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Author: Subject: Glass or teflon stop cock?
octave
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[*] posted on 12-6-2008 at 19:02
Glass or teflon stop cock?


What are the downsides of having a glass stop cock as opposed to having a Teflon stop cock and vise versa?
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[*] posted on 12-6-2008 at 20:10


I much prefer Teflon. It doesn't require grease and won't "freeze up" due to corrosion with alkali. I wouldn't buy glass unless it was unavoidable.
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[*] posted on 12-6-2008 at 20:21


Teflon stopcocks shrink and expand greatly with temperature, and as such can easily come loose if they get cold, or get stuck if they get hot. It does not take very much temperature change for this to happen. You have to constantly adjust the nut at the back of the stopcock to keep the right amount of tension. They are also somewhat porous (like ANY polymer) and will absorb chemicals even though they do not react. It is very difficult to get such absorbed chemicals out of teflon stopcocks. I had to boil one of mine for several hours to get rid of the red tint of NO2 when I was distilling RFNA with it. Really it's easier to just have a different one for each chemical if you are concerned about contamination. I find teflon stopcocks to be much more annoying than glass ones for these two reasons.

Glass stopcocks require lubricating oil, and there is no lubricating oil that is as chemically inert as teflon, period. Even the fancy fluorocarbon greases can react with certain strong oxidizers and contaminate some solvents, but ordinary petroleum based ones are much worse. Petroleum jelly works fine as long as you're using it with compatible chemicals. I use a Dow Corning PTFE/fluorosilicone grease (Molykote 3452) with my glass stopcocks. It was expensive (like $40 for a small tube) but is worth it imo for distilling strong acids and solvents and such. Also, when glass stopcocks do get stuck, it is much more difficult to get them out without breaking anything, even though they are less likely to get stuck than teflon ones. Usually the best way to free a glass stopcock is to carefully heat the outer joint while trying to avoid heating the stopcock, whereas the best way to free a teflon stopcock is to cool the entire thing, causing the stopcock to shrink more than the glass. Never use glass stopcocks (or any glassware for that matter) with alkali hydroxides or hydrogen fluoride of course.

[Edited on 12-6-2008 by kilowatt]




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[*] posted on 12-6-2008 at 21:15


I have managed to break two separatory funnels with teflon stopcocks. I live in a cold climate and the temperature changes are a bit too much for it to handle without special attention. They were low quality, but still, I would go with all glass if I ever get another one.
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[*] posted on 12-6-2008 at 22:50


Please define low quality.

Also an obvious workaround is to store the sep funnel with teflon stopcock removed when not in use.

How about the use of ptfe tape (the thin flexible sort used for sealing threads) on stopcocks (glass) A little at the top and a little at the bottom, just as grease would be applied. Worth a try.

Obviously, both glass and PTFE stopcocks have their good and bad points. Select the best for particular applications. Glass for reaents and mixtures that will contaminate teflon, teflon for applications absent temperature extrema or very aggressive reagent, and for times when grease contamination would be a major nuisance (like prep of samples for analysis.)

I'd resolve any issues of breakage of low quality glassware by using non-crapola glassware to start with. Such glassware is never a bargain.




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[*] posted on 13-6-2008 at 05:22


I have had nothing but trouble with glass, although admittedly I've never had to worry about contaminating the PTFE. I have bought nothing but Teflon for myself, and do not regret having to pay a little extra. As a solution to this, I have been using Teflon plumbers' tape to wrap once or twice around the glass stopcocks I own to prevent freezing in my reagent bottles over time.

I am unaware of Teflon coated Glass stopcocks as a workaround for the different thermal expansion coeff. causing damage... Perhaps the cost and manufacturing problems have prevented them being commercially available?




