Sciencemadness Discussion Board - Preparation of Nitric Acid

Preparation of Nitric Acid

Magpie - 2-12-2009 at 22:03

Preparation of Nitric Acid

by Magpie

12/2/09

Introduction
This procedure is for the preparation of 125 mL of azeotropic (68wt%) nitric acid using Rooto sulfuric acid and pottery grade KNO3.

KNO3 + H2SO4 = KHSO4 + HNO3

CAUTION Toxic NO2 is generated during the early stage of this procedure. Therefore it must be done outside or in an efficient fume hood.

Procedure

A. Reagent Preparation
Weigh out 225g of KNO3 in a beaker, crushing any lumps to attain a uniform powder. Set aside.

In a separate beaker carefully add 100mL of Rooto sulfuric acid (96wt% H2SO4) to 48 mL of water with stirring. The beaker should be cooled in a container of cold water during the addition of the acid.

B. Equipment Set-Up
Place a 500mL round bottom flask (RBF) as pot in a simple distillation set-up using a 250 mL RBF receiver. Seal all ground-glass joints with a thin coat of silicone grease. Place a Claisen adapter between the pot and the distillation adapter as shown in the photo below. This is to mitigate any foaming. Insert a plug in the unused short Claisen port. Install joint clamps at all joints except that for the plug. The plug will serve as a pressure relief port in the event of a pluggage. Provide cold water (or ice water) to the condenser.

Place the pot on a 500mL, 270 watt Glas-Col mantle (or equivalent). Install a thermometer capable of reaching 120°C in the distillation adapter. Use of a PTFE thermometer adapter is recommended as rubber is attacked by hot nitric acid vapors. If necessary the thermometer could be omitted and replaced by a plug.

Temporarily remove the pot and add the diluted acid prepared above. Using a powder funnel add the KNO3. Occasionally stir by swirling and using a stirring rod. A viscous mass will result. Reconnect the pot to the Claisen adapter and bring the heating mantle into position.

Wrap the pot with an insulated blanket. Wrap the Claisen adapter with fiberglass insulation.

C. Distillation

Turn on the cooling water. If using a hood, turn on the fan. Set the mantle at 70% of maximum power and turn on.

As the system heats up it will first fill with brown NO2 gas. This will escape through the vacuum adapter port. A piece of Tygon tubing attached to this port to direct the flow of NO2 away from the cooling water rubber hoses is recommended. After a few minutes the generation of NO2 will cease. The vapor temperature at the stillhead will reach and hold solid at 120°C.

A steady generation of 68% nitric acid will follow. Very little attention is needed until near the end of the distillation. At this time the Claisen adapter will begin to fill with foam. Terminate the distillation at this time.

CAUTION
Excessive foam generation and saltcake carryover will likely plug the condenser. The safety relief plug will blow and saltcake will flow down over the insulation and mantle. NO2 will escape out of the relief port. The product may become contaminated with saltcake.

Product
The receiver RBF should contain about 125mL of 68% nitric acid at the end of the distillation. It will be tainted brown with dissolved NO2. The color will eventually disappear if the lid on the storage bottle is not too tight fitting.

References
1. "Preparation of Nitric Acid," by Eleusius
2. "Equal weights of 96% H2SO4 and NaNO3" from traditional recipes.

no NO2.jpg - 71kB

[Edited on 3-12-2009 by Magpie]

NO2 gas.jpg - 68kB

[Edited on 3-12-2009 by Magpie]

reagents (2).jpg - 62kB

[Edited on 3-12-2009 by Magpie]

no insulation.jpg - 72kB

[Edited on 3-12-2009 by Magpie]

powder funnel.jpg - 66kB

[Edited on 3-12-2009 by Magpie]

[Edited on 3-12-2009 by Magpie]

Aqua_Fortis_100% - 3-12-2009 at 04:43

Magpie, do you tried to harvest and purify the saltcake KHSO4?

Its a interesting by-product since you could make other K salts.. Im interested in simple substances like K2CO3 and also in KOH, and here the only OTC salts are KCl and KNO3(much less OTC).

One time I did this and found that was difficult, since I used concentrated acid without dillution and the resulting cake formed was a very hard mass, dissolving it was difficult.

