Sciencemadness Discussion Board

Need 2" stopper (tapered rubber/silicone) for HNO3 and NH3 (both in hot gaseous form)

RogueRose - 21-10-2017 at 21:02

I have a SS distillation apparatus and the end of the column has a rubber bung/stopper (with hole for thermometer). This is fine for ethanol and most essential oils but I don't think it will stand up to gaseous ammonia or nitric acid (both being distilled).

The column isn't going to be under pressure but it still needs to hold in place. The OD is 2" and ID about 1.9" or so.

I was planning on making a tapered silicone stopper (from RTV caulking type silicone) but it looks like this wouldn't be resistant to either substance at the temps needed fro distillation. I was hoping to make something from items on hand for a temp fix but I'm not sure what would be suitable for this.


I do have some 1/4" PVC which could be stacked and then shaped into a stopper and it seems that this may be suitable for HNO3 and NH3 though it doesn't state temps in this chart.

http://www.aleco.com/chemical-chart.html

another option I was thinking which is a little unconventional is using wood (Oak, cherry, maple, black walnut, pine, MDF, press board and plywood - are available). The temps should be fine but I'm not sure if it would contaminate the gases at all.

What would be the ideal material for this stopper once I finally order one? I can get thick HDPE, LDPE, UHMEPE, graphite, may possibly be able to order a piece of PTFE (1" thick by 2" x 2" square..?) or I may be able to have a glass maker make a Boro stopper but I'd have to drive up to see him and do it.

Any suggestions for an immediate solution and thoughts on a more permenant option?




Deathunter88 - 21-10-2017 at 22:33

Quote: Originally posted by RogueRose  
I have a SS distillation apparatus and the end of the column has a rubber bung/stopper (with hole for thermometer). This is fine for ethanol and most essential oils but I don't think it will stand up to gaseous ammonia or nitric acid (both being distilled).

The column isn't going to be under pressure but it still needs to hold in place. The OD is 2" and ID about 1.9" or so.

I was planning on making a tapered silicone stopper (from RTV caulking type silicone) but it looks like this wouldn't be resistant to either substance at the temps needed fro distillation. I was hoping to make something from items on hand for a temp fix but I'm not sure what would be suitable for this.


I do have some 1/4" PVC which could be stacked and then shaped into a stopper and it seems that this may be suitable for HNO3 and NH3 though it doesn't state temps in this chart.

http://www.aleco.com/chemical-chart.html

another option I was thinking which is a little unconventional is using wood (Oak, cherry, maple, black walnut, pine, MDF, press board and plywood - are available). The temps should be fine but I'm not sure if it would contaminate the gases at all.

What would be the ideal material for this stopper once I finally order one? I can get thick HDPE, LDPE, UHMEPE, graphite, may possibly be able to order a piece of PTFE (1" thick by 2" x 2" square..?) or I may be able to have a glass maker make a Boro stopper but I'd have to drive up to see him and do it.

Any suggestions for an immediate solution and thoughts on a more permenant option?



Just wrap your existing rubber stopper with some PTFE plumbers tape. Make sure there are no gaps and you are good to go.

Sulaiman - 21-10-2017 at 23:45

Is stainless steel compatible with nitric acid ?

If you are distilling near-anhydrous HNO3 then I guess the ss would be passivated,
but I think that hot dilute nitric acid will eat ss.

RogueRose - 22-10-2017 at 01:38

Quote: Originally posted by Sulaiman  
Is stainless steel compatible with nitric acid ?

If you are distilling near-anhydrous HNO3 then I guess the ss would be passivated,
but I think that hot dilute nitric acid will eat ss.


The concentration will be less than 30% and from the few tables I checked it said that this was fine for the grade SS I had, even at high temps. The problem is that each chart listed many of the same materials and there was conflict between them and whether they are adequate with various chemicals, so I have to do some more research into the nitric acid issue before hand.


I have some Teflon non-stick spray (only ingredients are the propellents and petroleum distilllates, doesn't mention PTFE though..) I found the MSDS for the product and it doesn't mention PTFE in it anywhere! I'm wondering if a couple coatings of this long the pipe and inside of the vessel would allow for more or adequate protection from the HNO3?


Attachment: DuPont Non-Stick Dry Film Lubricant Aerosol_US GHS SDS_English_15_April 2015.pdf (194kB)
This file has been downloaded 510 times

Melgar - 22-10-2017 at 05:06

Closed-cell LDPE foam actually works pretty good for this and it's easy to find. I rolled this up and covered it with heat shrink once, and it worked surprisingly well. RTV silicone can make a decent stopper, but the trouble is that the surface is slippery against glass and often polished metal as well.

Another option is you just get some vinyl tubing of various diameters and put them one inside the other. Or you get a really durable, rigid material for the inside, then cut a piece of vinyl tubing that you can push over that when it's soft. Will it hold up for very long? Of course not, but it'll hold up for a single distillation, and two feet of vinyl tubing is cheap enough that you can just cut off a new piece every time you need to do another run and/or your last piece doesn't look like it'll last much longer.

edit: I used to have trouble with my thermometer gaskets always failing, until I got the idea to just cut thin slices off of a piece of vinyl tubing every time there was a failure, or a failure looked imminent. Also, during the distillation, I think what happens is that NOx and other nitrogen species end up being absorbed by the gasket material, and discolor it. This doesn't affect its structural integrity immediately, but those things will stay in there, breaking it down gradually over time. Thus, even though you do expect it to fail eventually, if you cut a new piece each time, it probably won't fail during any of your distillation runs.

[Edited on 10/22/17 by Melgar]