hissingnoise
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Electrolysis!
Dilute sulfuric acid is concentrated by electrolysis(disociation of water), but does the same hold true for dilute hno3, I wonder? If it does, it
should produce strong "white" hno3 without having to distil. Anode material might be problematic, though. 'Sounds good, so it probably
won't work. I speak from experience.
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Marvin
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Unfortunatly conc nitric acid is strongly oxidising, so release of hydrogen doesnt happen. Instead the nitric acid itself gets reduced to water and
oxides of nitrogen.
A method using compartments and a diaphram is possible, and concentration occures through a different in ion migration rates. The nitrogen oxides
from one side need to be returned to the other side, and one half of the cell becomes more dilute and the other more concentated.
It doesnt seem worthwhile when studied in detail.
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hissingnoise
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Thanks for the reminder, Marvin.
I simply forgot hno3's oxidizing power.
At least, it works for h2so4 and removes water without boiling.
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chloric1
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Hmmm interesting
Here is some data on the electrolysis of HNO3 I have From Electrochemistry by Creighton and Koehler. I will let all concerned draw there own
conclusions for suitablity.
Enjoy! 
Attachment: nitric electrolysis.doc (394kB)
This file has been downloaded 849 times
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Marvin
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Please dont post text as images in word documents, they are a right pain to view.
The Creighton that wrote the book is quite probably the same Creighton that did the experiment, and so he's more than a little biased and he was
doing this probably in the hope that electrolysis would become the industrial method for concentrating az nitric. (This was before distilling from
sulphuric or magnesium nitrate mixtures became adopted for chemical plants).
What he's not telling you, is that concentration is happening through ion migration, not electrolytic decomposition. Whats being decomposed, is
actually the acid, which is why you need to channel the toxic nitrogen oxides to the anode compartment.
Allthough the anode compartment gets more concentrated the cathode compartment gets more dilute and to maintain this you need to keep changing the
catholyte. So to produce a small amount of 99% nitric acid from 71%, you need a lot of current over a long period of time, and you also need a much
larger amount of 71% acid which you make more dilute.
If the method was able to seperate the acid from the water properly, I might consider it workable, if not actually useful becuase of the large
currents/times required. There really is no reason anyone would want to make conc nitric (and a lot more of slightly diluted az nitric) this way.
The hydroxlamine info people might find interesting, but unless they can get mercury and are willing to play with it, the Rashig method is probably
the way to go.
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chloric1
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I apologize I am not too good at scanning. And besides for whatever reason I cannot get it to scan directly into a word document.
I also would think the process has quite low yeilds and would not yield a quality material. I just thought it was a novel section on the book.
Sometimes the old books can be fun to read.  Besides I would rather mixed HNO3
with H2SO4 and then do 7 extractions with methylene chloride. Then I would simply imerse the mixture in dry ice and acetone.
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axehandle
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Concentrating dilute sulfuric acid by electrolysis sounds much better than boiling it (considering the dangers involved in the latter process, as well
as the inevitable loss of some of the sulfuric).
But what concentration can you reach by electrolysis? Anyone who knows?
I'm thinking of simply using two electrodes of a suitable material and connecting them directly to the mains AC, 220V. Bad idea?
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Saerynide
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You need DC to do electrolysis effieciently or you wont really get anywhere. In AC which stands for alternating current, the polarity is constantly
reversing, so it'll take a long time.
Edit: 220 v is WAY too much
[Edited on 12-2-2004 by Saerynide]
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axehandle
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I should have mentioned it in the first post, but I thought everyone would asssume I was going to insert a full bridge rectifier before the
electrodes.
And please! This is not meant as a flame, but I've studied physics at university level, and know full well the difference between DC and AC...
I was thinking of using a simple diode rectifier, giving an RMS of approx. 100V?
And using a thermometer to monitor the temperature so that it doesn't exceed, say, 80C?
[Edited on 2004-2-12 by axehandle]
[Edited on 2004-2-12 by axehandle]
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Saerynide
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Sorry, didnt mean to offend you. Just thought you might've been someone who would stick a cut lamp cord into the solution and plug it in 
I guess it would work, and youll get your products extra fast, but that things gonna heat up pretty bad real fast. The last time I passed 5v at 22
amps through a solution without a bridge, it pretty much boiled in 5 seconds 
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axehandle
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I'm not offended.
I was actually thinking of putting the electrolysis vessel in a cold water bath and see how it goes from there.
As a side issue, once I electrolyzed a solution using a salt bridge made out of toilet paper, at 180V, I don't know how many amperes. The damned
toilet paper started to boil and then carbonized! That's what you get when using potassium nitrate as the bridge I suppose...
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Saerynide
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ROFL
Last time, I was about to put the contianer in an icebath, but I doubt it wouldve helped much. I would need a huge supply of ice to replace the
melted stuff. I dont have that much space in my freezer 
Maybe its time we looked for unglazed flower pots Amazingly, paper towels can
actually stand up to dilute H2SO4 for a long time!
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axehandle
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But obviously not saturated KNO3 at something like 180V, 3A...
Edit: Sometimes my brain seems to fly out the window... cheezus... A full bridge rectifier connectes to the 230V AC mains..... a recipe for disaster.
[Edited on 2004-2-12 by axehandle]
[Edited on 2004-2-12 by axehandle]
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Turel
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Capacitation Buffer
If connecting a source of pulsed DC, like from a rectifier, to electrodes, you will see smoother operation if you shunt the hot and ground with a
capacitor (rated for at least the operating voltage for those not familiar with electronics) to smooth the waveform.
This will give a more consistent DC by reducing RMS fluctuation. Better for the electrodes, too. But we know axehandle doesn't worry about
electrode decay, with his fancy shmancy Pt electrodes. 
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axehandle
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Harh! I'll only use 5V for my KClO4 production using a Pt anode, thank you very much. I wouldn't want the _expensive_ wire to melt on
me.....
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