Although strictly speaking “HF” is the gas form (hydrogen fluoride), typically when people say “HF” they really mean HF(aq), that is hydrofluoric acid.
Dissolves SiO2.
WARNING: toxic. Probably the most dangerous chemical you are likely to run into. Spilling high concentration on your skill can make you die a horrible death.
Recommend use of approved respirator. Ex: 3M 7800 with acid gas cartridges. Recommend use of gloves. Double gloving may be a good idea. Recommend keeping calcium gluconate around in case of exposure. Ex: Calgonate
Due to this chemicals extreme toxicity it may be difficult to acquire depending where you are. Some rust removers are HF. Whink brand rust remover is a weak HF solution. At this solution data sheets indicate that it is not so deadly but it should still be treated with care. Contact me if you need to get a stronger solution brand.
~2.5% HF solution can be found in Whink brand rust remover. Unfortunately, due to health concerns with HF, this product is not sold in the EU. You still might be able to order it from somewhere overseas (ie eBay) cheap enough.
| Chemical | CAS | Weight % |
|---|---|---|
| Water (H2O) | 7732-18-5 | 90 - 100 |
| Hydrofluoric acid (HF) | 7664-39-3 | 1.50 – 3.5 |
| Denatonium Benzoate | 3734-33-6 | 0.01 – 0.10 |
Denatonium Benzoate is a bitterant probably put in to make sure that nobody consumes the HF.
MSDS: http://www.alwilson.com/products/rustgo/RustGo%20MSDS.pdf
| Chemical | CAS | Weight % |
|---|---|---|
| Hydrofluoric acid (HF) | 7664-39-3 | ~12% |
| Ammonium bifluoride (NH4HF2) | 1341-49-7 | ~16% |
| Water (H2O) | 7664-39-3 | Remainder |
I've heard it can be sourced for glass etching. The stuff I've seen is usually hydroxide based, and rarely some HF in paste form.
The most pure/concentrated HF I was able to easily acquire was RustGo. However, while more concentrated than Whink, it has a large amount of impurities.
This states that 12% HF freezes at about 5C or so. Could the NH4HF2 be frozen out? Its very soluble in water though at 63 g / 100 mL @ 20C. For better or worse NH4HF2 also attacks SiO2.
This was my first attempt (05/01/2013). It looks like the NH4F2 may have froozen out but the HF was slushy enough that its hard to tell. Had to get it pretty cold, used immersion cryocooler…alcohol solution was at maybe -50C. This may just have been from bad heat transfer though.
A straight up distillation is tricky. First, significantly heating it would release lots of toxic vapors. Second, it eats glass, prohibiting the use of standard equipment. [US4936955] notes that while stainless would probably work, semi industry avoids it for fear of introducing metal contaminants. Since at this time I'm mostly looking to do destructive analysis I'm less concerned with this. [US4936955] also notes use of vacuum which is what I was thinking would make the most sense to cut down on heat as well as better contain the hot acid. I was thinking H2SO4, which concurs at least with the title of [Extractive distillation].
As far as materials, [US4936955] says that the “…apparatus for said process is comprised of a plastic wherein said plastic is a fluoropolymer plastic.” (that is, PTFE, PFA, etc). For short runs PP will probably work fine, although fluorinated plastics are ideal. [Ultrapurification HF, 1] notes that stills are typically made of platinum, polyethylene, or polypropylene.
I think I read before that industrial HF production is done in copper. As long as the flow rate isn't too high a (CuF2?) passivation layer forms that prevents further etching. Experience has shown that etching solutions are very copper sensitive so I'm hesitant to try this.
Sometime before 05/21/2013 I have tried two distillations. One using a HDPE wash bottle and the other using a Polypropylene volumetric flask. The first was simply too fragile and collapsed too easily. The second was working but I got too aggressive with the vacuum/pressure and the flask eventually collapsed. I had it around maybe 80C which jives with [Ultrapurification HF, 1 per Coppola/Hughes] which suggests 70-85C (under 1 atm?). Although PP doesn't melt until 115C or so it does soften a little before then which must have been enough with the vacuum.
[Ultrapurification HF] describes a simple technique to basically force HF(gas) from HF(aq) at room temperature. They say to basically put one beaker with 35 M HF and then place a second beaker with equal volume distilled water. After two days the second beaker should have 12 M acid. If the acid in the first beaker is replenished it should get the second beaker to 25M after another 2 days.
I've concentrated HCl(aq) by adding NaCl to weak mixture to force out HCl(gas) and I imagine that I could do similar using NaF. I have a couple of PP flasks and PFA tubing would should be good for experimenting.