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[*] posted on 13-6-2008 at 07:59


Before storing idle glassware having a teflon plug , always loosen the nut which compresses the o-ring against the keyed washer so the compression force on the taper is removed. When old o-rings harden and lose their resiliency, replace them. Teflon has a semi-solid waxlike property about it, and gradually flows under pressure,
creeps even under slight tension enough for the deformation to be noticeable on precision taper joints
which are wished to remain movable, if they are left
assembled under significant compression for extended time. A compressed o-ring is enough force to do the
deed even after a days exposure to a tightened retainer nut.
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[*] posted on 13-6-2008 at 08:28


Speaking of frozen glass, I have a huge (4L) sep funnel that has a frozen glass stopcock.... I've tried solvents and knocking it.... I don't have access to a torch, so I cant really heat just the outside joint. Any one have any other ideas? :(



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[*] posted on 13-6-2008 at 08:59


Quote:

I don't have access to a torch, so I cant really heat just the outside joint.

Just go to the hardware store and buy a cheap propane torch. They are invaluable.




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[*] posted on 1-7-2008 at 19:06


a good japanese supply company website once posted a fix for frozen ground glass joints that i use often now, basically it just involves soaking the join in carbonated water (soda water) then slightly heating it, the CO2 in the water in the join expands the pops the joint out.



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[*] posted on 2-7-2008 at 01:32


Quote:
Originally posted by Ramiel
I am unaware of Teflon coated Glass stopcocks as a workaround for the different thermal expansion coeff. causing damage... Perhaps the cost and manufacturing problems have prevented them being commercially available?


Just a thought here. I know you can get these PTFE spray cans which spray a thin film of PTFE onto a substrate to make it non stick. Could you spray a coating onto your glass stopclocks as an alternative to the PTFE tape winding technique? Might be worth a try to see if it works - the spray cans are quite cheap.




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[*] posted on 2-7-2008 at 08:13


Quote:
Originally posted by Panache
a good japanese supply company website once posted a fix for frozen ground glass joints that i use often now, basically it just involves soaking the join in carbonated water (soda water) then slightly heating it, the CO2 in the water in the join expands the pops the joint out.


Oooohhh... I will try that :D




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[*] posted on 2-7-2008 at 13:02


Teflon any day of the week.

No grease and more air tight.

If you already have glass stopcocks, buy yourself a tube of Krytox grease. It's insoluble in the VAST majority of organic solvents, so you don't have to worry about contamination. It's bloody expensive though.
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[*] posted on 2-7-2008 at 13:43


Don't use a burner, use a hair dryer. Much more gradual heat which won't crack your glass and it works just as well.



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[*] posted on 2-7-2008 at 16:15


Krytox is worth every penny if you want to get decent NMR! Still, good old cheap silikonfett works fairly well for most things and contamination can be minimized by *not slopping it on*.

Oh, I prefer PTFE whenever possible.

I have found that sonication will frequenly un-stick frozen TG joints.

Cheers,

O3




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[*] posted on 3-7-2008 at 00:48


Quote:
Originally posted by DrP
Quote:
Originally posted by Ramiel
I am unaware of Teflon coated Glass stopcocks as a workaround for the different thermal expansion coeff. causing damage... Perhaps the cost and manufacturing problems have prevented them being commercially available?


Just a thought here. I know you can get these PTFE spray cans which spray a thin film of PTFE onto a substrate to make it non stick. Could you spray a coating onto your glass stopclocks as an alternative to the PTFE tape winding technique? Might be worth a try to see if it works - the spray cans are quite cheap.



Hi - guys. I wouldn't mind knowing what you actually think of this idea. Positive or negative? I no longer have a vac line or get involed with organic prep anymore so I wont be able to try it out. Will it work? It may be unnecessary as I know the tape and the greese works, but just idea - what do you think?




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[*] posted on 6-7-2008 at 07:01


i generally prefer glass stopcocks because teflon is too easily damaged by microabrasives, i only ever use teflon tape as lubricant for the tap unless the tap/bodies are number matched.

'Spray on PTFE' i imagine would also spray on a complex solvent mix, i hold no respect for that product as it uses the PTFE for marketing reasons. At best it would contain very finely divided teflon particles, which to all intents and purposes would not stay on the substrate very long.

