This is an old revision of the document!
Table of Contents
Applications
Dissolves SiO2.
Safety
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
Sourcing
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.
Whink rust stain remover
~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.
RustGo
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 |
Glass etching
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.
Purifying
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.
Freezing
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.
Distilling
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.
References
- Sulfuric acid extractive distillation process for the recovery of anhydrous hydrofluoric acid from by-product aqueous hydrofluoric acid: Preliminary process design and economic study (Extractive distillation): http://www.amazon.com/Sulfuric-extractive-distillation-anhydrous-hydrofluoric/dp/B0007HL7VW
- US Patent 4936955: http://www.google.com/patents/US4936955
-
- Makes standard taper fluoropolymer “glassware”