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Author: Subject: Ground glass valves freezing
Quince
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[*] posted on 2-10-2005 at 18:50
Ground glass valves freezing


I find that the T-joints on my Gregar extractor freeze during use and I can't turn them (sometimes for example it's useful to drain the upper section by opening the bottom valve, to prevent the thimble overflowing due to clogging by too fine particles; draining makes more gravity push on the solvent column in the thimble).
I can't see how I can put Teflon tape as I do on the joints, without making the valves leak. What's the standard procedure? I've read about greasing with glycerin etc., but that doesn't work with solvents such as acetone etc. or HNO3 extraction.




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mick
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[*] posted on 3-10-2005 at 11:29


You could try lapping it in.The old glass taps were sold as a matching pair. Good quality fine wet or dry wrapped around the barrel with WD40 might get a better fit. A smear of silicone grease should survive vacuum and organic solvents for a few months.
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[*] posted on 3-10-2005 at 12:06


I'd go with fine valve lapping compound (as in engine cylinder head valves) rather than the sandpaper. Maybe some jewelers rouge.

I'd also second the silcone grease.

If they get really stuck I find the heatgun rarely fails to get ground glass joints to part ways.
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Quince
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[*] posted on 3-10-2005 at 15:55


I ended up using a torch until the sulfur melted, and a few whacks with a mallet to get it out. I'm never putting sulfur in this again, what a pain to clean out...I had to scrub the outside of my glassware with steel wool to get it all off.

[Edited on 3-10-2005 by Quince]




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neutrino
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[*] posted on 4-10-2005 at 02:26


Nitric acid will work for this purpose. It oxidizes sulfur to sulfuric acid, removing it as an aqueous H<sub>2</sub>SO<sub>4</sub> solution.
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DrP
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[*] posted on 4-10-2005 at 05:27


Silicone greese is excellant for glass joints - inert and hold very goog vacuum.
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[*] posted on 9-10-2005 at 20:23


grease is definitely the way to go. Silicone will be fine if you don't run into high-temps and expecially highly reactive conditions.

When silicond doesn't cut it, there are greases made by Apiezon (spelling probably off) that have excellent resistance to both heat and chemical attack. I think back on both H and M types working quite happily.

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Quince
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[*] posted on 10-10-2005 at 02:55


Can someone suggest the most common things that silicone is NOT good for?

I have a spray can of "Heavy Duty Silicone Multi-use lubricant" and I'm wondering if it's usable. It says it contains heptane, propane-isobutane, and dimethylpolysiloxane. I guess the first two are propellants, and the last is the silicone.

Also, how do you remove it afterward?

[Edited on 10-10-2005 by Quince]




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12AX7
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[*] posted on 10-10-2005 at 13:29


Quote:
Originally posted by Quince
Also, how do you remove it afterward?


As I recall, that's a very good question, which is one of the reasons silicone oils are weary to use.

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DrP
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[*] posted on 13-10-2005 at 06:58


Buy a tube of silicone grease. Don't ust too much on the joint and just put it round the top. Wipe it off afterwards with some tissue wipe. Bet you'll be fine with this. Silicone grease is quite expensive though.
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Quince
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[*] posted on 13-10-2005 at 10:02


Hmm, it doesn't seem to just "wipe off".



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[*] posted on 13-10-2005 at 12:58


Fine, if you're worried about the 3 microgram residue left behind...carefully burn it in a bunsen flame and wipe off the SiO2 with a bit of sandpaper. Satisfied?



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Quince
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[*] posted on 13-10-2005 at 14:21


Er, I thought glass was mostly SiO2 also.



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neutrino
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[*] posted on 13-10-2005 at 16:38


I think vulture was being sarcastic.
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Quince
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[*] posted on 13-10-2005 at 16:51


Oh... but to what end?



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12AX7
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[*] posted on 13-10-2005 at 17:13


Silicones burn with a yellow flame (at least the last polymerized acetic polysilicone I burned), leaving a large amount of white ash, silica fume.

I would imagine the residue would wipe off, rather than requiring sand paper, but I could be wrong.

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[*] posted on 14-10-2005 at 11:46


12AX7 is right and I wasn't being sarcastic. If high enough temperatures are applied, silicone will burn leaving very pure SiO2. Sometimes it will cake or widen the joint in which case it wont fit anymore and that's when you need sandpaper.



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neutrino
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[*] posted on 14-10-2005 at 13:49


Does sandpaper damage the joint in this case?

I assume the cooling must be done carefully to avoid cracking?
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[*] posted on 15-10-2005 at 11:20


The commercial product used in glass stopcocks, dessicator flanges etc is usually called stopcock grease, but I've also seen the German word Exsikatorfett (dessicator grease) used for it. The last variety I ran into was advertised not to contain silicon, it was thick and sticky grease obviously of very high boiling point. The question that interests me is where is such grease used outside laboratory (OTC acquisition), but I haven't looked into it. It's a very useful product. The typical silicone grease works but it is not as thick and not as good. Here is a link about these products.

http://www.sas.org/E-Bulletin/2002-09-20/labNotesCoyne/body....
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Quince
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[*] posted on 15-10-2005 at 17:48


According to this link, silicone grease should not be used with vacuum as it deposits a film on the inside of the glassware; in heat it burns completely into the glass, and the removal method given is soaking in a base. Overall, pretty nasty! Given the problems with removal of the fluorinated grease as well, I guess PTFE sleeves or tape remain the best option in most cases.

[Edited on 16-10-2005 by Quince]




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