Brotato
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Fluoroantimonic acid, Anhydrous Hydrogen Fluoride, and Antimony Pentachloride
Fluoroantimonic acid has very little practical applications however one very interesting use case is the formation of an intermediary adduct in the
production of BN17, an experimental explosive containing one atom of boron and 17 nitrogens ! Anyways lets get into it, starting from anhydrous HF
which has an extremely large number of applications (read Ullmann’s Encyclopedia of Industrial Chemistry to learn more).
Hydrogen fluoride produced by the traditional fluorospar + sulfuric acid process is the most economical for large scale production in rotary kilns but
is labelled as crude as it contains certain silicates that form SiF4, an issue for fluorine cells which need greater than 99.9% pure HF in order to
run without the necessity of nickel anode pre electrolysis. Industrially this is solved via additional distillation until a sufficiently pure product
can be obtained. However on the lab scale, this can be done far more efficiently.
NaHF2 is a commercial product that is sold in the anhydrous state as an industrial side product of scrubbing HF waste gasses either from fluorine or
in waste towers. This product therefore can be obtained in its already anhydrous state from chemical suppliers for a reasonable price (15 USD/ 500
grams). Heating this compound to 250C both melts it and liberates NaF and HF as a gas, this gas can be separated off and liquified directly to afford
a totally anhydrous and pure product.
All operations were carried out in a controlled atmosphere glovebox with scrubbers and emergency external venting. Including a layer of butyl gloves,
an inner liner of Honeywell North Silvershield gloves and an additional layer of butyl were worn. Both a fire extinguisher and calgonate (calcium
gluconate) gel were kept at a hands reach throughout the described procedures. All reagents were ACS grade and used as received.
The setup for distillation included a 500 mL steel reactor vessel with an output barb. This barb was connected to a PFA tube which was led through a
tight fitting (must be the perfect size !) glass liebig condenser which was cooled to -30C. This PFA tube was led into a steel sample storage cylinder
with a rated pressure of 350 bar which was submerged in a dry ice/acetone bath at -78 C.
Attachment: phpt208y0 (7.7MB)
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Overall 150 grams of NaHF2 was decomposed and 48 grams of anhydrous HF was collected as a fuming liquid.
Also pictured here is the vessel after washing and drying and some slight etching due to formation of a passivation layer is noted but no significant
degradation is noted.
Attachment: phpoedrdQ (4.5MB)
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Antimony Pentachloride →
50 grams of Antimony Trioxide was placed in a 250 ml flask fitted with a dimroth condenser and a drying tube is cooled to -20C. 39 mLs of thionyl
chloride was slowly added (NOTE: The addition must be extremely slow and in very small portions, the formed antimony trichloride will begin refluxing
immediately). Afterwards, a black solution is obtained which is refluxed for an additional 6 hrs then subjected to distillation to obtain 68 grams of
a slightly yellowish solid, antimony trichloride. (NOTE: The reaction is so vigorous that a slight amount of antimony pentachloride was likely formed
which decomposed quickly to chlorine gas and the trichloride thus the color). This obtained solid was then melted and subjected to a dry stream of
chlorine gas until it gained the theoretical mass. ~89 grams of a yellow liquid was obtained, upon cooling to 0C it solidified completely.
Antimony Trichloride: Attachment: phpwwEXXs (1.9MB)
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Antimony Pentachloride: Attachment: phpWmfRCW (2.3MB)
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Fluoroantimonic acid →
To a 25 mL PTFE bottle (pre chilled to -78C) was added slowly an excess portion of anhydrous HF, then extremely slowly small drops of antimony
pentachloride were introduced causing extremely vigorous bubbling (analogous to adding dry ice to room temp acetone) of HCl gas. A slight excess of HF
was used corresponding to a 1:1 v/v solution of the two liquids.
The obtained fluoroantimonic acid fumes so vigorously in air that it is difficult to obtain pictures from the top of the bottle however it is visible.
Two such batches were formed and it was stored over soda lime pellets to avoid HF leakage into storage areas. It is crucial to make sure that this
reaction is conducted properly in an adequate size vessel with extremely slow addition. Too prompt of an addition rate leads to explosions involving
the HF vaporizing out of the tube like the exhaust of a souyz rocket.
