# Why is nitrous acid prepared in situ?

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?

• 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. – airhuff May 5 '17 at 1:59
• in situ means within the reaction. – Mitchell May 5 '17 at 3:21

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