I bought some sulfuric acid drain cleaner to clean an Erlenmeyer flask that had crud stuck to the bottom that I couldn't get off with anything else. It did a pretty poor job at first (but it did kinda work), but then I heated it and it worked great. That was a couple of weeks ago.
A plastic carboy that I used to ferment some beer had a white streak on the outside that I couldn't wash off. I thought it was yeast from when the beer foamed out the top of the air lock, but I couldn't figure out why it was stuck so tight. Tonight I decided to use the ol' H2SO4 on it. The carboy is PET plastic, and that's acid resistant, right? I used a Q-tip to apply the acid (the acid ate the cotton off the end right away but the stick part still worked) and it seemed to be working but awfully slowly. I thought that was just cuz the acid was cold, but I couldn't use hot acid for this. After it looked like it was all gone I washed the carboy with some water, and the whole area turned white. I scrubbed it a bit and a lot of the white came off but it still looks like *expletive deleted*it. Since it's on the outside and doesn't go very deep, it's still usable it just looks awful. What happened? (rhetorical) PET doesn't contain any oxygen in the molecule for the acid to attack (along with 2 hydrogens), does it? I looked it up.
D'oh!
I thought it was just huge chains of carbon and hydrogen, but that's polyethene (and polypropylene).
PS, the original streak was probably from sulfuric acid when I opened the bottle and spilled a drop w/o realizing it. I opened it right there by where the fermenter was sitting.
PPS, sulfuric acid will etch the 420j2 stainless steel tip of your Buck knife when you use it to cut the plastic seal on the bottle.
