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Author: Subject: Phosphoric Acid nitration mixtures and nitrocellulose

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[*] posted on 28-1-2019 at 19:50
Phosphoric Acid nitration mixtures and nitrocellulose

I stumbled on this paper:

which mentions:
A mixture of nitric and phosphoric acid in a ratio of 1:1 to 1:3 (nitric acid:phosphoric acid) produced nitrocellulose with a nitrogen content up to 13.7% [3]. By this way, the final product was a highly stable nitrocellulose, but phosphoric acid corrodes iron and steel, being a problem for nitrocellulose manufacture [2, 5].

I've seen mentions of use of phosphoric pentoxide for dehydration in mixed acids before, and some reference to that dissolved in a phosphoric and nitric acid system in this thread:

But not phosphoric acid alone, or anyone experimenting with concentrated (or varying dilute concentrations of) phosphoric acid alone in mixed acids, or use of phosphoric acid and nitrate salts for that matter.

It'd be interesting if concentrated phosphoric acid alone, or possibly polyphosphoric acid or some other mixture (mostly important for dehydration properties, so a variable mixture of polyphosphoric acid species might be useful here where they'd be undesireable in most other circumstances) made from heating or vacuum evaporating more dilute phosphoric acid.

That and a nitric-phosphoric acid nitrate-phosphate (or hydrogen-phosphate) system created by dissolving a nitrate salt in phosphoric acid. (given phosphates and hydrogen-phosphates also tend to be highly hygroscopic, they might be more useful as part of the nitration bath than hydrogen-sulfates produced in nitrate salt and sulfuric acid baths)

OTOH, with H3PO4's lower acidity, double displacement and equilibrium with Nitric Acid would be more complicated than the sulfuric acid, and searching a bit, I don't see any mention of folks experimenting with making nitric acid by this route (for mixed acids or to for distillation of nitric acid), but it seems like it would be possible, even if yields might be poor compared to sulfuric acid.

I haven't dealt with such mixtures much myself, but some very crude experimenting with hardware store grade phosphoric acid (39% Jasco Metal Etch solution based on the old MSDS) somewhere over a decade ago led to a notably exothermic reaction with potassium nitrate prills in a round-bottomed glass bottle, though no nitrogen oxide fumes were visible and the small amount used (probably 100-150 ml of acid and 10-20 grams of KNO3) didn't evolve enough heat to reach boiling, just barely hot enough to be uncomfortable to touch.

I tried submersing cotton twine in that solution and letting it sit for several hours, then rinsed and dried it. A burn test showed it behaving unlike cotton, but also unlike highly nitrated cellulose: it had an odd sputtering quality to it, though in hindsight it may have also been contaminated with small amounts of hydrogen-phosphate salts and I think those tend to act as fire suppressants (particualrly in their hydrated state), but it may have been slightly nitrated cellulose with other contaminants mixed in. (with such dilute acid, I'm surprised much of any reaction occurred, but what it seemed something happened, and from what I recall, didn't behave like cotton string simply contaminated with a small amount of potassium nitrate, and didn't produce excessive ash like nitrate-impregnated cotton tends to)

It was soft and felt like cotton, and very slightly tackier than it should be and was discolored somewhat grayish, and phosphate contamination seems more likely than nitrate salts due to the final tacky texture but general softness. (some phosphoric acid may have remained too, which also would've added to that texture) I thought I'd dried it fairly well at the time.

I never experimented with that again, and never delved into nitration reactions otherwise (mixed acids, etc), but it seemed like an interesting anecdote in any case, also not a particularly dangerous thing to repeat (just a waste of acid and nitrates if anything ... and a bit of cotton) but nothing I tried to repeat for whatever reason, even though it was kind of odd and interesting.

If a nitration (even a highly incomplete one) did occur with that mixture, I'd hazard a guess that the potassium-hydrogen-phosphate and mixed acid complex had a pretty potent dehydrating action. The potassium nitrate itself wouldn't do a whole lot on that end, but once in solution, the resulting phosphate salts could've begun forming hydrates and/or complexes of some sort and pulling a lot more water out of solution than would otherwise be the case. (ie doing things that 40% sulfuric acid probably wouldn't do in a similar situation)

The current equivalent to that seems to be 'prep and etch' which lists 37.5% concentration and which appears to be a higher concentration of Ethylene glycol n-butyl ether (that or the older MSDS simply omitted the concentration of that entirely)

Given how easily oxidized ethers and glycols tend to be, I imagine that would've been consumed in an oxidation reaction with any initial HNO3 formed and probably contributed to the exothermic reaction observed during dissolution of the KNO3 prills.
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