This is the most potent form of acid known to alchemists. A single dose will last an hour without deteriating and will eat holes 20' through stone floors. If the acid is splashed against skin, it does 1d10 damage per round, and unless removed will continue doing damage for the full hour! 1 pint of vinegar or water will reduce the acid such that it no longer causes injury.
The reason this acid is called "unstable" is because no known container, not even magical, can hold it for very long. A glass container is typically used for its storage, and even then it must be poured into a new container within 6 hours. Obviously, it can be very expensive in container costs to keep the acid around for very long.