Nice pics, nice write-up.
Lovely yield of HNO3

[Edited on 3-12-2009 by Aqua_Fortis_100%]

Magpie - 3-12-2009 at 08:52

Thanks AF. No, I have not tried to recover the KHSO4. I'm not sure how pure it is; it may contain unreacted KNO3. The saltcake is, as you say, not all that soluble. It seems to take a lot of hot water to wash the hard saltcake out of the RBF.

chemrox - 3-12-2009 at 21:53

A clarifying question: the 2% or so water in hardware store H2SO4 is sufficient to keep the mass from becoming too hard to still the HNO3 out? Could one use reagent H2SO4 and add a small amount of water?

Another idea: use a two neck flask and rig a "downspout" with a gooseneck adapter with a female joint-cap over the end. Or an RB with a glasswool plug between the down end of the adapter and the RB (?).

Last: anybody know what a literature version of this looks like?

Magpie - 3-12-2009 at 22:27

Quote: Originally posted by chemrox

A clarifying question: the 2% or so water in hardware store H2SO4 is sufficient to keep the mass from becoming too hard to still the HNO3 out? Could one use reagent H2SO4 and add a small amount of water?

I have no experience doing this. I don't see why HNO3 of concentration higher than that of the azeotrope (68wt%) could not be produced that way, however. I think others have done this. They may know the upper limit of H2SO4 "dryness," if any.

Quote:

Another idea: use a two neck flask and rig a "downspout" with a gooseneck adapter with a female joint-cap over the end. Or an RB with a glasswool plug between the down end of the adapter and the RB (?).

The advantage of the Claisen adapter is that it provides extra capacity for containing the foam before it reaches the condenser. In the condenser the foam will solidify and plug the vent path.

I wouldn't trust a glass wool plug to contain hot nitric acid vapor, if that is what you mean.

thereelstory1 - 17-4-2010 at 17:39

please excuse me not knowing the answer to this:

so if you wanted to further purify the HNO3 from azeotrope to as close to 100 percent pure, how would you go about breaking up this azeotrope effectively?

Magpie - 17-4-2010 at 17:51

I don't know off-hand how you would do this. I suspect a different procedure should be used to make nitric acid of higher concentration than azeotropic, ie, a procedure involving less water. Someone else can surely give you a better answer.

12AX7 - 17-4-2010 at 18:00

I heard distill it from sulfuric a second time. Probably with vacuum.

Tim

Magpie - 17-4-2010 at 18:27

Here's a NurdRage video showing how to make pure, or fuming, nitric acid. It's on the last part of the video.

http://www.youtube.com/watch?v=2yE7v4wkuZU

Here's a better one, more to the point of your question:

http://www.youtube.com/watch#!v=CtdX5YmOdcs&feature=rela...

[Edited on 18-4-2010 by Magpie]

mnick12 - 17-4-2010 at 18:35

I have made RFNA a number of times, no vaccum needed. Simply distill a nitrate salt with conc H2SO4, so do what Magpie tells you to do at the beginning of this thread but skip on the water.

thereelstory1 - 17-4-2010 at 19:07

thanks all of you. so, rooto is considered concentrated enough?

rrkss - 21-4-2010 at 10:01

Just ran your experiment as written. Did make nitric acid but upon titration I got 12 molar nitric acid and not the 15.8 molar of concentrated reagent grade. Still made 52% nitric acid this way but I think when I run it again, I am going to reduce the amount of water to 34 mL to bring the concentration of the distillate closer to 70%.

DJF90 - 21-4-2010 at 10:28

mnick12: You're right, you don't need a vacuum for making RFNA, but for WFNA, which is essentially as pure and concentrated a nitric acid you can get, a vacuum is indespensible. Tim is correct on this issue; distillation of azeotropic nitric acid from an equal volume of concentrated sulfuric acid under reduced pressure is what is needed to get as close to 100% WFNA as possible (tends to be ~95%+ HNO3.) - A second distillation would almost indefinately get you to 99%+ I suspect.

Magpie - 21-4-2010 at 13:20

Quote: Originally posted by rrkss

Just ran your experiment as written. Did make nitric acid but upon titration I got 12 molar nitric acid and not the 15.8 molar of concentrated reagent grade. Still made 52% nitric acid this way but I think when I run it again, I am going to reduce the amount of water to 34 mL to bring the concentration of the distillate closer to 70%.

