Bit of help

[DougBrummell]

Hey, let me preface this by acknowleding that this information is probably somewhere on the internet, but if one of you has a good, quick reference I would love to know the equilibrium dissociation constant for distilling KNO3 and H2SO4 (the final value at STP would be great). I am obviously making my own nitric acid. It is for my final in chemistry, hence my hurriedness. Additionally, any tips you might have are welcomed.

 

P.S. I know there is a thread on this, it wasn't helpful. It was actually frustrating. . . but i guess thats what happens when you're used to your science coming from a finely written book rather than a post written in the usual internet fashion.

 

Thanks in advance. . . ever since I found this site this morning I've been excited.

 

-Doug

[DougBrummell]

Okay, I'll be more specific, the Keq of 2(KNO3)+(H2SO4)<->(K2SO4)+2(HNO3).

Or. . . am I just being an imbecile and no such value exists. . .

[woelen]

Indeed, I have the idea that such a value does not exist. What really happens is that KNO3 dissolves somewhat in sulphuric acid, giving K(+) and NO3(-) ions. The NO3(-) reacts with H2SO4 giving HNO3 and HSO4(-). For that reaction there probably will be a Keq, but I doubt you will find that somewhere on the Internet. I don't know its value, but it will be mostly on the HNO3 side I think. It can even go further:

 

HNO3 + H2SO4 <--> H2NO3(+) + HSO4(-)

H2NO3(+) <--> H2O + NO2(+)

 

So-called nitronium ions are formed in the acid mix. These ions are strong nitrating agents.

 

Mostly, however, you will have plain HNO3 in the solution. You can distill off the HNO3 as a pure acid (99+ %), but this is accompanied with a lot of decomposition to NO2, O2 and H2O. What you really get is red fuming nitric acid, which is a fairly concentrated solution of NO2 in HNO3, with a few percent of water in it as well. I don't think you want that, it is real crap and quite useless stuff. Not suitable for nitrating purposes (it just is a very strong oxidizer, oxidizing every organic to crap and not selective at all).

 

If you want nice clean nitric acid, then mix some water with H2SO4 (1 part of water with 2 parts of acid) and then add the KNO3. Using this, you can distill the azeotrope of water and HNO3, with 68% HNO3. This will yield acid, almost free of NO2. When distilled carefully, the acid only is pale yellow at most, indicating that only a small amount of NO2 is in it.

[DougBrummell]

Thanks a lot. I'll let you know how it goes.