# Nitric Acid Boil Over - OOPS!



## pdamiant (Dec 30, 2021)

Happy New Year folks,

I am making my own nitric acid using sulfuric acid and potassium nitrate. I had a boil-over and some of the contents of my boiling flask ended up in my receiving flask. Has this ever happened to you? My thoughts are to simply pour the contents of my receiving flask back into the boiling flask and start again. But do I need to? I can see the potassium nitrate on the bottom of the boiling flask and can filter that out, but what about the sulfuric acid mixed in with the nitric after I filter? Will it matter. I am using the nitric to work on gold.

Thank you.


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## Martijn (Dec 30, 2021)

I think its best to pour everything back and start over. 
Turn down the heat once distillate is formed in the condenser to one drop every few seconds.


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## Grumbleact (Dec 30, 2021)

This hobby has some exciting moments usually due to a moment of inattention or lack of experience. Quite a lot of things are going on during distillation of acid and a basic understanding of what is in your reaction vessel and the sequence of what happens when, is useful. 

A boil over happens in this case as either gas evolves rapidly (either steam or nitrogen dioxide) and the reactants convert from aqueous solutions to molten salts. 
Martin is correct that you can simply pour it all back and start again. 

However all is not what it was. This restart must be watched more carefully as another distillation process is more likely to boil over again. 

I find sulphuric acid very dangerous especially when hot and so use Sodium Bisulphate (Pool pH Up) and because it is very hard to buy Sulphuric acid at times. 
When I started making Nitric Acid I would have a boil over and or suck back on every batch. 
Do not attempt to make commercial amounts of acid at a time. 
Things I learned :
1. Never set and forget - always watch the reaction
2. Small batches in your equipment - run around 30% of reaction vessel capacity. 
3. Watch your collection vessels- water comes over first
Dilute Nitric acid next, Azeotropic acid and lastly Red Fuming Nitric Acid. Each of these should be kept separate if you understand that salt or HCl may contaminate the reagents or water fractions and will distill over before the Nitric acid fractions. 
4. Start slow, you can speed up after red fumes appear. But when no more acid drops appear, then stop heating, as I have melted cheap glassware.
5. Allow everything to cool before the next batch. 
6. Always do this outside or in a fume hood with a bucket of water and a packet of bicarb soda handy. I use a big fan as well. 
7. The Darwin Awards get new candidates every year. There are good reasons authorities frown on backyard chemists. 
8. Personally I try to limit the amount of red fumes floating around and add about 10 ml of Hydrogen peroxide to any Nitric acid I have to re-distill. I collect red fumes through Rashig tube filled condenser and a distilled water trap and use that as feedstock in any future batch. 

Hope that helps - keep safe 

Regards Grumbles


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## pdamiant (Dec 30, 2021)

Grumbleact said:


> This hobby has some exciting moments usually due to a moment of inattention or lack of experience. Quite a lot of things are going on during distillation of acid and a basic understanding of what is in your reaction vessel and the sequence of what happens when, is useful.
> 
> A boil over happens in this case as either gas evolves rapidly (either steam or nitrogen dioxide) and the reactants convert from aqueous solutions to molten salts.
> Martin is correct that you can simply pour it all back and start again.
> ...





Grumbleact said:


> This hobby has some exciting moments usually due to a moment of inattention or lack of experience. Quite a lot of things are going on during distillation of acid and a basic understanding of what is in your reaction vessel and the sequence of what happens when, is useful.
> 
> A boil over happens in this case as either gas evolves rapidly (either steam or nitrogen dioxide) and the reactants convert from aqueous solutions to molten salts.
> Martin is correct that you can simply pour it all back and start again.
> ...


Thank you for sharing your experiences. I am new to this so your information really helps. 

I also noticed the time between drips at my receiving flask was directly proportional to the temperature on my thermometer. As the time between drips increased, so did the temperature on the thermometer. This was right at the end of my distillation process and when the boil over happened. I had the temperature regulated perfectly throughout the entire process and thought I had everything under control. Towards the end of the distillation process the temperature started to skyrocket in the boiling flask and it completely caught me off guard. Any ideas?


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## pdamiant (Dec 30, 2021)

Martijn said:


> I think its best to pour everything back and start over.
> Turn down the heat once distillate is formed in the condenser to one drop every few seconds.


Good stuff. Thank you Martijn.


