Meth cooks are getting exchange propane tanks from places like Wal-Mart,
Kroger, etc. and emptying them of the propane. Then, they are filling them
with anhydrous ammonia (which they now have a recipe for by the way). After
they are finished with them, they return them to the store. They are then
refilled with propane and sent back for you and me to buy. Anhydrous
ammonia is very corrosive and weakens the structure of the tank. It can be
very dangerous when mixed with propane and hooked up to our grills, etc.
You should inspect the propane tank for any blue or greenish residue around
the valve areas. If it is present, refuse to purchase that one.
You should check out the following website for more details. They also have
pictures you can show.
Anhydrous Ammonia Safety Alert - National Propane Gas Association