Proper/Safest Heat Source Question

[TORCHed]

I have navigated this forum for about a year now, and have never asked a question, but rather used the search to find some kind of explanation to any questions I've had. However, I seem to find conflicting information on the question below...

I am wondering what is the proper/safest way to heat the the KNO3 and sulfuric acid mix. I have a 250ml distillation retort setup I'm planning on using.

I have 99.9% KNO3 powdered, and more than likely will be experimenting with over the counter drain clearing products of 93% purity. The process is not foreign to me, but the heat source is what I want to address.

I have read tutorials on this site as well as others all varying on the heat source to properly and safely heat in order to distill the nitric acid. I have read of an open heat source such as a Bunsen burner, heating plate, using an oil bath, glass fiber insulation, etc.

Which is the most commonly used setup do you use and/or recommend?

Also, related to the use of an oil bath idea.... I can see using an oil such as olive oil or related cooking oils to be a possibly a flame risk if in contact with a flame. However, I wonder if synthetic (automotive) motor oil can be used as a substitute. That is just an idea coming from someone who is a gearhead.

This may be a stupid question that probably was answered many times, but any help would be gracious! Thanks guys!

[Masonjar Chemist]

I always use oil baths for distillation. Usually, I use a "cooking hot plate" with a small aluminum pot filled with vegetable oil. At the bottom of the pot, I put a small piece of a paper towel so that the distillation flask never actually touches the aluminum pot. I have used a camping stove in place of the hot plate before; however I prefer the hot plate as temperature control is much easier with it.

I'm not sure about synthetic motor oil, but I have heard that using conventional is a bad idea, and a quick way to stink up your whole neighbor hood. I have used antifreeze in place of vegetable oil, but at the temperatures that are required to distill nitric, you will end up boiling most of it away. Another medium to consider are sand baths, I hear they are the easiest to use in terms of temperature control, I imagine clean up is MUCH easier, and as an added bonus it totally inert! Worst case senario, if your flask breaks in an oil bath, the hot nitric acid will likely ignite the oil. Where as with a sand bath, the worst thing that could happen is that you loose your sand container.

[JouMasep]

You did not state if you distil under vacuum.
For lower temperatures sunflower oil is good enough. In time it will polymerise and become sticky though –giving off rancid aldehyde like fumes on heating.

For temperatures higher than c.a. 180 C it’s best to use sand.

Much better than either for lower than about 220 degrees C, is silicone oil.
It’s expensive though.

In truth, I have used the classical Bunsen burner plus asbestos / steel gauze mat as well.
Especially for smaller quantities, it’s clean.

First prize for anything but very high temperatures is a heating mantle. Big disadvantage is that besides the high price tag, one needs a different mantle for each size of flask

One may think about DIY construction here, It’s not hugely complicated. One of the challenges lies in neatly sewing up the woven glass fibre mats for the lining.

I have never taken one apart, but I’m quite sure that beneath the lining I would find a nichrome type of resistive element wire running within a flexible encasing of interlocking porcelain segments. That stuff can be found. I’ve seen it often enough in electrical appliances.

Heating mantles rock!

[Emil]

Personally I do not see the need of setting up an oil bath for nitric acid distillation. Reguardless of a vacuum distillation or not, an open flame will do quite sufficiently. Start with a medium flame, and then regulate as necessary when you come near to a boil.

I use a camping style gas burner with a steel gauze mat. Hot plates are ok, but I am not a huge fan of them. I find they take a long time to heat up your liquid, and I find an open flame is much easier to regulate beings as you can completely stop the flame in less than a second. A heating plate will stay hot for a long while.

The reason I use a camping stove over a bunsen burner is because the flame is alot wider on the stove. It's basically a cooker ring. This allows a wider heating area as I don't want to ever crack one of my beautiful flasks just from bad heating.

If you are able to do so, I do recommend a vacuum distillation. This allows for a wider "zone" when it comes to heating. I'm not sure of the temp I distill at, as I do not have a thermometer incorperated in my rig. At a guess I would say around the 50 degrees C mark, as it does take a fair ammount of heating before boiling. It doesn't really make too much of a difference though in this instance. You'll know when your acid is about to boil so that is all that matters. This results in some very nice ever so slighty yellow fuming Nitric, with not one sign of a NOx fume anywhere during the whole lab.

Judging from your post, you mentioned a 250ml retort, so I assume you will be doing at a normal atmospheric pressure. I personally have never tried without a vacuum, but let us know how it goes. All I would suggest is to be a great deal more picky with the heating at atmospheric.

[JouMasep]

Vacuum distillation is tops for nitric acid –the lower the temperature –the less decomposition and yellowing oxides.

What will also aid in reducing the dissolving of said nitrous oxides into your product, is the introduction of a fine, fine stream of air.

Usual practice to achieve this is to take a thin glass tube and put this into the place where normally your thermometer would be.

This tube is drawn out over a flame to a very fine capillary and the broken off end is submerged into the liquid. Not much vacuum will be lost as a result of this as the amount of introduced air is minimal.

Apart from getting a cleaner product, this type of set-up also eliminates “bumping”. (Even if bumping is less of a problem due the presence of solid KNO3 / K2SO4 crystals. Generally, the more smooth and even a distillation goes, the more pure your product.

When using a retort, this set-up would not be possible. I have not used retorts since a very long time; it is what the alchemists used –but better than nothing. If you only wish to do incidental experimentation, on a smaller scale it is a decent and cost effective way to go. Otherwise get to ground glass joints A.S.A.P. Should this be too costly, subsequently to discarding (breaking!) my retort a very long time ago, I “progressed to making silicone stoppers (from sealant) and then covered them in miles of PTFE tape. It was laborious and eventually the vapours got through anyway. Just make sure that if you’d decide to go this route, you use pure PTFE plumbers tape, not something cheaper that contains PVC –or what have you.

Whatever glassware you use, Emil is right: a spread out heat source –such as a ring burner- is better than a single point burner.

Remember: the less concentration of heat on any point of your mixture -and therefore least amount of localized heating- the less decomposition will take place, and the better your product! (The chance of glass breaking is also minimized of course.)

[Charles Owlen Picket]

From an empirical standpoint the above has been my experience as well. However the strength level of vacuum did not appear to affect the product as much as the temp. (80 C)

A friend had left me with his expensive pump until he could put a lock on his shed and said I could use same to my heart's content. I had a half-ass pump and did several distillations. My pump sucked. His pump sucked much better! But both appeared to achieve the same results in the final product.

I was careful with my temp and received the same very light lemon clear bright nearly anhydrous acid from both. The virtually clear bright acid looked sweet and pure and the morning dew. The tiny touch of lemon yellow could be brought to total clarity with less than a gram of Urea.

However, that level of temp control could be maintained best (IMO) with a hot plate of fair quality as anything else could not meet the same levels of standardization. One thing I have found is that when working with HNO3 / nitrations outdoors (the safest method for lack of fume hood); I work at 0400hrs as that time has the least UV and yet enough light to see well (false dawn). Setting up at "zero dark hundred" is worth the hassle as it's free from disturbances and intrusions as well.