If you collected distillate before the still head temperature reached 120C then you would have a concentration less than 68%, assuming a slight surplus of water. I probably should have clarified that in the procedure. As you say, cutting back on the water should correct that.

thereelstory - 21-4-2010 at 18:48

so can you recycle the h2so4 once you have distilled the hno3? AND how much water would the concentrated h2so4 absorb from the hno3?

DJF90 - 22-4-2010 at 03:19

Well if you start at 68% and end up in the mid 90's, the the sulfuric must end up absorbing most of the water right? Doesn't take a genius to work it out :/

Natures Natrium - 19-11-2012 at 19:10

Just followed the write-up here, with the following changes:

Used flat bottom flask on a stirring hot plate with a stir bar.
Changed reagent ratios from "1.86mol H2SO4 : 2.23 mol KNO3" to "2.25mol H2SO4 : 2.23mol KNO3"
Added another 20% water by volume. (58mL total).
Used a different brand of sulfuric acid, which had a density indicating 97% and which was a translucent amber color.
Used stump remover grade potassium nitrate.

The addition of the salt initially froze the stir bar. Upon heating the stir bar was able to resume, which caused a bit of mild bumping that ejected a small quantity of salt into the condenser, but did not clog it. After that the distillation was smooth. It did however produce quite a large quantity of NO2, and the acid collected was a deep orange. After several days, it is finally beginning to be more yellow than orange, although there is still a visible atmosphere of NO2 in the bottle.

Fortunately, my fumehood (powered by a whole-house ventilation scroll fan) was just able to keep up. I didn't realize that this would produce so much NO2, and should I ever repeat this I will definitely set up a gas scrubbing train.

The flask received around 100mL of liquid, but 5mL were left behind to keep the salt out of the storage flask. The 95mL that were recovered weighed 135.8g on a triple beam balance, for a density of 1.43g. This would seem to indicate a ~70% solution, although between dissolved NO2 and measurement inaccuracies, 68% seems reasonable.

I also titrated 5mL of the above acid, mixed with 95mL of dH2O, with 1M NaOH (prepped by myself some weeks ago) and calculated a concentration of 60%. This doesn't equate to the density data, and given the age of the 1M solution (kept in a quart mason jar), it seems likely that this titration is inaccurate. I might repeat with a fresh batch of 1M NaOH at some point.

Overall it would seem that some or all of the modifications I made resulted in a lower yield despite using more reagents. Should there be a next time, my plan is to follow the reagent ratios listed in this write-up more exactly.

As an aside, I poured 100mL dH20 into the reaction flask (the gaseous NO2 was absorbed very quickly), and plan on attempting to recover the KHSO4 at some point.

smaerd - 13-1-2013 at 13:08

I followed this preparation today using some sodium nitrate I made after generating ammonia gas from ammonium nitrate. Worked great, I have yellow fuming nitric acid about 150mL or so.

I also plugged a thermometer adapter with teflon tape(plumbers tape) then fixed it down with some duct-tape. Held up perfectly fine, not that I really recommend it.

Just wanted to report my findings and say thanks to magpie for this write-up.

binaryclock - 19-5-2013 at 20:43

Sorry to revive this thread, but it seems like a good one to continue this conversation.

Q: What would be a good scrubber setup for removing the toxic NO2 gases or the fuming from the acid gases when they exit the vent port? I don't use a vacuum. Could a diluted 10% NaHO + H2O solution neutralize the gases if I hooked up a hose from my vacuum port? Also, I suppose my latex/rubber hose wouldn't be good for this.. what type of hose would you suggest?

