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Various types of nitric acid have been used in rocket fuels as oxidisers (RFNA is red fuming nitric acid, WFNA is white fuming nitric acid) as they are often hypergolic with a wide variety of fuels.

RFNA is "pure" nitric acid with a lot of dissolved $\ce{N2O4}$ and WFNA is supposed to be "anhydrous" nitric acid, $\ce{HNO3}$ .

But early research found a variety of problems with suitable nitric acid oxidiser mixes that seemed to depend on their water content. Ignition delays and storage problems both seemed to be influenced by water content.

The chemists developing the fuel mixtures spent a great deal of effort trying to produce nitric acid that did not contain any water but struggled despite taking considerable efforts to prevent the mixtures absorbing water from the environment. But they kept observing water in carefully produced products which, superficially, seems odd for "anhydrous" WFNA acid or RFNA.

So why is water still present in those mixtures? Or, more generally, what sorts of species are present in highly concentrated nitric acid mixtures?

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  • $\begingroup$ Nature abhors purity as much as it abhors a vacuum. $\endgroup$ Dec 20, 2022 at 14:53
  • $\begingroup$ @OscarLanzi I perhaps should have added more about why that is not,apparently, the major reason especially for RFNA. $\endgroup$
    – matt_black
    Dec 20, 2022 at 16:39
  • $\begingroup$ While these solutions are highly concentrated, nobody claims they're 100%. Not to mention even then there would be traces of H2O. $\endgroup$
    – Mithoron
    Dec 20, 2022 at 20:51
  • $\begingroup$ @Mithoron If you read some of the history of rocket fuel development, they certainly thought they should be able to get essentially water free products and they had a desperate need to do so for multiple reasons. It looks far more interesting than a simple view would expect. $\endgroup$
    – matt_black
    Dec 20, 2022 at 22:09
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    $\begingroup$ Hmm, I don't think I got your point in there. If you ask why they don't actually make 100% even if it would be better, than it would be good if you made it more straightforward. $\endgroup$
    – Mithoron
    Dec 20, 2022 at 23:36

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Aside of hygroscopicity and residual initial water, additional water is formed at $\ce{HNO3}$ decomposition to $\ce{NO2}$ ( it would not be red with just $\ce{N2O4}$ ). It is also created by the acid autoionization and due forming nitryl cation. Note that due excess of the acid, practically all water molecules are protonated.

\begin{align} \ce{HNO3(l) + H2O(solv)&-> NO3-(solv) + H3O+(solv)}\\ \ce{6 HNO3(l) &-> 4 NO2(solv) + O2 + 2 H3O+(solv) + 2 NO3-(solv)}\\ \ce{2 HNO3(l) &<=> H2NO3+(solv) + NO3-(solv)}\\ \ce{H2NO3+(solv) + HNO3(l) &<=> NO2+(solv) + H3O+(solv) + NO3-(solv)} \end{align}

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    $\begingroup$ Those reactions are a much better explanation than hygroscopicity. Is there any recent work on other "internal" equilibria that generate other species even in closed containers of fuming acid? I know some was done in the 1950s. $\endgroup$
    – matt_black
    Dec 20, 2022 at 16:41

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