Why is water present in "pure" nitric acid and RFNA and WFNA?

Question

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?

Answer 1

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}