Thanks

Bot0nist - 20-5-2013 at 02:53

Bubbling through a hydroxide solution will scrub most of the acidic vapors, keep it cool. Vinyl or HDPE tubing will work fine, but it will get corroded. If your condenser is very cool, and long enough, you shouldn't get to many acidic vapors on the exit end. Don"t overheat the mix either. Distill in a place, where if the apparatus has a catastrophic failure (read: it all goes to shit, worse case scenario) you and your property will be OK. Plan for contingencies, like a fire, a cracked flask, a RFNA spill and the escape of Noxious gasses. Proper ventilation (NOx will KILL you) and fire mitigation is priority...

binaryclock - 20-5-2013 at 19:46

Thanks for your reply! I moved all the equipment up to my garage when distilling nitric. This way, if "it all goes to shit" then I can open up my garage door and deal with it. I don't feel comfortable distilling nitric in my basement, even with a fume hood.

RRMarsh - 1-7-2013 at 23:14

Thanks all for the insight. Got to know the appropriate procedure and concentration for each constituent.

Hellafunt - 2-4-2015 at 10:28

I recently switched to this method for producing nitric acid, and Im sure you guys don't need me to tell you that it works great. Last night , however, I ran into a slight problem. I was excited that I figured out a solution.
I had wrapped some heat proof pads around the top of the reaction flask and the Claisen adapter to keep it warm. Then I decided to let it run a little longer than usual to try to get every last drop of nitric acid out. I knew there was some foaming going on towards the end, but I didn't realize how much. Anyway, I ended up with my Liebig condenser completely clogged with the salt cake. Like a solid rock tube the whole length of the inner glass tube.
Of course I tried to wash it out, and to gently chip away at it with metal implements of destruction. It seemed that I was going to break the thing before too long. Then, I put it in the microwave for about 15 seconds and stood back. When I opened the microwave, the salt cake had mostly liquefied and ejected itself from the inside, and the rest was easily washed out with hot water. What a relief. It is nice to be divorced and to be able to use the microwave when needed.

Loptr - 2-4-2015 at 12:03

Quote: Originally posted by Hellafunt

I recently switched to this method for producing nitric acid, and Im sure you guys don't need me to tell you that it works great. Last night , however, I ran into a slight problem. I was excited that I figured out a solution.
I had wrapped some heat proof pads around the top of the reaction flask and the Claisen adapter to keep it warm. Then I decided to let it run a little longer than usual to try to get every last drop of nitric acid out. I knew there was some foaming going on towards the end, but I didn't realize how much. Anyway, I ended up with my Liebig condenser completely clogged with the salt cake. Like a solid rock tube the whole length of the inner glass tube.
Of course I tried to wash it out, and to gently chip away at it with metal implements of destruction. It seemed that I was going to break the thing before too long. Then, I put it in the microwave for about 15 seconds and stood back. When I opened the microwave, the salt cake had mostly liquefied and ejected itself from the inside, and the rest was easily washed out with hot water. What a relief. It is nice to be divorced and to be able to use the microwave when needed.

I was kicked out of the kitchen when I attempted to dry magnesium sulfate in the oven.

Molecular Manipulations - 2-4-2015 at 12:06

You could have just soaked it water, potassium sulfate and bisulfate are soluble, takes a while though. I wouldn't have thought of using the microwave, nice.

Monte Carlo - 22-6-2017 at 18:25

Persulfate for cleaner WFNA from atmospheric distillation (and as NO2 scavenger): US3113836

Ammonium persulfate is an excellent NO2 scavenger, useful both during distillation from sulfuric acid (whether using a nitrate salt or azeotropic nitric acid) and for clarifying and stabilizing the distilled product.

The persulfate reacts with water and NO2 to make bisulfate and nitric acid, with a net reaction as follows:

(NH4)2S2O8 + 2 H2O + 2 NO2 ==> 2 NH4HSO4 + 2 HNO3

I have found that adding a small amount of ammonium persulfate (less than 1% of the weight of the nitrate source) to the pot seems to significantly reduce the NO2 that comes over during fractional distillation at atmospheric pressure. (It also helps to exclude light from the parts of the apparatus where HNO3 is hot -- the pot and the column -- by wrapping them in aluminum foil, which also acts as heat shielding for better fractionation.)

The distilled product still has a yellow tinge which can be clarified to water-white either with a small amount of persulfate or urea.

The advantages and disadvantages are,
* Urea slightly dilutes the acid when scavenging NO2 (water is a byproduct), but it does not introduce any contaminating ions
* Persulfate actually scavenges water, dehydrating the acid while removing NO2, and even a tiny excess of persulfate also stabilizes the white fuming nitric acid product during storage. However, persulfate does introduce a small concentration of contaminating ions. For most WFNA uses, this has no impact.

