[kamikaze762x39]

Maybe this should have gone in citizen chemist... 

First off, I'm a total noob, but I have always been interested in chemistry.  I have been trying to understanding some of the concepts of fractional distillation and have been studying how azeotropes affect the boiling points of mixtures.  For some reason, I just can't seem to put into a concrete understanding of the results.

I made nitric acid by reacting potassium nitrate, copper, and HCL and bubbling it through 3% peroxide.  The resulting solution will bubble when copper is placed in it, but is too weak to etch silver, so I need to concentrate it.  I bought a 2000ml boiling flask, a fractionating column, thermometer, condenser, drip adapter, and collection flask. 

So I'm all set to go, and here is what I know about this process.  From what I read, I am expecting to get mostly water in the residue, with the distillate being more concentrated?  So HNO₃ boils at 83C and water boils at 100C.  But the azeotrope boils at 120.5C. 

So... HNO3, being the more volatile, should vaporize first, right?  I am assuming this is regardless of whether the azeotrope is negative or positive. 

I am also assuming that the temperature at the condenser neck should ramp up until it reaches 83C and hold close to that range until the less volatile compound starts to reach the top? 

Part of it I can't quite understand, though.  If I am liberating the more volatile compound, does this not mean that my residue is actually moving AWAY from the azeotrope?  And by this, I should be getting ideally a 68% concentration in the distillate... but by this, the boiling point should be 120.5C... at which I'm going to be getting a ton of water instead of the HNO₃...

So my understanding of this process is going in circles without actually adding up to an understanding.  Can someone shoot some holes in my explanation here and show me where where I am going wrong?  Thanks!

[Arkcon]

You've written a long missive of ifs and buts, and its hard to work with it quickly.  But leats try.

First of all, concentrated nitric is difficult to handle safely, and that's why its difficult to buy.  So if you can't buy it, you shouldn't have it.

I made nitric acid by reacting potassium nitrate, copper, and HCL and bubbling it through 3% peroxide.

I simply don't recognize this reaction.  Any nitrate reacts with any acid to make nitric acid.  The copper is weird, and the household peroxide is without meaning for me.

There's lots more in your posting, but I wanted to start with what's simple.

[kamikaze762x39]

Nurd Rage posted a video showing how to use this reaction.  No arguing with the results.  It produces copious amounts of nitrogen dioxide, which is pretty unmistakable.  I think nitric acid is formed in the solution from the nitrate salt, heat, and HCL.  The copper then reacts with it immediately to produce the gas.  As to why the hydrogen peroxide is better than water for reacting the gas, I am assuming it oxidizes the NO₂ more readily than the water. 

And yes... Nitric acid is nasty stuff.  The dissolved NO₂ alone is deadly if inhaled, it can make gloves burst into flame, and all that assorted evilness.  Unfortunately, nothing else can dissolve copper or silver and buying it is expensive.  I do these reactions outside, upwind from the vapors, with water on hand and a sodium bicarbonate trap to catch any unreacted gas.

So... We could completely remove the nitric acid and still be left with the same question concerning negative and positive azeotropes, how to watch the boiling points, and why different concentrations of mixtures gravitate to and from the constant boiling mixture.

[Arkcon]

Well, returning to to nitric acid, good, you understand that nitrogen dioxide reacts with water to make nitric acid, and now you want to concentrate it.

How azeotropes form is  a complex idea, I don't really know it, it comes up here often, because it is confusing.  But as often as now, we get bad logic about an azeotrope could be broken.

[kamikaze762x39]

I'm not necessarily interested in how to break the azeotrope.  68% is just fine for my purposes.  I just don't fully understand the boiling points involved here.  I mean, supposedly we can't go any more concentrated than the azeotrope.  But in order to produce azeotropic HNO₃, it's vapor temp would have to be 120.5C or higher, else it would condense into a liquid mix, right?  But I'm still left with the idea that nitric acid vaporizes at 83C and would necessarily have to reach the top of the column before the water would.  These ideas seem to be contradictory.

Distillation involves pushing the more volatile chem to the distillate and leaving the less volatile chem in the residue.  That seems simple enough.  Consequence:  the more volatile chem reaches the top first, or rather the vapor will have a higher concentration of the more volatile chem. 

Well, that more volatile vapor won't be 100% nitric acid because there is an azeotrope involved.  And this azeotrope happens to boil at 120.5C... which means it wouldn't survive in vapor form... I just don't how it would ever work at all with these consequences.  I must have something wrong because it doesn't add up.

[kamikaze762x39]

This is why peroxide is better for the reaction...

3 NO₂ + H₂O → NO + 2 HNO₃

The NO left over from the reaction with water readily combines with excess oxygen to form more NO₂, which again reacts with the water.