Hi!

My guess is because it's passivated. The oxidation potential of 60% HNO3 is such that cause the MnO2 superficial layer to prevent dissolution. Try more diluted acid (10 or 20% should do it) and, if necessary, some heating.

Cheers

Hello ,

I will try this .

thank you Mário Silva

MnO2 dissolves in hydrochloric acid. To remove chlorides, you can add nitric acid and evaporate the solution to low volume or to dryness. Orthophosphoric acid can also be used to dissolve MnO2.

thank you Zbigniew Jonca .

Hi..

I agree with mr mario. A diluted HNO3 known has higher oxidator than the concentrated one. So.. You can try the a diluted HNO3.. With heat would be faster i guess.

The product possibly could be as Mn(NO3)4 or Mn(NO3)6.

Metal oxide is dissolved oxidatively or reductively. HNO3 is an oxidant, however it can not oxidize Mn4+ (s). So, MnO2 is not dissolved in aqueous HNO3. The addition of reductant such as Cr 2+ may dissolve MnO2.