Sciencemadness Discussion Board

Correct setups for distillation

ManyInterests - 14-11-2021 at 00:45

I've been trying to make nitric acid for a while. I had my first 3 attempts fail due to a combination of sodium hydroxide being sucked into my receiver flask (destroying my product), or the heat being too high when my mantle's thermometer not being able to fit through the adapter, making the heat just go insane. My 4th attempt was a limited success when I managed to get my thermometer into the adapter correctly... by forcing it in. The problem is, the seal was completely broken after that and I could not get it to correctly go on. I got nothing in my receiving flask after 4 hours and I think it's because the nitric acid is escaping through the adapter somehow. I greased the joints with sulfuric acid, I also wrapped the top part of the adapter as tightly as I could with aluminum foil. I guess that wasn't sufficient to get a good seal.

I will post pictures of my setup later. Anyone who ever distilled with a heating mantle like this I would like to know what you did.

What sealant would work to keep the thermometer where I need it and is chemically resistant? Would epoxy work?

Sulaiman - 14-11-2021 at 04:04

You have a few things to sort out, so I suggest the following;

1) whenever you have a new/modified/untried still, start by distilling some water,
it is a good and reasonably safe way to test your still

2) in my opinion, trying to control heating power based on vapour temperature is an incorrect approach,
because increasing heating power will affect the rate of vapour production (boiling) but NOT the vapour temperature.
The vapour temperature is a function of the vapour content, and pressure (atmospheric in most cases)

3) the heating mantles that you pointed to are quite low power, so can be run at full power continuously.
To do this, electrically connect your temperature sensor to the heating mantle, but do not put it in the liquid or vapour, just leave the sensor on the bench or in the air.
Or, put the temperature sensor in the vapour path and set the temperature control to a temperature well above the expected vapour temperature.

4) seal the adapter with aluminium foil as you have already done, with or without the sensor inserted.
For water distillation a small leak will be of negligible concern, because the path to the condenser is much easier for the vapour.

I have done similar when distilling conc. sulphuric and (separately) nitric acids. The foil will blacken and corrode a little but it works.
Later you may want to buy a glass stopper to replace the thermometer adapter,
or a new thermometer adapter and thermometer or thermocouple with meter.
(or maybe with the existing temperature probe the display will show actual temperature)
If you plan to distil sulphuric acid sometime, then a glass thermometer pocket would be more suitable as plastic would melt due to the temperature.
Similarly, many solvents will also degrade plastics and rubbers.

at the moment I can't think of an adhesive that is compatible with hot nitric acid.

PS your heating mantle may be under-powered for a column (eg Vigreux),
so if your kit uses a column then for initial testing purposes, insulate it well, or at least with a loose wrapping of aluminium foil or cloth or foam sponge etc.
This will convert your fractionating column into a plain pipe with negligible fractionating effect, but you should at least get some distillate.

PPS some temperature controllers will shut down if heating power is applied but no temperature change is detected,
I doubt that the controller in your mantle is that sophisticated.

[Edited on 14-11-2021 by Sulaiman]

ManyInterests - 14-11-2021 at 08:18

Leaving the probe out would work (I can seal the top joint with my glass thermometer. It fits quite snugly... or it did before I put the mantle thermometer on it). But the problem is that the heat does not stop rising without the reading.

For example: in the past when I was concentrating hydrogen peroxide by heating it at 78-80 C until the water was evaporated off, it remained at that temperature throughout. I tried to heat up some water recently without the probe in the solution and the temperature kept rising.

My mantle is powerful enough to reliably and quickly start boiling sulfuric acid. That means it absolutely can reach very high temperatures. It has a digital display with a 1 degree precision. I highly doubt it is underpowered for any home chemist application.

Also I did successfully make 70% nitric acid previously. This time failed quite spectularly (I have only a few drops of nitric acid in my receiver flask) and I am not sure why. The only difference is that I put in the thermometer a bit deeper in. That's it. Same heat settings as previous. The only other difference is I added a bit more reagents.

Edit: this nitric acid situation is what is annoying me the most. It is basically the final thing I need to finish off the experiments I want, yet it is proving to be far more difficult than anything I've ever done.

[Edited on 14-11-2021 by ManyInterests]

SWIM - 14-11-2021 at 08:45

350 Watts is actually about normal for a 1 liter mantle.

My 1 liter Glas-col mantles (the very archetype of professional lab equipment) are rated at 380 Watts.

Mantles are very efficient compared to hotplates and only require a fraction of the wattage for heating.

However I do agree with Sulamian on pretty much everything else he wrote.

ManyInterests - 14-11-2021 at 09:32

OK I will try again later with the sensor out of the top joint. I took apart most of my apparatus and cleaned the joints with bicarbonate water and regular tap water to remove all acids. They're drying now. I will also tilt it slightly towards the receving flask since I noticed some acid was pooled towards the ends of the condenser but did not go into the flask.

I WILL succeed at this. I've been doing all of this little by little for more than a year and I will not give up on this. As frustrating as it is, I hope to just succeed at being able to consistently make nitric acid of all concentrations.


OK I need some advice on this. I am using a joint like the one linked above. The problem now is that after I put my mantle's thermometer in there, the seal on the top cap is completely gone. If I put my glass thermometer in there, it will fall right through.

