In the language of chemistry,in situ means The reaction mixture.Here,in case of HNO2,it is too unstable.Here,if we see the reaction,
HNO2→HNO3+NO+H2O
The oxidation no of central atom N:
in HNO2-+3
in HNO3-+5 (oxidation,as oxidation no increased)
in NO-+2(reduction,as oxidation no decreased)
If we observe,we can see that,here at the same time, the oxidation and reduction of N occurs.When in any compound,the oxidation and reduction of any element occurs at the same time,we call it Disproportionate reaction instead of oxidation-reduction reaction.
HNO2 has an intermediate oxidation state of +3.Thats why,it tends to be disproportionate into more stable oxidation state.
Instead of this,to prepare Nitrous acid,we take the mixture of NaNO2 and HCl.
NaNO2+HCl→NaCl+HNO2
Through this process,HNO2 is prepared in the labratory in need