Why is nitrous acid prepared in situ?

I am not sure about the above question. First of all, I don't exactly understand what is meant by "in-situ". Secondly, how would preparing nitrous acid in-situ, when amine reacts with nitrous acid, make it less toxic or unstable?

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    $\begingroup$ When you're completely lost, Wikipedia can give you a good start on an amazing array of topics. In situ, with respect to chemistry, nitrous acid, preparation. These don't answer your questions (especially the HNO2 one), but they're a good place to start. $\endgroup$ – airhuff May 5 '17 at 1:59
  • $\begingroup$ in situ means within the reaction. $\endgroup$ – Mitchell May 5 '17 at 3:21

Well, if you don't make it in situ, you would have to add the nitrous acid you have already kept in your laboratory shelf, and surprise! It's not there! Because, it has already decomposed.

Nitrous acid ($\ce{HNO2}$) has an intermediate oxidation state of +3, and hence, it tends to disproportionate into more stable oxidation states:

$$\ce{2HNO2 -> NO2 + NO + H2O}$$

$\ce{NO2}$ itself dissolves in water to give $\ce{HNO3}$ and $\ce{HNO2}$. This gives an overall reaction in aqueous solution as:

$$\ce{3HNO2 -> HNO3 + 2NO + H2O}$$

This should give you a good idea on why we make nitrous acid within the reaction mixture. You can't store it beforehand. It must be made when it's time to use it.

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Yes, HNO2 is unstable, however, per Wikipedia the gaseous anhydride N2O3 is described as a:

deep blue solid[1] is one of the simple nitrogen oxides. It forms upon mixing equal parts of nitric oxide and nitrogen dioxide and cooling the mixture below −21 °C (−6 °F):[2]

$\ce{NO + NO2 ⇌ N2O3}$

Dinitrogen trioxide is only isolable at low temperatures, i.e. in the liquid and solid phases. At higher temperatures the equilibrium favors the constituent gases...

And further:

Nitrite salts are sometimes produced by adding N2O3 to solutions of bases:

$\ce{N2O3 + 2 NaOH → 2 NaNO2 + H2O}$

So, equivalently, one may be able to employ stable stored cold N2O3 and not work with in situ formed nitrous acid.

So, on the question, "Why is nitrous acid prepared in situ?", one possible answer is that it is actually an elective (not to employ N2O3), and not always a requirement.

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    $\begingroup$ You would have to chill the N2O3 as you say, including purchasing it and having it transported chilled. It is much easier logistically and economically to use the in-situ route. $\endgroup$ – Oscar Lanzi Mar 5 at 20:46

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