Sublimatus
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Polydimethylsiloxane to silicon dioxide?
What follows is completely speculative, since I know very little about silicon. Any insight into the event I'm about to describe is appreciated.
Recently I was synthesizing nitric acid by way of heating sulfuric acid with sodium nitrate and condensing the resulting vapors. The grease I use for
the ground glass joints is the typical dielectric tune-up grease you can pick up at an automotive store (which I believe many users here also use).
This grease is mostly polydimethylsiloxane and some silica gel.
Anyway, after completing this particular run of acid I unstoppered extra necks on the flask and still head, blew out the uncollected acid vapors with
air from my pump, and then left the apparatus to be cleaned up when I had more time like I usually do (my first mistake). Once I got around to tearing
the rig down, I found that the joints closer to reaction flask were seized.
Packing the inside of the joint with some dry ice and then warm water on the outside got most of them apart without much fuss. The reducing adapter
bridging the reaction flask to the rest of the apparatus was hopelessly frozen, however. I left this to soak in NaOH and ethanol for a week, but it
was clear the interface in the joint had somehow fused.
What I'm wondering is whether it's possible that the heat and nitric acid oxidized the polydimethylsiloxane to silic acid (the conjugate acid of
sodium silicate, water glass), which then broke down to silicon dioxide/silica gel, fusing the joint?
Has anyone else ever experienced this? I find it curious, since I've run this reaction the same way many times and never encountered this.
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unionised
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The simple answer to your question is yes. It's perfectly possible that the nitric acid oxidised the silicone grease to silica.
It could be a simple hydrolysis or it could be oxidation (or both)
Edited to remove reference to deleted spam posts.
[Edited on 18-6-2012 by ScienceSquirrel]
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Mailinmypocket
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That happened to me once although it wasn't seized that badly, from then on I started using sulfuric acid to grease the joints (only when distilling
HNO3) and that works like a charm, just be careful!
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Sublimatus
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Thanks for the response.
Are there any other greases that might be inert toward nitric acid? I think even Dow Corning vacuum grease is some variety of silicone.
Does anyone have experience with Krytox (perfluoropolyether and PTFE based grease)?
I've used PTFE tape in the past, but I found it hard to get a good seal, and some of the things I've read about thermal expansion make me nervous.
[Edited on 6/18/2012 by Sublimatus]
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Zan Divine
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Don't worry about the thermal expansion aspect. Teflon deforms and flows under moderate conditions. The stopcocks of modern sep funnels and addn
flasks are solid Teflon.
Those perfluoro greases are very costly and almost impervious. I did vacuum line work with anhydrous GaCl3 and it attacked the silicone greases and
even interacted sowewhat with the Apiezon greases we had. The perfluoro grease we resorted to, Fluorolube, was untouched.
A better option might be the formed Teflon sleeves that Aldrich and other places sell. Just slip it into the joint before assembling.
[Edited on 18-6-2012 by Zan Divine]
He who makes a beast of himself gets rid of some of the pain of being a man. --HST
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Sublimatus
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Yeah, I've looked at those sleeves before. Cool stuff.
I've never experienced it myself, but I recall reading here that sometimes PTFE tape will swell under heat, fracturing the female joint. Perhaps the
tape was just too thick, or wrapped too many times in those instances.
I think I'll get a smaller tube of the PFPE grease and use it for those special occasions. 
Anyway, thanks for the input. This thread can float off into the archive, where it'll remain for the next poor soul with cemented joints.
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99chemicals
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Quote: Originally posted by Mailinmypocket  | | That happened to me once although it wasn't seized that badly, from then on I started using sulfuric acid to grease the joints (only when distilling
HNO3) and that works like a charm, just be careful! |
Has anybody else used H2SO4 for a grease before? If so should I use concentrated or use diluted?
I need some nitric acid for gold refining I plan on doing this summer.
Thanks
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Mailinmypocket
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Use concentrated, and only a very small amount. I have only used it to distill fuming nitric as well as bromine now that I think of it... Again, just
be mindful when dismantling your setup to not touch the joints and rinse them off with lots of water. I think there might be mention of using sulfuric
acid as a joint lubricant somewhere on this forum, just use the search and it should pop up!
[Edited on 18-6-2012 by Mailinmypocket]
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