New, safer method of nitric acid production / Chemistry / Forums

Forums

4hv.org :: Forums :: Chemistry

« Previous topic | Next topic »

New, safer method of nitric acid production

1 2 next

Move Thread

LAN_403

Shaun

Thu May 10 2007, 12:10AM

Registered Member #690 Joined: Tue May 08 2007, 03:47AM
Location: New Jersey, USA
Posts: 616

Okay, this is my first post and first thread on 4hv, so bear with me. I'm 17 years old and I take my AP chem test in less than a week, and I'm gonna get a five.

Anyway, anyone whos ever been into the pyro category has thought about how to get/make nitric, and If you are one of these people you know it is VERY HARD AND VERY DANGEROUS, unless you only need small, very dilute batches.

My method was cobbled together based on what I learned from other methods, so dont be surprised if anything sticks out. It is partly based on electrolysis, partly double replacement/precipitation. There are two variations to the method, and each have their advantages and drawbacks. Neither involve heating liquids.

First, I set up a basic electrolytic cell. As a salt bridge I used a heat-bent piece of half inch PVC pipe with paper towel wads on both ends. The salt bridge and both cells were then filled with a saturated KNO3 solution. As the anode, I used a stripped piece of big ass 4 gauge copper wire, left over from my Tesla Coil's ground (about 6" stripped). The cathode material doesnt really matter, but I used a lage galvanized iron bolt. The power supply was a 19.5V 5.13A laptop charger. Now I know this was probably way more volts than I needed, but since the solution itself is a fairly poor conductor, i figured Ohm's Law would kick in and allow more amps to flow through a constant resistance as voltage is increased. I'm not sure if Ohm's Law can be used here because the solution would be moving ions, not electrons. (interesting note, the salt bridge actually stays warm to the touch; i guess it acts as a resistive heating element)

Anyway, I let this setup run for about a week, and I usually end up replacing the anode three times.

At the anode the Cu is oxidized to Cu(NO3)2, while KOH forms at the cathode (tested w/ litmus paper).
Another way to test is to mix a drop or two from each half-cell in a cup, and a dark blue Cu(OH)2 precip forms.
When it's done, evaporate the anode side, yeilding a copper nitrate hydrate, probably with some KNO3 impurites, but these won't matter.

Now that you have the Copper nitrate (this is the new part), you can heat it strongly to decompose it into NO2 gas, water vapor, and copper oxide. Allow the NO2 to pass through water, and you have HNO3! The drawback here is that a gas such as this dissolving in water is very exothermic, so a larger volume of water should be used for safety, but it would yield a less concentrated acid.

The variation to this method, which I have yet to attempt, would utilize a pure silver anode (old sterling spoon, maybe?), which would be oxidized to silver nitrate. The beauty of this is that the silver ion is notoriously insoluble, so you can mix it with nearly any other acid to safely yield a precipitate and HNO3. It would be wise to do some stoichiometry before this part, so you are have minimal left over Ag+ or A- (from the initial acid) in your product.
The only limit to the concentration of your product would be the concentration of your inital acid. Muriatic acid would be good, its cheap and high molarity (AgCl is insoluble). H2SO4 could not be used in this method because silver sulfate is soluble.

Of course, you could use a lead anode, it would be cheaper, but Pb+2, while also extremely insoluble, is VERY BAD FOR YOU!
Ag+, on the other hand, is non-toxic, but it can stain skin and if ingested, will turn you blue!

Have fun, good luck, and BE SAFE!!

PS I dont know if its aginst the rules here to discuss making nitric, if so, sorry.

Oh my god I just realized I misspelled my thread title. *sigh*

[Edit: Use the edit button then you don't have to double post.]

Chris

Thu May 10 2007, 12:36AM

Registered Member #8 Joined: Thu Feb 02 2006, 04:34AM
Location: Harlowton, MT, United States
Posts: 214

I have distilled RFNA from a slurry of KNO3 and concentrated sulfuric acid several times. We have another thread on it. That's much easier and more more direct, with fewer steps, and requires less heating than your method. The aparattus is not any more complex either, and it doesn't use up lots of copper. It does generate a lot of NO2 as well, which is a highly toxic gas, but that could be prevented using vacuum distillation, driven out and run through water similar to your method, or maybe you could just add water to the RFNA and bubble air through to convert it completely to HNO3.

The spent potassium sulfate can be reduced with coke (or any relatively pure form of carbon) and silica at high temperature to yield sulfur dioxide and potassium silicate (a precursor to silica gel which could be useful). Sulfur dioxide can then be catalytically oxidized over vanadium pentoxide/KOH catalyst at 500C to yield sulfur trioxide, which forms oleum when added to concentrated sulfuric acid. Oleum is then added to water to further yield concentrated sulfuric acid, recycling it. This method is used instead of directly adding SO3 to water because such an addition is too highly exothermic and forms a lot of acidic vapor/mist.

