Sciencemadness Discussion Board

Problems with nitroglycerin synthesis

Rich_Insane - 2-4-2022 at 10:51

So I tried synthesizing some nitroglycerin in the lab yesterday, but both times I noticed a lot of NOx evolution and black material (presumably from the H2SO4) show up in the reaction vessel. Now, I keep the vessel on ice. First I add the nitric acid (13 mL 70%; 15.9 M) and chilled it on a beaker to reach a temperature around 5 degrees C. (~10-15 minutes). Next, I added 41 mL of 18 M sulfuric acid incrementally so the temperature would not pass 15 degrees. Once the solution has cooled to 15 C again, I add glycerol dropwise, maintaining a temperature below 30 degrees C. For the first two trials I did not calculate how much glycerin I needed. Now that I do the calculations, it appears I need 6.2 mL -- I had been adding 10 mL (slowly, not rapidly).

The problem with this is that the temperature does not rise very high initially after adding the glycerol, so I continue adding all of it in over a period of 2-3 minutes, carefully monitoring the temperature such that it is below 30oC. However, within 5 minutes, the mixure begins fuming with NOx and the beaker overflows with this black sludge. The temperature remains below 30 oC until this point, but after stirring, the black sludge forms, and the mixture begins to foam and boil. Addition to NaHCO3-water solution yields two layers, one with the sludge and a bottom later that is a clear red color. What am I doing wrong? Could excess glycerol be causing the problem? Or perhaps excess acid? I calculated the moles of nitric acid.

I did some calculations and found that there are approximately 0.11 mol of glycerin in 10 grams (8 mL). Tripling this value to 0.33 mol (3HNO3 per glycerin) gives the amount of nitric acid necessary -- 21 mL (0.3257 mol). What is confusing me is the requirement for so much sulfuric acid. I was under the impression that sulfuric acid is simply a catalyst that promotes formation of the nitronium species (and a dehydrating agent) and is not consumed in the reaction. Is this not correct?

Fulmen - 2-4-2022 at 11:06

The first thing you can try is to heat the acid to 25°C. Low temperatures can delay the initial reaction, tricking you into adding too much.

Rich_Insane - 2-4-2022 at 14:39

Should I heat the acids separately? I'm using HNO3 and H2SO4. I notice that when I mix them, there is some kind of a reaction (fumes form, temperature rises)?

SWIM - 2-4-2022 at 20:25

Forming the nitronium ion makes water at the same time, so you need a lot of extra sulfuric acid to keep the water tied up to get a healthy nitronium ion concentration.

I don't do this myself much, but I'm pretty sure that's the answer

Herr Haber - 5-4-2022 at 06:24

I dont think heating is a good idea. Just add the glycerin over a much longer period of time stirring continuously.

B(a)P - 5-4-2022 at 15:17

Are you sure of the purity of your glycerine?
You also need an excess of nitric acid to make sure your glycerine is fully nitrated.
In the past I have had good yields using approximately 1.5 x the volume of nitric acid of sulfuric acid.
So to nitrate 10 mls of glycerine, add it to mixed acid consisting of 30 mls of HNO3 and 45 mls or H2SO4.
I would also recommend keeping your reaction mix cooler, no more than 20 C and start your additions when your mixed acids are well below this temperature.
For 10 mls of glycerine being added to mixed acids in a salted ice bath you should expect the additions to take approximately 30 minutes with good stirring.

Rich_Insane - 5-4-2022 at 18:10
Quote:

Are you sure of the purity of your glycerine?
You also need an excess of nitric acid to make sure your glycerine is fully nitrated.
In the past I have had good yields using approximately 1.5 x the volume of nitric acid of sulfuric acid.
So to nitrate 10 mls of glycerine, add it to mixed acid consisting of 30 mls of HNO3 and 45 mls or H2SO4.
I would also recommend keeping your reaction mix cooler, no more than 20 C and start your additions when your mixed acids are well below this temperature.
For 10 mls of glycerine being added to mixed acids in a salted ice bath you should expect the additions to take approximately 30 minutes with good stirring.


That's exactly the mixture I used (30 ml of conc. HNO3 and 45 ml of conc. H2SO4). I did not salt the ice bath though, and I think that might be a good idea. I was using old ice from the ice machine. I'm just stunned at how violent the reaction was. There was a cloud of NOx released and the mixture turned red then pitch black, eventually splitting into two layers. It happened very suddenly, the temperature was at around 15-18 degrees C

The glycerin I used was lab-grade at the least, and might've been ACS reagent grade.