Attachment: phpJeb1tu (1.8MB)
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Attachment: phpc5YU7W (2MB)
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Future →
Fluorine production is the next goal as anhydrous HF can be obtained in high purities as well as KHF2 which will be purchased in >99% purity. More
concrete details are being worked out in design software, but the main difficulty seems to be sourcing non graphitic carbon electrodes. Fluoride ions
permeate the sheet structure of graphite at a significantly higher rate than with petroleum coke or vitreous carbon electrodes leading to swelling and
more importantly anodic polarization or “the anode effect” (formation of CFx films at the interface of the anode and the electrolyte). Lithium
ions work in the electrolyte at this boundary to help lower this effect but it seems to depend greatly on the current density as well as the anode
material. (I will talk about these things more in a post dedicated to fluorine electrolysis if I am successful in this endeavor)
Small Edit (11/30/25 4:30 EST)--> Nickel anode pre electrolysis is a common technique used to introduce both Nickel fluoride and dry the
electrolyte. However sources indicate that if the HF introduced and electrolyte is totally anhydrous, this pre-electrolysis at high voltage with
current limiting is unnecessary and can be completely omitted. Therefore this should make the process simpler and stresses the excessive importance of
totally anhydrous conditions
Currently I'm investigating YBC carbon or any sort of non-graphitic carbon material but they seem to be difficult to find due to the widespread
abundance of graphite and the specific use case. If anyone knows or works at a company involved in fluorine electrolysis or electrode production,
please reach out, I would be interested in discussing these details!
[Edited on 30-11-2025 by Brotato]
[Edited on 30-11-2025 by Brotato]
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Radiums Lab
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Antimony(V)fluoride should be used right? Is HCl a side product? Won't chloride form HSbCl5F?
Water is dangerous if you don't know how to handle it, elemental fluorine (F₂) on the other hand is pretty tame if you know what you are doing.
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Brotato
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No the HCl bubbles out of solution and with excess HF like I used here this leads to only the formation of Fluoroantimonic acid without those side
products.
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Radiums Lab
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Was SbCl5 more viscous than SbF5 at room temp?
[Edited on 30-11-2025 by Radiums Lab]
Water is dangerous if you don't know how to handle it, elemental fluorine (F₂) on the other hand is pretty tame if you know what you are doing.
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Brotato
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Was hard to judge the viscosity of the Fluoroantimonic acid as it was fuming HF and a PTFE vessel was utilized but the Antimony pentachloride was def
more viscous than water.
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Niklas
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I advice to properly citing all your sources at the end of your posts, you briefly mention Ullmann‘s encyclopedia, but no volume or page, making
this info fairly useless (wouldn’t cite a JACS paper by just saying it’s from JACS either).
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bnull
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Be careful when using Ullmann's. It points to the wrong sources here and there (see the thread on saccharine). I don't know if it was by laziness on
the part of the editors or some typo when researching, preparing or writing the book. Still, the errors are there.
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Brotato
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Not a citation to back up a point or statement, but rather just further reading for the curious ppl. I see your point tho, I'll edit the post to
include the formal citation.
Oh I was unaware that Ullmann's had these issues, thats good to know. For fluorine production and a lot of related compounds, a majority of the
literature is in the form of patents so yeah theres some degree of inherent unreliability when reading anything about fluorine past the 60s-90s. And
admittedly it's been hard to pin down where these come from. Also Japan has a lot of literature on Fluorine chemistry (AGC chemicals, were the
sponsors and chemical providers for the perfluorocubane synthetic group) but a lot of it is in Japanese and was never digitized so only physical
copies are available. Later for fluorine gas production, Ive read quite a few papers so I'll make sure to compile them if I am successful.
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chloric1
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Great read! Thankfully there are some chemical hobbyists that were brave enough to work with fluorine gas that posted on you tube! I hate to sound
like an armchair chemist but this is where the cost of the equipment and lab ware far exceeds the cost or value of the products! Not to mention you
only have one chance to do this safely!
Fellow molecular manipulator
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chempyre235
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I don't know if this has practical application, but I came across a paper that showed this acid's ability to protonate cyanides and nitriles. The
compound on which this property was demonstrated was Mn(CN)84- to Mn(CNH)84+. I thought it was
interesting, in any case.
https://onlinelibrary.wiley.com/doi/full/10.1002/anie.202002...
"However beautiful the strategy, you should occasionally look at the results." -Winston Churchill
"I weep at the sight of flaming acetic anhydride." -@Madscientist
"...the elements shall melt with fervent heat..." -2 Peter 3:10
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chloric1
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Is this not the acid that can actually protonate a paraffin candle? Kind of like how brick an concrete can catch fire in chlorine trifluoride.
Fellow molecular manipulator
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Niklas
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The infamous story of a paraffin candle being dissolved is for magic acid which is a mixture of antimony(V)-fluoride and fluorosulfonic acid, if magic
acid can do it I would assume hexafluoroantimonic to be able to do it too though.
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chempyre235
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I believe it'll do fullerenes, too. If we could get to a +10 buckyball cation, things would get very interesting, as this would have spherical
aromaticity, IIRC. Adamantane's +2 cation has this. B12H12 clusters are aromatic as well.
"However beautiful the strategy, you should occasionally look at the results." -Winston Churchill
"I weep at the sight of flaming acetic anhydride." -@Madscientist
"...the elements shall melt with fervent heat..." -2 Peter 3:10
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