As for other methods that use NO2 generation directly as a precursor to HNO3, you could carry out the Ostwald Process. Over a platinum catalyst at 800C (I wonder if a catalytic converter core would work) ammonia is oxidized to NO and further (on cooling) to NO2 in an excess airflow on the order of 100psi. Upon bubbling this gas with the excess air through water nitric acid is formed. Due to the high temperatures and corrosiveness, this must be done in stainless or nickel clad aparattus.

If you can get ammonium nitrate, you can do both:
a) Add alkali to ammonium nitrate to generate ammonia, which is run through an Ostwald processor.
b) Add resultant metal nitrate to sulfuric acid to distill RFNA.

With any of these methods, remember how toxic NO2 is. It doesn't take very much to kill you, with symptoms of lung adema not developing until several hours after exposure. Even though it's a pungent gas on short exposure, you can quickly become desensitized to it and not know you are inhaling dangerous amounts. Either use an aparattus that doesn't vent any (difficult) or use lots of forced ventilation, like a fume hood or something.

...

Thu May 10 2007, 12:59AM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

I remeber taking AP chem last year (sophmore) and thinking about how much safer 'playing with wires' as my friends call it is than chem... At least when you touch a hv wire you KNOW that you got exposed

I any case, definantly be safe about the NO2, it is just such a nasty gas to be playing with. To get a 5ppm concentration (enough to render your nose useless) in a 10x10x8ft room only takes 2mg leaking out somewhere...

You know, you can get a 2nd hand SCBA on ebay for like $50.... And a cheap oilless air compressor could be had for under $100 as well... Then you can really freak out your friends :p

Shaun

Thu May 10 2007, 04:12AM

Registered Member #690 Joined: Tue May 08 2007, 03:47AM
Location: New Jersey, USA
Posts: 616

The reason I stay away from distillation is because the HNO3 eats through nearly anything except glass and teflon, any making a still from those materials would be very tough for me. Also, the idea of piping hot nitric acid and nitric acid vapor is not very appealing to me. <sarcasm> Of couse, I have plenty of platinum and vanadium oxide catalysts not rusting in my backyard, so the ostwald process shouldnt be a problem </sarcasm>.

But seriously, I think the method I described is much less dangerous than a distillation, and I'm not sure how it would require more heat than distilling HNO3 or catalyzing reactions at hundreds of degrees C, as Chris said. To clarify, the only heating involved would be maybe 20g of Cu(NO3)2 in a test tube or something.

Also an important note for anyone planning to perform sensitive nitrations- Red nitric acid, which is produced by distillation and in the method of passing NO2 through water, is NOT suitable at all for a nitration. However, Red can be converted to White by holding it under a moderate vacuum for an hour or so.

This is why I like the precipitation method. No hazadous gases, no heat, no fumes, produces WHITE nitric, and is capable of high concentrations and large volumes. On the money side, its cheap and can be done with very simple, common materials. If you value nitric acid more than a silver anode. This is more of a teenage amateur method (cause that's what I am!).

BTW, thanks for telling me about the edit button (I edited in this statement), Chris

Chris

Thu May 10 2007, 05:03AM

Registered Member #8 Joined: Thu Feb 02 2006, 04:34AM
Location: Harlowton, MT, United States
Posts: 214

So, you don't want to distill because piping hot nitric acid vapor is difficult... yet the method you propose involves piping even hotter NO2 (170C decomp for copper nitrate). What's the difference? Either way you are generating a hot oxidizing gas in one vessel that will corrode nearly everything except glass and teflon and ignite most plasics, and piping it over to another vessel for collection. The RFNA distillation can be carried out in just as simple of aparattus, it does not really require fractional distillation since you are separating compounds with wildly different boiling points (though I did use a complete reflux rig for my distillations).

This is why I like the precipitation method. No hazadous gases, no heat, no fumes, produces WHITE nitric, and is capable of high concentrations and large volumes. On the money side, its cheap and can be done with very simple, common materials. If you value nitric acid more than a silver anode. This is more of a teenage amateur method (cause that's what I am!).

No hazardous gases? I thought your method involved decomposing copper nitrate to NO2, a very hazardous gas? No heat? You spoke of heating copper nitrate to decomposition, which is at least 170C.

For the second method, I don't think many people would value nitric acid more than a silver anode either, unless you only need very small amounts.

PS I dont know if its aginst the rules here to discuss making nitric

What would be the point of even having a chemistry forum if you couldn't discuss making the most important and useful chemical reagents?

Shaun

Fri May 11 2007, 12:09AM

Registered Member #690 Joined: Tue May 08 2007, 03:47AM
Location: New Jersey, USA
Posts: 616

Now that I reread my first reply I realize it may have sounded a little rude, that was not intended.

No heating and no gases are involved in the process where silver nitrate is used for a simple double replacement reaction to yield HNO3 and a precipitate, to clarify that part.

Bored Chemist

Fri May 11 2007, 06:10AM

Registered Member #193 Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022

I can't help thinking that people are being a little harsh on the newbie.
The ideas are interesting, particularly the silver based method. (Though it would take a while to get more AgNO3 than KNO3). Rather than writing "Of couse, I have plenty of platinum and vanadium oxide catalysts not rusting in my backyard, so the ostwald process shouldnt be a problem " you could have done some research and found out that NH3 can be oxidiesd to NOx with a hot copper oxide catalyst. Even a used exhaust catalytic converter would (as suggested) work.

and re. "What would be the point of even having a chemistry forum if you couldn't discuss making the most important and useful chemical reagents?"
Well, most professional syntetic chemistry fora wouldn't bother because they would just buy the stuff.

