I have old Aqua regia, don´t excatly know how long it´s been stored, but a guess is for several years. Now I have to neutralize it, beacause the chemicalwaste-firm dont take Aqua regia. But I´m not sure how to do it properly. Any suggestions? I have several amount of KOH -pellets, can I use them? What can happen? What are the risks?

here's a link

 

also here's an MSDS so u know the hazards you're dealing with.

potassium hydroxide should work...

 

i would assume nitric acid actually behaves like a proton donating acid when mixed with a base, and not so much as an oxidizer...

 

even baking soda should work according to http://www.umanitoba.ca/admin/human_resources/ehso/media/Concentratedacidmixtures.pdf