prole - 2-11-2005 at 11:49
I have a Teflon coated magnetic stir bar which I just used to distill water from battery acid. Upon finishing my distillation and recovering the bar,
I noticed that some of the Teflon coating had 'washed' off. Now, Merck and many of the posts here mention that PTFE is chemically inert,
but my stir bar says otherwise. I can now see the metal under the coating, although it's not that bad.
The acid went from @30% to @95+%, and a max vapour temp of 106 C, for several hours, and the bar was completely white with no blemishes prior to
beginning. It's now covered with little grey spots that won't go away. Has anyone had this happen to them, and is PTFE sensitive to these
temps in this acid?
[Edited on 11/2/2005 by prole]
garage chemist - 2-11-2005 at 12:11
Teflon is heat resistant until 260°C.
Boiling concentrated H2SO4 is about 330°C hot.
It was the extreme heat that damaged your stir bar, not the acid.
For those temperatures you can use a steel rod inside a glass pipe sealed at both ends.
Such high- temperature stirring rods can also be bought, but are harder to find.
Making one yourself would be the best solution here.
prole - 2-11-2005 at 14:29
Ah, I didn't realize that the liquid was really that hot. The dial was set to 250 C, but I didn't think this was very accurate.
Forgive me, I don't understand, if I was distilling water from acid, why was it necessary to heat it so hot? At any temp much lower than this
setting, not much water would come over. Is this because H2SO4 is loath to give up water? I know it's hygroscopic as all get out, but come
on...
12AX7 - 2-11-2005 at 14:44
Ah sure... take calcium sulfate for instance. It chemically combines with water in two phases: the dihydrate is stable to about 250°C, when it
decomposes to the hemihydrate (used in Plaster of Paris) which is stable out to red heat! And after that, the sulfate ion won't begin to break
down for another thousand degrees! (F, that is... yellow heat is about what, 1200C, and red around 550C.) Not only that, it won't decompose
appreciably until *white* heat!
Tim
prole - 2-11-2005 at 16:02
hmmm, the light grows less dim in prole's mind...
Am I correct in assuming, then, that my hard-won conc. H2SO4 is now contaminated with decomposition products from PTFE? The acid, clear and
colourless in the begining, is now a lovely golden colour
. And if so, would they
be negligible, or pesky bitches in future reactions?
Supposing I can't get around to fabricating a glass-entombed steel bar. I still have boiling stones, but alas! they too are Teflon. What about
cracked porcelain? Would they react with the acid? Or what about broken glass shards as boiling stones? Back to the lab, I guess...
Man, this chemistry stuff is frustrating! I love it!
12AX7 - 2-11-2005 at 16:08
My store-bought H2SO4 is a light gold color. I wouldn't worry about it.
Porcelain and glass can certainly be used. Eventually, I suppose the ions inside will be eaten away, or at least on the surface, but that'll
take some time. (Porcelain is composed of silicon, aluminum, calcium and sodium/potassium oxides. Silicon obviously isn't going anywhere, but
the rest can be leached, at least from a powder.)
Tim