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## Grumbleact (Dec 30, 2021)

@ pdamiant

Boil over = the ‘oh s
Moment when stuff goes awry



It has been many years since physical chemistry and I were intimate in a hot afternoon lecture theatre- but here goes

This has lots of complications : duh and I hope my simplification is not too painful 

The heat of enthalpy is a principle of phys chem that begins to describe how chemical reactions can abruptly go from a steady state into excitement.

Eg a certain amount of energy is required to push a fluid into a gaseous state - this energy input is quite high for water and different for each relevant set of reactants and azeotropes (mixtures which act as a single molecule)
And so the energy required to boil water is much higher for water than conc nitric acid (68%) and very much more for RFNA ( Red FumingNitric Acid) thus the transition between boiling water and boiling Nitric and then RFNA will fall over a cliff as the boiling point for one fluid is replaced by the next which needs less energy then the previous and so must quickly use the energy that is now in excess.

So the heat required changes as each component is boiled off.

There are also issues of geometry and reaction states. But simply put: it is a geyser in action - superheated steam pushes through a column of fluid. As energy is added to the bottom.

Thus a steady state becomes a recirculating reaction.

This is why it is important to allow everything to cool before disassembly as cool fluid on superheated molten salt or solutions can be explosive.

Thus your control is not at fault:

It is the nature of both the physics and chemistry that these transitions between the azeotropes and end states are abrupt and energetic.

Thus the cautions:
1. Do not rush
2. Do not seal the system
3. Monitor closely

Regards Grumbles


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## pdamiant (Dec 30, 2021)

Grumbleact said:


> @ pdamiant
> 
> Boil over = the ‘oh sh1t’
> Moment when stuff goes awry
> ...


That was perfect. I actually understood it and it all made sense. Well done and thank you. 

Now if you could just explain why a 24/40 joint wont come apart. The 24/40 connection between my boiling flask and my tee- spout appears to have welded together. lol. Not the end of the world, but I can't exactly hit it with a hammer to knock the glassware apart.


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## olawlor (Dec 31, 2021)

pdamiant said:


> Now if you could just explain why a 24/40 joint wont come apart. The 24/40 connection between my boiling flask and my tee- spout appears to have welded together. lol. Not the end of the world, but I can't exactly hit it with a hammer to knock the glassware apart.



I've accidentally used thermal contraction to almost weld ground glass joints together: a cold inner joint and hot outer joint, once assembled, is nearly impossible to remove because the cold inner joint expands and the hot outer joint contracts.

Heating the outer joint while cooling the inner joint can reverse the process. Good borosilicate can take a surprising amount of heat as long as it's applied slowly and smoothly.


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## Grumbleact (Dec 31, 2021)

Ah an easy one!

The outside has shrunk onto the smaller one therefore make the inside smaller or (usually) make the outside bigger. 
Fit your best asbestos glove to one hand and then wave a heat source over the joint.
I use an electric heat gun. Usually releases in seconds. 

Regards Grumbles


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## Dorcas Nimrod (Dec 31, 2021)

Sulfuric acid is often recommended to seal the joints in this case, but I've always used teflon thread tape. It works well and precludes any possibility of joints freezing.


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## Jado (Dec 31, 2021)

Grumbleact said:


> This hobby has some exciting moments usually due to a moment of inattention or lack of experience. Quite a lot of things are going on during distillation of acid and a basic understanding of what is in your reaction vessel and the sequence of what happens when, is useful.
> 
> A boil over happens in this case as either gas evolves rapidly (either steam or nitrogen dioxide) and the reactants convert from aqueous solutions to molten salts.
> Martin is correct that you can simply pour it all back and start again.
> ...


When you use the Bisulphate reaction, do you have a use for your waste product? Do you separate the salts? If so, do you use methanol or have you found an easier way to do it small-scale?


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## Grumbleact (Dec 31, 2021)

@ jado

I use the Bisulphate reaction because it is easy to do a small batch with minimal risks - enough for one or two batches of gold or silver. Apart from the Nitric acid (which I use) the waste is sodium or potassium sulphate And an excess of sodium bisulphate added to force the equilibrium reaction to nitric acid. 

Eventually I will reclaim nitric acid from my waste Copper Nitrate - but that is a future adventure  those wastes are more complex. 