Chris

Fri May 11 2007, 03:54PM

Registered Member #8 Joined: Thu Feb 02 2006, 04:34AM
Location: Harlowton, MT, United States
Posts: 214

I don't think anyone was particularly harsh, just pointing out various fallacies. There's nothing wrong with that.

Looking further at the precipitation method, it is apparent that silver would be far too expensive to be of any real use. Lead nitrate as you mentioned would be a far more likely choice. The toxicity of lead is really of no issue whatsoever here, since even soluble leads compounds (yes, lead nitrate is soluble in water, (52g/100mL) making it far more toxic than other forms of lead) pose little hazard when compared with concentrated nitric acid. I wasn't able to find anything about the solubility of other nitrates in HNO3, but I know that lead nitrate is insoluble in it as long as it's an anhydrous mixture. You will need to dry it out from the electrolytic bath, but this is quite easy as it forms no hydrates.

It would seem that the procedure to follow that would be nearly the same as the KNO3/sulfuric acid method prior to distillation. It may even require heating for the reaction to proceed fully, and the anhydrous stochiometry will involve a heavy paste of lead nitrate and H2SO4 from which it will be difficult to separate the HNO3. Even the anhydrous procedure with KNO3/H2SO4 involves a heavy paste in which it can be difficult to get the reaction to proceed fully until it has been heated enough to dissolve most of the nitrate forming a thick reddish solution. I always use a large excess of H2SO4 (one time I didn't and the paste, with which the aparattus was highly loaded, ended up clogging the reflux column, resulting in 3rd degree chemical burns, a very bad situation). With your precipitation you will likely need to stir the mix vigorously, and then somehow separate the HNO3 from the paste with pressing or a centrifuge. This is not like a typical precipitation where a small amount of solid falls out of a bulk solution; rather it is quite the other way around. If you use an excess of H2SO4 to provide more liquid reaction medium, you'd have to use distillation anyway to separate it. Perhaps you could use air pressure to force the relatively small amount of liquid HNO3 through a filter. Due to capillary action however, it will be difficult to remove it all, and your lead sulfate may need washed afterward.

The reaction will be much easier to carry out for lower concentrations of nitric acid, since the soluble lead nitrate will react easily forming insoluble lead sulfate, and the large amount of water will provide plenty of liquid medium out of which sulfate will precipitate. To gain azeotropic nitric acid from this diliute solution would, of course, require distillation, while gaining WFNA from it would require vacuum distillation with sulfuric acid. It would work well if you only needed a low concentration though. I think that is where this method could be the most useful. In either the aqueous or anhydrous case the resultant nitric acid will probably contain some amounts of lead and sulfuric acid impurity which could only be separated through distillation.

One thing that is neat about this method, is that lead sulfate can be electrolyzed right back to sulfuric acid (like in a lead-acid battery), unlike the other metal nitrate methods in which recycling sulfuric acid requires a coke reduction followed by full implementation of the Contact Process utilizing exotic catalyst. In light of that convenience, lead nitrate would probably be a superior reagent for the standard sulfuric acid/distillation procedure for RFNA/WFNA than potassium or sodium nitrates, though I maintain that distillation is likely the easiest method for separating HNO3 at high purity from the anhydrous system.

Shaun

Fri May 11 2007, 07:46PM

Registered Member #690 Joined: Tue May 08 2007, 03:47AM
Location: New Jersey, USA
Posts: 616

Yes, I alway knew I would need a distillation to get true anhydrous HNO3. I'm not sure if I mentioned it but this method (precipitation) absolutely works better making dilute (<80%) acid. (safer too)

By the way, during the Cu/KNO3 electrolysis, a strange reaction occurs within the salt bridge, more specifically in the paper towel plug. The cathode (KOH) side plug turns slightly yellow, but the Anode side (Cu(NO3)2) plug becomes covered at the entrance with a crusty orange substance. The substance can easily be pulled off, revealing a black powder behind it with a color and texture resembling ash. I know the PVC pipe is not reacting because it is still totally intact (I can see the file marks from where I cut+smoothed it). Both of these things occur on the anode side ONLY.

I have absolutely no idea what the orange substance is, but I think the black ashy substance could be organic compounds in the paper that have somehow decomposed to form carbon and H2O.

Anyone heard of this before?

ramses

Tue Jan 22 2008, 09:34PM

Registered Member #1208 Joined: Thu Jan 03 2008, 05:30PM
Location: Chesterland, OH
Posts: 154

I haven't tried this, but the improvised munitions handbook (issued the the US military) seems to have an OK guide:

seems easy enough, just leave the immediate area when the fire/heat becomes involved. you could probably use an electric hotplate just to be safe(ish)

1 2 next

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob