less Fluoride is consumed and you
might even get a small catalytic effect.
- If for example Calcium is present in your mineral CaF2 might precipitate which can be dissolved in stronger acids again.
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bolbol
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Interesting stuff. I have tried concentrated nitric acid on a (Ca,Na) aluminosilicate (Labradorite) and after 3 days there didn't seem to be any
changes or reactions occurring. So the HF must be definitely doing something. I am going to try with HF, HNO3, and HF/HNO3 once again to see how the
reaction rates change. Do you know any general book concentrating mainly on the behavior and reactivity of HF?
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