Sciencemadness Discussion Board

Distillation Apparatus question

Camroc37 - 22-8-2015 at 17:08

I bought a 3-way adapter to make nitric acid, but realized the plastic cap may dissolve. Not certain though since it is laboratory equipment. Any help/thoughts are appreciated. I asked the seller and he didn't know. Or you could say he didn't HNO! Ha. Ha. Sorry...

3wayadapter.jpg - 541kB

greenlight - 22-8-2015 at 19:27

Definitely looks like chem resistant PTFE plastic that comes with other glassware but its hard to tell.
If it is PTFE plastic, it will hold up against the HNO3.
If it is cheaper no-name glass item you never know.
Can always go to the hardware store grab a roll of plumbers PTFE thread tape and wrap its threads as extra protection then just give it a go.






[Edited on 23-8-2015 by greenlight]

hissingnoise - 23-8-2015 at 02:03

C37, normally there would be a rubber o-ring with PTFE protectors top and bottom ─ what you seem to have is a poorly made rubber stopper seemingly without provision for inserting the needed thermometer . . .

But as GL said, PTFE tape is a boon in dealing with oxyacids; on joints, too, it's a reasonable alternative to stop-cock grease!

greenlight - 23-8-2015 at 02:16

Damn, must be a chinese knock off.
Looks the same colour as the Schott duran plastic on their glassware.

Sulaiman - 23-8-2015 at 03:39

I don't have huge experience here but in my opinion;
as is,
the nitric fumes/vapour will only get to the inside of the glass and rubber washer,

a thermometer can be used by piercing the central thin part of the rubber,
in which case
the inside of the glass, part of the thermometer and a very small area of rubber will be exposed to the vapour.
should be no problem.


I see no point in using ptfe tape in this case.
the threaded areas are just to allow the mechanical compression of the seal,
they do not and are not meant to form a gas tight seal.

worst case; the rubber will swell a little and over much use decompose,
cut a new silicone rubber washer or replace with a rubber O-ring.
common butyl rubber will be ok, fluorinated rubber (e.g. viton.) better.

The old guys used huge rubber bungs etc.
the small area of rubber exposed in this case is negligible,
so go ahead and use as-is.


[Edited on 23-8-2015 by Sulaiman]

Magpie - 23-8-2015 at 07:29

Selection of chemically compatible materials of construction is crucial. The Cole-Parmer Chemical Compatibility Database is very useful for material selection:
http://www.coleparmer.com/Chemical-Resistance

When dealing with hot concentrated nitric acid the correct selection of elastomer is important. I've had black rubber O-rings of unknown composition completely destroyed when making con HNO3.

Black rubber O-rings are cheaply available at my local hardware store (as well as black rubber stoppers). But I don't know the rubber composition: is it natural rubber, Viton, EPDM, or what? By buying from a supplier like McMaster-Carr I can specify the rubber composition.

Camroc37 - 23-8-2015 at 10:20

Quote: Originally posted by Magpie  
Selection of chemically compatible materials of construction is crucial. The Cole-Parmer Chemical Compatibility Database is very useful for material selection:
http://www.coleparmer.com/Chemical-Resistance

When dealing with hot concentrated nitric acid the correct selection of elastomer is important. I've had black rubber O-rings of unknown composition completely destroyed when making con HNO3.

Black rubber O-rings are cheaply available at my local hardware store (as well as black rubber stoppers). But I don't know the rubber composition: is it natural rubber, Viton, EPDM, or what? By buying from a supplier like McMaster-Carr I can specify the rubber composition.


It is cheap china. (Didn't specify type) I will try using as-is, but in worst case scenario I can have PTFE at hand and ready to use.

[Edited on 23-8-2015 by Camroc37]

DFliyerz - 24-10-2015 at 20:06

A little late, but do not use this. I used this with nitric acid, and the thermometer fell in and broke inside my flask.

UC235 - 24-10-2015 at 20:51

Nitric acid destroys rubber. The only o-rings I know of that will mostly put up with HNO3 are viton. Best approach is to just use a stopper and figure out the concentration later.

Sulaiman - 25-10-2015 at 01:33

Quote: Originally posted by DFliyerz  
A little late, but do not use this. I used this with nitric acid, and the thermometer fell in and broke inside my flask.


If you buy some cheap rubber O-rings that are a tight fit on your thermometer
put one on your thermometer a little above the plastic cap
even if the internal rubber corrodes or the plastic fails
the O-ring will prevent the thermometer crashing into the bottom of your flask.
Even a rubber band wrapped around the thermometer stem many times will work long enough to prevent disaster,
if you are attentive enough to notice the thermometer drop a little.

JJay - 25-10-2015 at 23:13

I just dropped a thermometer into a flask and broke the thermometer... it was tight when I assembled the apparatus, but thermal expansion loosened the adapter... fortunately, the thermometer was a cheap one that came with a mostly decent lab kit that was the cheapest available... and it just so happens that I bought an extra thermometer yesterday. Taking a look at the thermometer adapters I was using, they are junk too.. fortunately, I have some high-quality ones that I ordered from a reputable Chinese supplier.

My expensive 350 degree mercury thermometer (also Chinese) has a high-density plastic ring permanently mounted at the top. Seems like a good idea....