In a residential pool these chemicals are regarded as safe with dilution in water - and are dumped into storm or wastewater systems during Normal pool maintenance as per local ordinance. 

Therefore - I simply wash out after a batch in laundry sink. Using generous amount of cold water. 

I have found stump remover (Potassium Nitrate) has small amounts of chloride in which can give silver chloride in your gold recovery. Unless you carefully separate your early fractions during distillation. 

Hydroponic Sodium Nitrate is about 20-30% more expensive here and is pure and more soluble for clean up. 

Either way it is simple to re-distill with reflux to separate HCl and HNO3 if it is troublesome. 

Seems there are always swings n roundabouts - what is your time worth etc. 

Hope that helps. 

Regards Grumbles.


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## Jado (Jan 1, 2022)

Grumbleact said:


> @ jado
> 
> I use the Bisulphate reaction because it is easy to do a small batch with minimal risks - enough for one or two batches of gold or silver. Apart from the Nitric acid (which I use) the waste is sodium or potassium sulphate And an excess of sodium bisulphate added to force the equilibrium reaction to nitric acid.
> 
> ...


IIRC the K salts are reusable (both sulphate and bisulphate) but the Na salts would need to be removed. It’s been a while, but we had this as a topic back in school. Don’t get old, things slip quick. IIRC methanol separation was viable in small batches (cold to avoid noxious fumes), but there were some interesting discussions involving Cu and Fe nitrates.


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## Grumbleact (Jan 1, 2022)

@Jado 
Thanks for your comments. 
I am afraid it is too late to take your advice about not getting old - at this stage I am limiting my tasks at hand to not getting dead. 

I make mineral acids because I cannot buy them easily or cheaply.

Nitric acid because I must have it to remove copper and silver efficiently. 

Sulphuric acid in tiny amounts to remove lead and for electrolytic processes. 

I too am watching nitrate/nitric recovery from waste copper nitrate solutions with some interest. There appear to be three approaches (with variations)

1. Distillation from waste with fresh Sulphuric Acid as a primary consumable. 
2. Electro winning copper and sulphuric acid from copper sulphate with distillation of remnant nitric acid. 
3. Thermal decomposition of nitrate salts. 

All methods seem to be quite hazardous especially given very toxic heavy metals, toxic acid fumes and arsenic maybe involved. 

Sounds to me like lots of reading and preparation of PPE and fume scrubbers. 

While adding alcohol or ammonia to nitrate salts is intellectually stimulating I am loath to create energetic substances especially by accident. 

Again thanks for sharing your expertise , experience and time. 

Regards Grumbles


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## Yggdrasil (Jan 2, 2022)

Jado said:


> IIRC the K salts are reusable (both sulphate and bisulphate) but the Na salts would need to be removed. It’s been a while, but we had this as a topic back in school. Don’t get old, things slip quick. IIRC methanol separation was viable in small batches (cold to avoid noxious fumes), but there were some interesting discussions involving Cu and Fe nitrates.


Jado. What is IIRC?


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## BlackLabel (Jan 2, 2022)

IIRC stands for „if I remember correctly“.


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## FrugalRefiner (Jan 2, 2022)

And this is why we ask members not to use text lingo and similar abbreviations. Many members have to use translators to read the forum, and such abbreviations do not translate well.

Dave


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## pfg0200 (Jan 2, 2022)

pdamiant said:


> Happy New Year folks,
> 
> I am making my own nitric acid using sulfuric acid and potassium nitrate. I had a boil-over and some of the contents of my boiling flask ended up in my receiving flask. Has this ever happened to you? My thoughts are to simply pour the contents of my receiving flask back into the boiling flask and start again. But do I need to? I can see the potassium nitrate on the bottom of the boiling flask and can filter that out, but what about the sulfuric acid mixed in with the nitric after I filter? Will it matter. I am using the nitric to work on gold.
> 
> Thank you.


I find you don’t need to make nitric acid by distillation. Add the nitrate to the gold you want dissolve then add some water. Add the sulfuric acid


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## Jado (Jan 2, 2022)

FrugalRefiner said:


> And this is why we ask members not to use text lingo and similar abbreviations. Many members have to use translators to read the forum, and such abbreviations do not translate well.
> 
> Dave


my bad. will have to write up a sig with a disclaimer stating it’s been a while since university and my memory isn’t as great as I wish it was


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## orvi (Jan 3, 2022)

Grumbleact said:


> @ pdamiant
> 
> Boil over = the ‘oh s
> Moment when stuff goes awry
> ...