Would wrapping it nice and tight with foil be sufficient? Or is there some other way I can use to plug that hole? My nitric acid will just go right out of there instead of going into the condenser.

[Edited on 14-11-2021 by ManyInterests]

SWIM - 14-11-2021 at 14:23

PTFE plumber's tape works for this as well.

Before I had ground jointed thermometers I used to use it on regular thermometers and just jam them into the thermometer ports on all-glass equipment.

ManyInterests - 14-11-2021 at 15:09

I am looking at the websites of hardware stores near me. They teflon plumber's tape and PTFE 'sealant' tape. Any of these work? I know the answer is probably yes, but when I see a specific name, I tend to look for something with that exact name

SWIM - 14-11-2021 at 16:25

Teflon plumber's tape sounds like what you want.
It gets called different things in some places, but its basically just pure Teflon as a very thin tape and is wrapped around pipe threads to get a good seal.

Sealant tape is probably just another name for the same thing, but I'm not sure.

ManyInterests - 14-11-2021 at 16:58

That sounds good. I'll be buying some shortly. I will need to remove the cap in its entirety and wrap the threads and the hole completely shut in order to prevent any gasses from escaping.

And I can also the wrap the joints in the same tape? for an added seal? What I have on my receiving flask is this

I realize I don't need the vacuum. Can I plug the vacuum port with the tape as well, or will that possibly create a dangerous overpressure?

ManyInterests - 15-11-2021 at 16:53

I just want to say it here. I found some pure teflon tape in the plumbing section of my local hardware store. I was expecting to pay a decent amount of money for that stuff, but I was delighted to find out that they cost less than a cup of coffee! Now I can seal not only the end-cap of my distillation joint, but also seal the joints very firmly and permit so gasses to escape, save for the ones that come out of the vacuum joint (which should just be the NO2 gasses that are useless anyway).

ManyInterests - 19-11-2021 at 15:20

Oh wow. The teflon seal worked amazing! I distilled the leftover stuff from the last attempt (which failed) and left the thermometer out all together. So the acid boiled quite rapidly. I had to shut it off because of all the foaming... but wow! I mean wow! Within barely a few minutes I got a small (but not insignificant) yield of acid at around 1.428 specific gravity. It is well below room temperature and I need it to warm up before getting a good weight. But it is clear with no NO2 contamination.

I will be doing this again. The cap still needs a touch more teflon to fully seal it as it changed color from red to yellow, but other than that this is a very good seal. I hope my future attempts at getting azeotropic nitric acid will be successful. Then all I gotta go is make some WFNA in a bit.

Edit: I think it may even be higher... I need mix it a bit and let it warm up before taking a final measurement...

[Edited on 19-11-2021 by ManyInterests]

ManyInterests - 20-11-2021 at 13:44

OK I will make a final note on my nitric acid synthesis. While I believe I did get a good handle on it. I realize that the formulas given don't work for me for some reason. The solution would be to add an excess of sulfuric acid. Also I might need to do it in two phases. The first one being to get rid of the NO2 gasses, the second to get the nitric acid. The first one would have to involve boiling until all the gasses appear to be gone, but before any liquid gets over. Take apart the apartaus and clean the condenser and receiving joint and receiving flask before putting everything together again and drawing out the very clean azeotropic nitric acid.

Not everyone's method, but this should work for me from now on.

SWIM - 20-11-2021 at 13:59


You've adapted the standard process to work with the materials and equipment you have at hand.

That's what amateur chemistry is all about.

Edit: Yes, you pretty much always want to leave an open vent somewhere on a distillation apparatus because if pressure develops and forces a joint open it can be a real mess.

I have used teflon tape on joints sometimes, but I often just wet the joints with water or solvent or sulfuric acid (whatever works with the reaction you're doing.
For me, the tape seems to work best when it's just a single layer.

[Edited on 20-11-2021 by SWIM]

ManyInterests - 20-11-2021 at 15:29

Yep. I used multiple layers and created a tight seal. The teflon tape is just awesome.

The second batch I made yesterday appeared to have dropped to 65% but it should still work for the one last ETN synthesis (the other should be just fine, but I haven't tested it again yet). Like I said in another post. Magpie's formula is very good, but to make sure I get the nitric acid concentration I want, I would need to add around 15ml or so extra sulfuric acid. His formula is 225 grams KNO3, 100 H2SO4, and 40ml or so of distilled water. For me to get 68-70% the first time around I need 8ml of water (not more than 9) 115 or 120 ml of sulfuric acid, and dry the KNO3 and grind it to a fine powder. It must contain no errant grains or clumps due to moisture or any other reason.

And I need to replicate the process I did to get the clean acid to the best I can. The first succeeded because the hole in the joint let all gasses out and not in the condensor, and the thermometer was in, so the mantle didn't heat like crazy. Without the thermometer in, I need to make visual guesses as to what the heat is and turn it off and let it cool and heat up multiple times to get rid of the gasses and not destroy the nitric acid. Because once it's ready, the HNO3 will leave the boiling flask very, very fast.

While I should be working on WFNA. I will use the little KNO3 I have left on one final experimental batch, before making more KNO3 to use for other experiments.