Sciencemadness Discussion Board

KNO3 + H2SO4 -> HNO3 + KHSO4

Drunkguy - 9-10-2006 at 03:22

In Wiki the following is quoted:
Quote:

Nitric acid can be made from Copper(II) nitrate or by reacting approximately equal weights of potassium nitrate (KNO3) with 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 anhydrous nitric acid attacks cork, rubber, and skin, and leaks can be extremely dangerous.


Has anybody got experience with this. I'll be checking what Bromic said about it also. Needless to say this would be attempted outside, but removing the NOx from the fuming red liquid may not be possible since a water aspirator is not on hand and it sounds like it is some posioness nasty stuff that would damage a mechanically driven diaphragm pump.

YT2095 - 9-10-2006 at 03:28

IIRC, Urea (carbonylamide) can be added to this red acid to convert it to white.

solo - 9-10-2006 at 03:34

You might gain some insight by reading this thread and its links...............solo


https://sciencemadness.org/talk/viewthread.php?tid=1851#pid6...

Drunkguy - 9-10-2006 at 05:04

Quote:
Originally posted by YT2095
IIRC, Urea (carbonylamide) can be added to this red acid to convert it to white.


This might be handy. How does this work exactly on a chemical level?

YT2095 - 9-10-2006 at 05:09

I don`t know, I`m only a Hobby chemist, I just know it`s used and it apparently works by removing the NOx gasses that create that red color.
sorry!

woelen - 9-10-2006 at 05:58

The NOx and urea react to form H2O, CO2 and N2. It is important though that a precise amount of urea is added. Too little and dissolved NOx remains, too much and you are making urea nitrate. It is, however, easy to do so, because the end point of adding can be clearly seen (acid becomes colorless).

ordenblitz - 9-10-2006 at 13:03

It’s easy to add too much urea before the color dissipates. I have had this experience before. As the COPAE says: warming and bubbling nitrogen through while adding small amounts of urea is advisable.

Magpie - 9-10-2006 at 15:36

I don't have a vacuum pump. But when I was researching them I noted that some have wetted parts of Teflon or other material that may be OK for nitric acid vapor use.

Also a cold trap or chemical trap ahead of the pump might protect vulnerable pump internals. These would also trap the noxious vapors instead of exhausting them to the atmosphere.

not_important - 9-10-2006 at 21:42

Quote:
Originally posted by Magpie
I don't have a vacuum pump. But when I was researching them I noted that some have wetted parts of Teflon or other material that may be OK for nitric acid vapor use.

Also a cold trap or chemical trap ahead of the pump might protect vulnerable pump internals. These would also trap the noxious vapors instead of exhausting them to the atmosphere.


Better to use absorption traps - wash bottles with a solution of Na2CO3 or fairly dilute NaOH , or even urea. Still not the best thing to do.

When you're not actually trying to vacuum distill or such, but just want to draw gas through someting, the old tradition way was to have a large jug or small barrel with a connection for tubing at the top (1-hole stopper in jug mouth) and a tap at the bottom. Fill it with water, connect tubing to whatever needs gas drawn through it, open tap at bottom and as the water drains out it draws gases through the tubing.

Don't need N2 for bubbling through the HNO3. CO2 will work, or even clean air should be OK - pass it through several washbottles - 1st with Na2CO3 or NaOH solution to grab any HCl or similar in the air, 2nd with 5% H2SO4 or Na/K HSO4 to pick up any ammonia; this also removes dust but still start out with a cotton plug or some other dust filter at the air intake.

Drunkguy - 10-10-2006 at 09:41

Swim only wants the azeotrope so he'll not be needing to use vacuum. Since this distills at 120C presumably the end point wont be too hard to determine. Also what is the the contaminent that calls for a redistillation? Is it too make the acid more concentrated or is there a hidden reason?

What ratios should be employed? Is a 1:1:1 weight all right or is there an optimized quantity that should be employed in this reaction?