B(a)P - 5-4-2022 at 18:27

Quote: Originally posted by Rich_Insane  


That's exactly the mixture I used (30 ml of conc. HNO3 and 45 ml of conc. H2SO4). I did not salt the ice bath though, and I think that might be a good idea. I was using old ice from the ice machine. I'm just stunned at how violent the reaction was. There was a cloud of NOx released and the mixture turned red then pitch black, eventually splitting into two layers. It happened very suddenly, the temperature was at around 15-18 degrees C

The glycerin I used was lab-grade at the least, and might've been ACS reagent grade.


And you definitely employed good continuous stirring/swirling, with slow additions? That is indeed a strange outcome. What you describe sounds like a runaway reaction, but with no rise in temperature. The nitration of the glycerine should occur without the generation of NOx.

katyushaslab - 7-4-2022 at 01:31

Use a salted ice bath to get lower temps. I like to have my nitration mixes very, very cold to begin with. Been ages since I did this, but I'll drop some thoughts below. Used to do this a lot.

From the sounds of it, insufficient mixing was likely the culprit.

So the glycerol often doesn't mix in very well without a decent amount of swirling (or slowish stirring using a stirplate), and can form a layer or some "globs" that you may not notice, which can cause localised hotspots and trigger a runaway reaction. The overall temp will seem low due to the hotspot being localized to one place.

My usual process for this was mixed acids in a salted ice bath on top of the stirplate, stirplate on medium speed so there is a good amount of stirring going on, and dripping in the glycerol "down the side" of the beaker, so it gets "washed" into the mix quite well by the vortex-of-sorts.

You do want to let the temp rise somewhat, so the reaction can actually get going, but "not too much". It is a careful balance.

Once additions are done, I like to just leave it under slower stirring for quite a while. Let the ice bath melt, let it slowly, slowly increase in temperature to room temp over an hour and a half or so. I didn't find this longer contact time with acid to have any real detrimental effects on yield or quality of the product - though - mileage may vary, and some reckon you should get the product out after maybe 20 minutes. The last thing you want is "frozen" or "semi frozen" NG.

Some people have legitimate concerns around the use of mechanical stirring for this, but personally, I find the risk of local heating and runaways higher. NG in suspension/solution is not particularly sensitive at all, would take a lot to get it going.

zed - 14-4-2022 at 03:55

I would use a published procedure, and follow it carefully.

Moreover, it is very important to purify your final product carefully.

A buddy of mine, made a kilo or so, and stored it for a short while.

Upon later inspection it was found to be reddish and outgassing.

You don't want your Nitroglycerine doing that.

Alarmed, he disposed of it by pouring the material out into a small depression in the ground.

This also was unwise.

Perhaps in Winter, rains would have sufficiently diluted the unstable Nitroglycerine.

But alas, it was not Winter.

After a few days, it occurred to him, that he might have created a kind of non-mechanical "Land Mine" in his own front yard.

The area in Question, was not detonated by a careless footstep, but apparently it might have been.

Ultimately, he hosed hundreds of gallons of water onto the area in question.

There were no further problems, but there might have been.

As I recall, thereafter, there was a remarkable decline in his enthusiasm, for manufacturing Nitro-Glycerine.

This looks kind of interesting.

https://www.chemistryworld.com/news/industrial-nitroglycerin...

[Edited on 14-4-2022 by zed]

Fantasma4500 - 17-4-2022 at 02:51

i recall something about in germany they sold something like 60% or was it 50% HNO3- because then it would be practically useless for making NG
this could maybe explain your struggle, or maybe not, youre using 70% HNO3. this could maybe imply your glycerine is the problem, oftenly thats 85%, i vaguely remember that it can be dehydrated by simply heating it
if a reaction goes slow i would just dump it in the fridge, it might be shy. ive had nitrations in fridge going for +24H with great yields no problems.
i would also suggest hopping onto EGDN, i did that one with just 62% HNO3 long ago, its a bit stronger and a thousandfold easier to seperate as it sinks to the bottom so you can just tilt beaker and suck it out, also less volatile and less prone to going bad

now that im at it i do remember an accident in a NG plant where a worker added in hot steam into the NG, i cant remember which phase of the process it was, but it got heatshocked and blew up, it is possible that heating the nitration mixture with steam was done back in time?

i would be a bit shaky heating up a nitration mix with NG in it, if it does turn into runaway it could end up detonating by heatshock, even without it being full detonation you would have the added joy of boiling acids and nitrous fumes