Very nicely explained. Water is unique with its heat of evaporation, and it is overlooked in many instances. Cooking without a lid is one of the best examples  
This is why many accidents are well preventable. If person is well educated and can evaluate the hidden risks of the setup and procedure. It could be called over-thinking or over-education, but in terms of messy accidents, it is close to zero


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## orvi (Jan 3, 2022)

pdamiant said:


> That was perfect. I actually understood it and it all made sense. Well done and thank you.
> 
> Now if you could just explain why a 24/40 joint wont come apart. The 24/40 connection between my boiling flask and my tee- spout appears to have welded together. lol. Not the end of the world, but I can't exactly hit it with a hammer to knock the glassware apart.


Never give the salts chance to access the ground glass joint. With great surface area of the ground glass, they could solidify, filling all gaps, tightening the joint and making it nearly impossible to dissasemble. 
Basic solutions are another example - etching the glass slowly, tightening the joint together.

Basic thing you could do is grease joints with good quality silicon grease - but for distilling nitric acid, you cant do that. Nitric will decompose nearly any grease.
Greasing with concentrated sulfuric acid is advised instead.
Best solution is to use teflon joint spacers or lab grade teflon tape - but these are expensive. On the other hand, they will last for years 

To dissasemble freeze joint, one of best things is ultrasonic cleaner. If not in hand, heating technique is very helpful. Heat gun or torch will do the trick. Always wear good and thick gloves when applying force to the freeze joints. Cracked glass in the palm and fingers with simultaneous applying good bit of force create very nasty cut, very deep, in a very bad place - lots of veins :/ big bad. Seen few times in my lifetime, not pleasant at all.

When trying to loosen it, not only try to "turn" it, or only "pulling" the pieces. Very good technique is (with thick gloves on your hands) slightly waggle the inner part from side to side, changing the directions of applied force and simultaneously pulling pieces apart. The waggling should be only very gentle, and it requires some skill, but it work wonderful


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## Noggin (Jan 3, 2022)

I was once present at the moment of a boilover. It was rolling and I just turned the heat down (my old mantle without digital controls). The bubbling solution was teasing the bottom of the distillation column. Suddenly the vapor on the bottom lifted the whole mass and it just shot everything over nearly completely (thank you Keck, wherever you are).
Yes imo, you should pour back into your boiling flask and start over with a clean receiver. Sulfuric is hard on paper filters and can easily make an acid that's way harsher than nitric by itself. You will also create sulfates in solution which may be less soluble than the nitrates.
Nitric acid boils off at about 120-ish (technically should be less but I use 120 as guideline). You'll stay way below the angry boiling temperature of your sulfuric, which will cause bumping and boilovers. Hopefully you have an IR thermometer, and watch the temperature at your column. Covering the riser and column adapter with aluminum foil conserves heat and lets you keep temperatures lower.
Steel keck clips are worth investing in.
I've done several recoveries of nitric from my copper nitrate using sulfuric acid, so I'm familiar with the process. This is no joke, after your upper column temperature falls (because no more vapor is being brought up) you can turn off the heat and let it cool. When hot this stuff is savage.
There's a lot more to this that you may already know - I won't assume. Let us know if anything's unclear.


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## Noggin (Jan 3, 2022)

orvi said:


> Never give the salts chance to access the ground glass joint. With great surface area of the ground glass, they could solidify, filling all gaps, tightening the joint and making it nearly impossible to dissasemble.
> Basic solutions are another example - etching the glass slowly, tightening the joint together.
> 
> Basic thing you could do is grease joints with good quality silicon grease - but for distilling nitric acid, you cant do that. Nitric will decompose nearly any grease.
> ...


Orvi's right on everything, but I've also got a couple items for thought. 
Grease won't last very long against sulfuric acid. To get a tight seal you can consider sulfuric acid. It's got decent viscosity. 
Also for the frozen joints, I use my squirt bottle and put a few drops in the joint. It will wick through the salts if the glass is warm (like 70-80 c is when I disassemble my apparatus). I can't stress the thick gloves part enough


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## snail (Jan 4, 2022)

Lou recommended Krytox grease for the joints. Pretty pricey but you don’t use much. Being some sort of perfloropolyether compound I imagine it is pretty durable.


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