# Question about Potassium Nitrate solubilty in Nitric Acid



## Noxx (Jun 8, 2008)

Hello guys,
I was thinking about something.
Let say you add 150 grams of Potassium nitrate in 100mL of water at 35C. This is 50 grams more than the maximum solubility of KNO3.

Now let say that you slowly add concentrated sulfuric acid to the solution. This will convert the Potassium Nitrate into Nitric acid and Potassium Sulfate.

My question is, will this conversion gives more "room" to more KNO3 to dissolve ? Because this technique could be used to make stronger nitric acid. As soon as the dissolved KNO3 is converted, new KNO3 dissolves whithout water addition.

Thanks


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## Scott2357 (Jun 8, 2008)

Noxx,

I would not use any water at all. Particularly if you are after concentrated acid. I'll post details later when I have more time, but you should try a 2/1 ration of Potassium Nitrate to concentrated Sulfuric. Apply heat to the paste and collect the red Nitric acid fumes as they are produced. This will yield an equal volume of NA as KNO3 used.


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## Lou (Jun 8, 2008)

Eh, it doesn't quite work like that Scott. Not in my experience, or in theory. I don't know what equal volume of KNO3 will get you in terms of nitric, I'm sure no one knows as the volume of a powder depends on the nature of the powder. What you mean to say is it will give you an equal number of *moles*.

When you do that displacement reaction, there are many things at play (PM if you want to talk the chemistry) that will rob you of your yields. Also, applying heat to that is best done in a distillation setup, unless you have a death wish.


Noxx,

Think about this: you make a saturated solution of KNO3 in water at 35C, right? There's KNO3 sitting on the bottom of your flask, undissolved. When you add conc. sulfuric (I assume that's what you're using), the sulfuric is going to ionize in the water. It's mostly going to be HSO4-. When it does this, as you know, it produces heat. This heat is what will increase the solubility of the KNO3, not so much it reacting to give you stronger nitric.
If you want to keep nitric fumes to a minimum, I suggest you do this all cold.

The reason you can make nitric from KNO3 and H2SO4 but not using NaNO3 is that the solubility of the K2SO4 produced is very low. You can essentially crystallize out the potassium sulfate from your formed nitric acid by cooling the solution. This will remove 99% of the sulfate (actually, this is a good problem for students!!).


Louis


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## Noxx (Jun 8, 2008)

Louis, 
Yes like I stated in my first post, I'm using concentrated Sulfuric. 

With my calculations, making nitric acid using KNO3 and water at 20C (all KNO3 dissolved) will make nitric of a concentration of about 15%. This is a way too low for me. And if you make it at 70C, you are supposed to get nitric at a concentration of 35% but some nitric goes up in smoke as soon as you add the sulfuric.

So my questions was more; if it's possible to add not enough water to dissolve the KNO3 completely but still be able to use it all. I hope you understand what I mean...

Thanks


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## Lou (Jun 8, 2008)

Jean,

Frankly, I would just add sulfuric to the KNO3 paste/slurry. Then chill it and the K2SO4 will crystallize out. Filter it all cold, then chill it again, more K2SO4 should crystallize out. This should leave you with 40%+ HNO3.


Lou


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## Scott2357 (Jun 8, 2008)

Lou,

Yeah, moles is correct. I'm not up to speed on chemistry terminology... need to get some good books. Also, distillation is what I was talking about when I said "collect" the red nitric fumes. Thanks for making that more clear.


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## Noxx (Jun 8, 2008)

Louis, I'm still not sure to understand (pardon me !)

If you want to make nitric acid of a concentration of 40%, you need to dissolve your KNO3 in water at 75C (155g./100mL). But the problem is that when you add the first mL of sulfuric, it goes up in smoke.

Arggg, I'm having so much trouble to obtain nitric acid of a good concentration, this is supposed to be an easy operation ! I think I'll have to make a video tutorial for other noobs like me lol.


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## Noxx (Jun 10, 2008)

Hey guys, you should watch that video, this guy is making nitric acid using only KNO3 and Sulfuric acid (no water). There are some flaws in his setup but the concept is interesting. Will do some research.

Here's the link : http://youtube.com/watch?v=q07XzOf_Cyc


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## Scott2357 (Jun 11, 2008)

Yes, that's exactly what I was talking about although he's venting a lot into the air.


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