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--- chemical:hf [2013/05/03 04:54] – [Distilling] mcmaster
+++ chemical:hf [2013/10/20 14:59] (current) – external edit 127.0.0.1
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+Although strictly speaking "HF" is the gas form (hydrogen fluoride), typically when people say "HF" they really mean HF(aq), that is hydrofluoric acid.
 ====== Applications ======
 Dissolves SiO2.
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 ===== 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.  I was thinking H2SO4
+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.
+====== Lab notes ======
+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.
+====== Isothermal distillation ======
+[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.
 ====== 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: http://www.amazon.com/Sulfuric-extractive-distillation-anhydrous-hydrofluoric/dp/B0007HL7VW
+  * The Ultrapurification of Hydrofluoric Acid (Ultrapurification HF).  W. KWESTROO, J. VISSER.
+  * Polyethylene Still for Preparation of Pure Hydrofluoric Acid.  P. P. Coppola , R. C. Hughes.
+  * 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
+  * http://www.finemech.com/Merchant2/merchant.mvc?Screen=PROD&Product_Code=A155-36&Category_Code=f516&Product_Count=5
+    * Makes standard taper fluoropolymer "glassware"
+  * Good safety info: http://www.prevor.com/EN/sante/RisqueChimique/hexafluorine/fluorure.php
+  * A NaF/H2SO4 experiment: http://woelen.homescience.net/science/chem/exps/KMnO4+NaF+H2SO4/index.html
+    * Says it must be heated