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Topic: Questions about synthesizing Nitric Acid in the home lab.  (Read 18084 times)

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Offline alancj

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Questions about synthesizing Nitric Acid in the home lab.
« on: June 10, 2006, 06:59:47 PM »
Hi all, I’m new to this forum…

I’ve been looking into making nitric acid, mostly because it is rather difficult to buy it around here without ordering online at great expense. I’m going to be using it for the preparation of lead nitrate for the electroplating of lead dioxide anodes.

From Wikipedia (http://en.wikipedia.org/wiki/Nitric_acid) I find that:

“Nitric acid can be made from Copper(II) nitrate or by reacting 200 g of potassium nitrate (KNO3) in 106 ml of 96% sulfuric acid (H2SO4), and distilling this mixture at nitric acid's boiling point of 83 °C until only a white crystalline mass, potassium hydrogen sulfate (KHSO4), remains in the reaction vessel. The obtained red fuming nitric acid may be converted to the white nitric acid. Note that in a laboratory setting, it is necessary to use all-glass equipment, ideally a one-piece retort, because nitric acid attacks cork and rubber, and leaks can be extremely dangerous.

I have a few questions that hopefully some of you can answer about the process.

1.) When boiling the solution of H2SO4 + KNO3 in a retort, am I simply evaporating HN03 in the solution and just simply condensing nitric acid vapor in the retort’s side tube? Or do I get evolving NO2 or NO3 gas that needs to be plunged into water to turn into nitric acid? The Wikipedia entry states that you will get RFNA, which is highly concentrated, so I’m guessing the later isn’t true since you could never get that kind of concentration.

2.) Can hydrochloric acid be substituted for the sulfuric acid? I have read that Aqua Regia can be prepared by adding sodium nitrate or potassium nitrate to HCl which produces nitric acid in the solution… so why can’t this be used instead of sulfuric? I tried this the other week (with “muriatic” 33%) and I got an evolving orange/red gas that readily ate the (I think it was PVC) tubing I was using to direct the gas into a jar or water. It seemed to make the water acidic as indicated by the strong reaction when some baking soda was sprinkled on top. The gas leaks tarnished some nearby metal objects a reddish orange color; it did this to some steel and also to some nearby brass regulators.

3.) If sulfuric is the only method then can 33% battery grade acid be used, or must it be brought to a high 96% concentration by first boiling the water off? Does the concentration of the sulfuric acid affect the resulting nitric acid’s concentration? If not, then is there is any reason why diluted H2SO4 acid can’t be used?

Ok, well that should be a good start for now; I’ll probably have more questions later.


Offline woelen

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Re: Questions about synthesizing Nitric Acid in the home lab.
« Reply #1 on: June 12, 2006, 05:46:39 PM »
Making nitric acid from KNO3 and H2SO4 is perfectly possible. The only thing is that you need all-glass apparatus. HNO3 is very corrosive and if you use rubber tubes, stoppers and joins in your distillation apparatus, then these will be eaten away very quickly by the hot nitric acid vapors.

You can make red fuming nitric acid from concentrated H2SO4 and KNO3 at normal air pressure, but that is not pleasant at all. The red fuming acid is horribly impure, it contains a lot of dissolved NO2 and hardly is useful for synthetic purposes.

Best thing to do is to use dilute H2SO4 (e.g. 60%) and add KNO3 to that. That will give water at first, but at a certain point the azeotropic mix of water and HNO3 (containing 68% acid by weight) will distill. In this way, you'll get an almost colorless acid. The first part is mainly water, the last part is the azeotropic mix. When all of the distilled liquid is taken, then with 60% H2SO4 I expect acid of 40 - 50%, but you need to perform precise math to get the precise concentration. This acid is perfectly suitable for dissolving lead metal. For dissolving lead metal you need 40 - 50% acid, higher concentrations may lead to passivation and very slow dissolving of the metal.

If you have 33% H2SO4, then boil it somewhat, such that you get 60% acid. You do not want to boil it all the way down, such that you get 96% acid.

You cannot use HCl. When that is mixed with nitrate, then you get an aqua regia like liquid. The following reaction occurs on heating: KNO3 + 4HCl --> NOCl + Cl2 + 2H2O + KCl. The NOCl and Cl2 escape as (very toxic and corrosive) gases. You can recognize the formation of the NOCl and Cl2 by change of color. The liquid becomes yellow/orange and a yellow/orange gas mixture is formed.

A word of warning with all these experiments. Distilling HNO3 is a very risky thing. It can be done at home, but if you have never done a distillation before, then first try distilling something less risky, e.g. some water/salt mix, just to get the idea of how this works.
When you dissolve lead metal in HNO3, then initially the reaction seems not to run at all. A little heating may be needed. But, once the reaction runs, a lot of brown NO2 gas is formed and a crystalline white mess is formed in the acid. The NO2 gas is insidiously toxic and has delayed effects. At the moment of breathing you hardly notice its effects, but it may cause lung edemia after 15 hours or so. Be very careful and do this outside. Another serious risk is that when the lead is dissolved, many small droplets of the liquid are sprayed into the air. These droplets consist of HNO3, water and Pb(NO3)2. The HNO3 and water quickly evaporate and the Pb(NO3)2 remains as tiny dust specks. This also is very harmful for you health. So, if you perform the reaction, cover the beaker loosely with a paper tissue. This will capture small droplets of liquid, while the gas (NO2) will escape. Lead-salts are a cumulative poison. One exposure is "remembered" by your body and that is the reason why you need to be so careful with the lead salts. All this experimentation MUST be done outside on a windy day, or in a good fume hood.

Want to wonder? See http://www.oelen.net/science

Offline alancj

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Re: Questions about synthesizing Nitric Acid in the home lab.
« Reply #2 on: June 14, 2006, 12:23:11 AM »
Thanks, your advice is well noted.

I think I will order a retort off ebay. Will the nitric acid come out of the side tube as a liquid? I just wonder if a retorts tube is long enough to get the vapor to condense. I am eyeing this one http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&rd=1&item=7610017946&ssPageName=STRK:MESE:IT. He does state in another auction for the same retort that "It is also the apparatus of choice for the preparation of nitric acid from sulfuric acid and a nitrate." Not sure if it would be big enough, maybe I'd need something more like 500 ml. I guess it depends on how much acid I will need and how offten, which I'm not sure of yet. 

I obtained a gallon of sulfuric acid drain cleaner, the bottle doesn't say the concentration but according to what I have read they are from 92-95% concentration. The brand is "Bulldog" and the bottle was in a sealed poly bag. The bottle doesn't even have the words sulfuric acid on it, but the bag does on its warning label. Perhaps I could find out the concentration by determing the density and doing a little math...

The acid actually has some small black particles in it, like little flakes, I don't know what they are or what effect they might have in making nitric acid. Do you have any idea what they might be, or if they might be a problem? I wonder how one would filter it out, I suppose I would need an inert filter.

That is interesting that the nitrate plus HCL just makes an aqua regia "like" acid, and not the real thing. Now I know why people have said it won't work since there is no nitric acid formed in the first place! I thought that I read that it was exactly the same as aqua regia, which was from the people sell the sodium nitrate "sub-zero" powder.   


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