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teodor
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@Housane so, to stay legal, I suppose, we need either method of direct HNO3 distillation of required concentration into the reaction flask or usage of
salts like Pb(NO3)2 which liberates highly concentrated HNO3 by action of HCl/H3PO4 or application of NO/NO2 mixture in gaseous/liquid form or in a
solution (which is not formally HNO3 anymore). So, I suppose if we want to share our experience with members of general public we need rewrite
experiments of chemical demonstrations by usage on of these techniques.
[Edited on 5-11-2020 by teodor]
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woelen
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In many cases this is highly impractical. Imagine yourself making copper nitrate by distilling HNO3 directly into copper wire, with all the issues of
back-pressure, due to formation of NO2 from the metal. It also is difficult to control the amount of acid added.
What I personally do is distill HNO3 from H2SO4 and NaNO3 (and in the future I certainly will try the NaHSO4 + NaNO3 method), but I only make small
amounts (using a 100 ml boiling flask and a 50 ml receiver flask). This allows me to make some acid in a relatively short amount of time, and I make a
few tens of ml. That amount I use in a few experiments. No need to make huge quantities and keeping those around. The experiments, however, I do with
the prepared acid, I never distill straight into the reaction mix!
As an added nice side effect, this strategy also reduces the risk considerably. If an accident happens with my distillation setup, it is unpleasant,
but not a disaster. Things will be very different if you have half a liter of concentrated hot fuming acid and something nasty happens with that.
I also noticed that the foaming problem with distillations is dependent on the source of the nitrate used. I have NaNO3 (so-called 99% chili salpeter,
purchased as fertilizer in a 25 kg bag several years ago). If I use this directly, I get a lot of foaming and a lot of NO2. The granules are somewhat
greasy, I think they are covered by some wax-like substance to make them less hygroscopic. What I did is dissolve several hundreds of grams of this
NaNO3 in water, which leads to a turbid solution, with a lot of scum on the surface. I let the crap settle at the bottom and then I carefully siphon
90% or so of the liquid into a clean pan. The rest of the liquid (insoluble crap at the bottom + scum at the surface) I discard. The solution of NaNO3
in the pan I evaporate and let it dry (not perfectly, but for my distillation, this is not important, the acid concentration still is well above
azeotropic concentration). This cleaned NaNO3 only causes little foaming in the distillation. I do not use all this cleaned NaNO3 in one distillation.
I keep it around as clean stock of NaNO3 and if it is used up, I make a new clean batch. Up to now, I only have done this twice, so I have a life-time
stock of NaNO3.
[Edited on 5-11-20 by woelen]
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Fyndium
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It looks to me as a suitable method as conc H3PO4 is readily available. The yields did not seem to be great looking at some YT videos, but other
source states it needs 300C to run.
Using sacrificial flasks would be advisable as phosphoric acid apparently corrodes glass a little.
Does H3PO4 have any dehydrating properties? Could it be used to distill above azeo HNO3?
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karolus28
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afaik no, but (HPO3)n does, there's a thread on that from like 2008, basically you boil it in a copper crucible till it's kinda
like glass or something like that.
https://www.sciencemadness.org/whisper/viewthread.php?tid=44...
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Fyndium
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A further thinking.. I have a feedstock of KNO3 that has an impurity of water soluble phosphate. While being in benign amounts not effecting yields to
a significant degree, shouldn't it upon addition of H2SO4 form phosphoric acid, which further reacts with the nitrate to form HNO3 and the
corresponding phosphate, resulting in negligible losses?
What I was also thinking if there is an impurity of KCl with nitrate, should the formed HCl be distilled over with HNO3, or react with the nitrate?
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