As an aside, we all know teflon's main weakness is it's lack of mechanical strength, however if one seals something with teflon tape mechanical integrity can by added by overlaying the tightly wound and compressed teflon tape layers with a silicon self binding tape. That way you get the chemical resistivity underneath, and the mechanical strength on top. I use this combination often.




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[*] posted on 26-11-2008 at 07:34


Sorry to drag this up... What are peoples oppinions on the Rotoflow stopcocks? All the separatory/addition funnels I can find that dont have a glass stopcock have this kind. Personally I've always used a teflon tap at school for separating, as you can trap the phase boundary in the tap. However with these "rotoflow taps" this doesnt look possible.
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[*] posted on 26-11-2008 at 11:54


I own a Rotaflow burette and i have never had any problems with it, even when using it for conc acids, including alkalis.
I find Rotaflows are fairly expensive but usefull.
For a seperation funnel i wold use a stockcock, seperating with a rotaflow is possible but not as easy.
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[*] posted on 30-11-2008 at 08:50


Thanks, thats what I would have assumed. The look great for adjusting the rate of flow, which makes them perfect for burettes/addition funnels. I shall have to look around and see if I can find a sep funnel with a plain PTFE tap.
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[*] posted on 30-11-2008 at 09:51


Just some other thoughts. Always try to keep your stopcocks with whatever item they came with. They are swappable to some extent but they always seem to fit better when dedicated to a specific buret or whatever.

Also, glass stopcocks with grease hold a better vacuum. They are essential for high vaccum work (<.1 mm Hg) because at these pressure the leaking from teflon is noticeable. I routinely use Torians with sidearms and use the sidearm for relasing the system to nitrogen, and like I said above if I am going to high vacuum I use glass stopcocks.

Cleaning glass is not a problem, just get some pipe cleaners and make sure you have the grease availible to grease the joint correctly in the first place. With teflon stopcocks be sure you have enough rubber O-rings to replace them as they become frayed.

I have known some people who insist on greasing teflon stopcocks. Never tried it myself since that was always the positive benefit to using teflon in my eyes, but they contend that it allows them to hold a better vaccum and allows better cleanup for some things that stain them (such as nitro's).




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[*] posted on 30-11-2008 at 12:43


I agree; surely the whole point of having the stopcock made of a low friction (and inert) material is so that you don't need to grease it (and thus cause [minor?] contamination of your products). I also believe that glass stopcocks still have use in a lab where teflon taps are used in funnels and burettes. They are suited to vacuum work as mentioned, and the transferrance of grease from a vacuum stopcock into your product (when there is no direct contact as with a sep. funnel) is nil.
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[*] posted on 30-11-2008 at 22:58


Quote:
Originally posted by Panache
'Spray on PTFE' i imagine would also spray on a complex solvent mix, i hold no respect for that product as it uses the PTFE for marketing reasons. At best it would contain very finely divided teflon particles, which to all intents and purposes would not stay on the substrate very long.


Hah! I totally agree. That stuff is bunk. It stays on until you touch it. And then it all comes flaking off. Useless...




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[*] posted on 4-5-2012 at 14:38



You can purchase teflon sleves for the job

https://www.sciencelab.com/page/S/PVAR/23729/60-204141030

People have reported breakages if using plumbers teflon tape.
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[*] posted on 5-5-2012 at 17:51


Quote: Originally posted by Ramiel  
I am unaware of Teflon coated Glass stopcocks as a workaround for the different thermal expansion coeff. causing damage... Perhaps the cost and manufacturing problems have prevented them being commercially available?


There was at least one company that used to make a glass stopcock coated with a thin Teflon layer. It was a smaller company, not one of the biggies, can't remember the name. Unfortunately, I have not seen them much, and don't see such an item in any current catalogs. But they appeared to work well. I wish that they were still readily available.

Bob
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