He'll use an ice bath upon carefully adding the KNO3 to the H2SO4 followed by H2O dilution, since this is known to be exothermic.

not_important - 10-10-2006 at 20:00

Swimming while attempting this method is a bad idea; all the reactants and products are water soluble, and allowing cool water to touch the hot distillation vessel is not a good idea.

Instead I suggest that _you_ read the comments more carefully. There is no redistallation, just a removal of NOx from the nitric acid that includes bubbling gas through it. The only reasons to redistill is
A) to obtain constant boiling acid, the concentration of which can be calculated using the observed boiling point and reference tables.
B) You were sloppy during the original distallation and had some of the reaction mix splat over into the product.
C) if you used urea to remove NOx and added it in excess, leaving some urea nitrate in the acid. If that happened and the urea nitrate was a problem for the uses of the acid, you would either need to vac distill the acid away or generate enough NOx to destroy the excess urea - possibly overshooting in that direction. If you use some urea to remove most of the NOx, so that the acid is a fairly pale yellow and then bubbling/pulling air/CO2?N2 through the acid to remove the rest.

Reduced pressure distallation is used during heating of H2SO4 + KNO# to reduce the decomposition of the nitric acid formed. This is more important on a large scale than when making a 100 ml for hobby use. Some claim that vac distilling allows the reaction H2SO4 + 2 KNO3 => K2SO4 + 2 HNO3 to happen, I have not seen experimental evidence of that.

Note that light will cause the constant boiling acid to slowly break down, generating some NOx.

1:1:1 ? Of what, H2SO4 : KNO3 : H2O ? Slight mole excess of H2SO4, enough water to equal about half the amount of HNO3 produced. Can't tell you volume of H2SO without knowing what strength of acid you have, likewise the amount of water to include depends on the H2SO4 concentration. But it's simple maths to calculate, once you know the sulfuric acid concentration. Given that MW H2SO4 is 98, KNO3 is 101, 1:1 is roughly equal mole amounts. But an equal weight of water is way overkill, you'll be distilling off a lot of water before the acid starts to come over.

As it would be sad, and make us feel guilty, if you or SWIY were to be converted into hissing, oozing, slimely piles of acidified and oxidised protoplasm through a miscommunication between you and SWIY, it would be a good idea for SWIY to get their own account and communicate directly. Otherwise drop the nonsense, playing silly games with 'SWIM' and 'draming' isn't going to save your arse if the Law tumbles to you - they just do a global replace of 'SWIM' with 'I', as SWIM stands for "Someone Who Is Me", label you a major drug dealer or terrorist because you attempted to hide, and lock you away for a half century or longer.

YT2095 - 10-10-2006 at 23:53

does anyone know if Oxalic acid would work in place of urea instead?
I know it`s breakdown product is CO2, in fact it`s used in Hydroponics for some CO2 generators.
might it work as an alternative to urea?

woelen - 11-10-2006 at 00:18

Oxalic acid does not work. It also is oxidized by HNO3 itself, giving more NO2. The good thing of urea is that it is selective. It reduced NO2, while leaving the HNO3 untouched. Oxalic acid reduces all of it, or it does not reduce at all (when things are cold).

Drunkguy - 11-10-2006 at 00:52

Quote:
Dilute nitric acid may be concentrated by distillation up to 68% acid, which is an azeotropic mixture with 32% water.


So if one has dilute nitric acid, although the bp of pure nitric acid = 83C, the water will distill first, since the azeotrope doesnt distill until a temperature of 120C is reached.

So in a sense, if all one wanted to do is concentrate dil. HNO3, all they are doing is boiling off excess water and the nitric acid is not actually distilled at all?

http://www.chemguide.co.uk/physical/phaseeqia/nonideal.html

Edit: I found an interesting link in another thread.

100g KNO3 has a molecular weight of 101 = 0.99 mol
0.99mol x 63.01 = 62.3g HNO3 made upon reaction with H2SO4.

Thus ~38mL H2O should be adequate for distillation to get the azeotrope?

[Edited on 11-10-2006 by Drunkguy]