The stabilization is described in patent US3113836.

Chemstacks - 21-7-2018 at 22:20

I have a question: Why are you using 225g KNO3 and 100mL of H2SO4? I have tried to calculate the necessary reagents needed and got different values:

(125mL HNO_3)/1 x 68/100 x (1..51g HNO_3)/(1 mL HNO_3 ) x (1 mol HNO_3)/(63.01g HNO_3 ) x (1 mol H_2 SO_4)/(1 mol HNO_3 ) x (98.08g H_2 SO_4 )/(1 mol H_2 SO_4 ) x ( ml H_2 SO_4 )/(1.84g H_2 SO_4 )=108 mL H_2 SO_4

(125mL HNO_3)/1 x 68/100 x (1..51g HNO_3)/(1 mL HNO_3 ) x (1 mol HNO_3)/(63.01g HNO_3 ) x (1 mol KNO_3)/(1 mol HNO_3 ) x (101.1 g KNO_3)/(1 mol KNO_3 )=205.94 g KNO_3

The amount of sulfuric acid is close, but 8 mL difference is still a bit off. Also, I am not sure if my stoichiometry equation is right or its because you are using pottery grade KNO3 so you needed more of it to compensate for its impurity?

weilawei - 26-9-2018 at 05:33

I'm following MagPie's preparation, as written, with the addition of 1.18g (0.5% w/w% of the KNO3) ammonium persulfate. However, upon heating, the temperature rose to ~80°C, distilling over a liquid from 67°C to 80°C. Then, the temperature started to swing wildly, down to 70°C, back up to 80°C, etc..

Any ideas what's going on? I've only collected maybe 20mL so far, but its never reached past ~80°C in 7 hours on a sand bath with insulation. The sand itself reads 197°C, so that seems more than sufficiently hot even with heating losses. It was initially held at 135°C, but I was getting nowhere, so I pushed it up to around 197°C, which is the rated maximum setting on my hotplate for a sand bath (#3 on the dial, the 197°C is just where the temperature was measured). Eventually, I had to shut it down and go to work after pulling an all-nighter, so I'll heat it up again tonight.

Also, anyone have a vapor-liquid equilibrium chart with the temperatures and percentages marked on the axes for 760 Torr?

[Edited on 26-9-2018 by weilawei]

Magpie - 26-9-2018 at 17:25

You shouldn't have to heat it so much. The azeotrope comes over at 120°C. Did you add the required amount of water?

weilawei - 26-9-2018 at 19:15

I added 100mL of H2SO4 to 48mL of water in an ice bath and let it cool.

I reheated it today after insulating more thoroughly. Same heat settings, though I have some question as to the veracity of the sand bath temperature; the thermocouple probe looked pretty bad after I pulled it out. The last time a thermocouple got melted, the readings were unrealistically high.

I ran the distillation until it reached 114°C (density of 1.34g/mL at 28.4°C, 57.6% for the first fraction), then swapped receivers as it seemed to not be climbing. The temperature dropped to around 110°C and never rose again. I ended heating when I saw heavy red clouds of NO2 puffing into the condenser. Haven't measured density for the second fraction yet.

I'll definitely run this again. I'm thinking I'll do several runs, combine all the fractions, and do one final distillation to get 68% HNO3. I also think I'll switch to an oil bath. As to insulation, that seemed critical, so maybe I'll get out the glass wool. I'd used Al foil this time--ultimately in 3 layers.

Edit: Took the density of the second fraction, got 1.36 g/mL at 22.7°C, which is 61.0%. I'm satisfied with this as a first attempt. Certainly, my technique could stand improvement, but the reaction worked with relatively little fuss. I did not suffer major issues with foaming or clogging--I found it to be rather well behaved for such a solids-heavy slurry. I employed a dumbbell-shaped stirbar just in case. I found the microwave trick to be extremely useful in cleaning my reaction flask. As for the thermocouple: it did not survive. A couple pictures, the thermocouple and the combined fractions before the acid is transferred to an amber bottle.

[Edited on 29-9-2018 by weilawei]