I would like to know whether strong acids, like 99-100% sulfuric acid, 65% nitric acid or 98-100% formic acid, stored for many years (lets say over 20 years) deteriorate/change in such a way that it is preferred not to use them anymore? Or do you believe nothing happens to these solutions when stored properly in closed bottles?
If any of the containers have ever been openned, I would not trust them. Is the formic acid in a self-venting plastic container or glass? If it is in glass, again, I would not trust it; formic decomposes into carbon monoxide and water; if in glass, the bottle may be pressurized so be careful. If the sulfuric and nitric are in their original sealed glass containers, they are probably OK to use in the lab.
From my experience, sulphuric and nitric should be alright but a test for the specific application is worth.
I worked as a lab technician for several years. I found that say a 1.0M solution would be far more stable than conc solutions. Both bottles of solutions were treated the same way. As far as 20 year old acids, I would order new stuff. The fumes from the acid can and do deteriorate the caps over time.
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