Sciencemadness Discussion Board

Nitric Acid from a Nitrate Salt

rollercoaster158 - 21-4-2012 at 13:31

I have heard several times that Nitric Acid can be produced from electrolysis of a nitrate salt. For example:

KNO3 + H2O --> HNO3 + K + O + H

However, these were in video comments and unrelated forum posts, making me question the validity of this experiment. I don't have any saltpeter right now, but I will get some soon and try this out. My main question is whether this works, or if something else occurs, like my theories:

KNO3 + H2O = 2H + O2 + N + K
KNO3 + H2O = KOH + H + (anode material)NO3

KNO3 + H2O --> HNO3 + KOH but then
HNO3 + KOH = KNO3 + H2O

Let's figure this out once and for all.

Vargouille - 21-4-2012 at 15:48

I doubt it works. KOH and HNO3 are strong, so they'll almost certainly neutralize before they can be separated.

The ways I know of making it are with NO2 bubbling through water and H2SO4 reacting with a nitrate salt.

DerAlte - 21-4-2012 at 16:28

IT will not. See the thread

http://www.sciencemadness.org/talk/viewthread.php?tid=12941&...

Regards
Der Alte

rollercoaster158 - 21-4-2012 at 17:11

Quote: Originally posted by Vargouille  
I doubt it works. KOH and HNO3 are strong, so they'll almost certainly neutralize before they can be separated.

The ways I know of making it are with NO2 bubbling through water and H2SO4 reacting with a nitrate salt.


Yeah, KOH and HNO3 stabilize each other back into KNO3 and H2O. It's easier to use H2SO4 and KNO3 in a flask in a hot oil bath. I was just wondering. Perhaps we could make a dual-chambered electrolysis rig with a salt bridge and hope the nitric acid and potassium hydroxide don't meet and ruin each other.

Vargouille - 21-4-2012 at 19:25

The link DerAlte posts talks about forming nitric acid from a nitrate salt, and the problem of neutralization is solved by quickly removing the nitric acid from solution with a vacuum or the like, as far as I can tell.

What about ammonium nitrate instead of potassium nitrate? It'll still neutralize some of the nitric acid, but less than KOH would, and should work if some other aspect of the reaction hasn't slipped my mind.

rollercoaster158 - 22-4-2012 at 07:06

Let's see, what nitrate salts have hydroxides that don't react with nitric acid?

Fossil - 22-4-2012 at 10:14

This should work for you, all you need are some basic glassware and chemicals. It's real easy to do, yet it is quite inefficient, but the required chemicals are cheap.

It doesn't use electrolysis, but you might want to give it a look anyways.

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

rollercoaster158 - 22-4-2012 at 10:19

Actually I've already seen almost every video in that channel. It was more of a curiosity than anything, and if I wanted to make some, I would use the nitrate salt and sulfuric acid oil bath method. It would be cool though if this worked.

Vargouille - 22-4-2012 at 13:21

Although, come to think of it, if you have a transition metal nitrate, it should form an insoluble hydroxide, but if nitric acid gets near it they'll still react. The solution might be a combination of transition metal nitrate (CuNO3, for example) and something to remove the nitric acid from the solution.

I can't think of any hydroxide that nitric acid wouldn't be able to react with, mind you, or that would be my suggestion.

EDIT: From NurdRage's channel, I believe he mentioned in his vlog that he was trying to make nitric acid by electrolysis.

[Edited on 22-4-2012 by Vargouille]

rollercoaster158 - 22-4-2012 at 14:51

The vlog said it was an "abject failure". I guess we'll see.

bbartlog - 22-4-2012 at 15:06

The problem of the acid reacting with the hydroxide produced at the cathode is trivially solved. You use a divided cell. The far less tractable problem is that (AFAIK) there is no anode material that will not be rapidly eroded as it is oxidized, except platinum, which is expensive (and for all I know will still be gradually attacked - platinum(IV) nitrate supposedly exists). Now, maybe there is some voodoo product of advanced materials science out there that would do the trick, like doped silicon or diamond. Regardless, that is the crux.

rollercoaster158 - 22-4-2012 at 17:26

With how this thread is going, I'm gonna have to try it. I'll divide the cell, use a platinum wire electrode, and as much saltpeter as I can dissolve in the water. I'll post pics when I finish.

KNO3 + H2O = HNO3 + KOH

woelen - 23-4-2012 at 00:51

The reaction equation you post is not what happens. You get decomposition of water as well and oxygen is formed at the anode and hydrogen is formed at the cathode:

Below follow the idealized, simplified reactions:
On the anode: 2H2O - 4e --> 4H(+) + O2
On the cathode: 2H2O + 4e --> 4OH(-) + 2H2

The net reaction hence is decomposition of water with buildup of acid around the anode and buildup of hydroxide around the cathode.

You need a cell with a membrane or a suitable bridge between the anode chamber and cathode chamber. The bridge or membrane is needed to allow ion transfer between the chambers in order to keep them electrically neutral. If you use a bridge, then you cannot use a simple metal wire, because then you just construct two cells in series and the wire will act as anode in one cell and as cathode in the other. You need a U-shaped tube, filled with electrolyte, which dips in both chambers.

Do not expect an easy production of acid. The experiment is very interesting from an educational point of view, you really easily can show the formation of acid and base in the chanbers (e.g. with suitable pH indicators), but isolating a decent amount of acid from the anode chamber will be a lot of hassle and hardly worth the effort.

[Edited on 23-4-12 by woelen]

rollercoaster158 - 23-4-2012 at 13:17

It's always worth trying. I'll have to wait though, need to get all of the stuff for it. I'll come back to this and post my results when I finish, whether that be next week or next month.

Pyridinium - 26-4-2012 at 20:31

Quote: Originally posted by bbartlog  
The problem of the acid reacting with the hydroxide produced at the cathode is trivially solved. You use a divided cell. The far less tractable problem is that (AFAIK) there is no anode material that will not be rapidly eroded as it is oxidized, except platinum, which is expensive (and for all I know will still be gradually attacked - platinum(IV) nitrate supposedly exists). Now, maybe there is some voodoo product of advanced materials science out there that would do the trick, like doped silicon or diamond. Regardless, that is the crux.


Reading down through this thread I was waiting for someone to say this...

I was thinking you could do it with graphite if you didn't mind running through who-knows-how many electrodes....

chloric1 - 5-8-2015 at 02:30

Suppose I wanted the nitrate ion to attack the anode. If I wanted dilute ferric nitrate or copper nitrate in the anolyte, wouldn't the nitrate ion do this in preference of just discharging oxygen gas? I wanted some metal patina solutions from scrap metal remnants. I work a lot with steel and copper.