ave369 - 27-8-2015 at 10:21
Today I've found some OTC 80-85% phosphoric acid (sold as a soldering flux for steel) and conducted some experiments with it. Namely, I made some
hydrochloric acid by reacting phosphoric acid and table salt under heating, and dissolving the evolving HCl in water.
After that, I've read some reports that heating may cause phosphoric acid to attack glass. I examined my round bottom flask closely and found no
damage, but this was a definite "oh, shit" moment.
According to the reports, phosphoric acid attacks glass when heated to 80 degrees Celsius. Is it true?
[Edited on 27-8-2015 by ave369]
gdflp - 27-8-2015 at 10:37
Yes, hot concentrated phosphoric acid is capable of attacking borosilicate glass. However, solutions of alkali hydroxides are also capable of
attacking borosilicate glass. At 80°C, I doubt that much would happen to the flask, but many repeated exposures would thin the walls of the flask
and eventually cause to fracture due to a loss of mechanical strength. The attack is not quick, but I would not habitually boil phosphoric acid in
glass. See threads such as this to get an idea of the severity of the etching that phosphoric acid can cause. Overall, I would not worry about a single exposure at 80°C
and would treat that flask as any other normal flask.
ave369 - 27-8-2015 at 10:44
If it's the same rate of damage as alkalis do, then it's okay. But anyway, if I'll need to boil phosphoric acid every day, I'll use some Orky medicine
bottles and pickle jars which it won't be a shame to lose, rather than proper glassware.
[Edited on 27-8-2015 by ave369]
byko3y - 27-8-2015 at 14:46
Regular phospohric acid does not attack glass, but its polymeric forms do. The more acid is dehydrated - the more it attacks glass. It forms
phoshpate-silicates on the surface of the glass, thus making it resistant to HF, but mechanical strength is lost. AFAIK, phosphoric acid attacks glass
only slowly below 200°C and the rate is negligible below 100°C.