12AX7 - 19-7-2005 at 21:13
What is the reaction for phosphoric acid etching glass?
Tim
Tacho - 20-7-2005 at 08:40
Interesting. I have boiled 85% phosphoric many times without noticing any etching in my glassware. Does the pure acid etch glass?
12AX7 - 20-7-2005 at 09:17
I don't know. It just says so on the MSDS. I've had only sparing results for anything related on Google.
Tim
Tacho - 20-7-2005 at 09:39
I doubt you can do any practical etching with phosphoric acid. It's such a common and cheap chemical. It's even a food additive. If it could
do glass frosting easily, why would people bother with HF or sandblasting.
S.C. Wack - 20-7-2005 at 10:08
Not sure that I understand the question. Hot concentrated H3PO4 dissolves SiO2, not fast.
Tacho - 20-7-2005 at 10:32
Sorry, I realize now that you are interested in the reaction, not in practical considerations about glass frosting. My brain is decomposing faster
every day.
I tried to edit that post, but got a blank page in my browser.
What I read
chloric1 - 21-7-2005 at 09:42
In Phosphorus and its compound I read very briefly about phosphate glass. My understanding is phosphates can polymerize along with long chain
silicates. My guess is this is what happens with Hot Phosphoric acid. I once noticed minor etching on a beaker I used to distill HBr from sodium
Bromide using 85 % Phosphoric acid about 13 years ago. I wonder if quartz would be affected the smae way?
12AX7 - 21-7-2005 at 12:29
Hm, interesting. My understanding is phosphate (P2O5) is rather insoluble in silicate melts (glass) and hence is often used for example to make
translucent and opaque ceramics and glass (bone china is half bone ash, calcium phosphate).
Tim
chloric1 - 22-7-2005 at 09:23
Well, that might be so, as I did not do a thorough examination of that chapter. It would be a reasonable reason to explain the corrosion of glass by
hot phosphoric acid. It wouldn't be because of extraction of alkali becuase HCL and H2SO4 would do the same. I have dissolved copper in boiling
H2SO4 and the beaker did show any change in appearance. Concentrated sulfuric acid is stated to be an excessively acidic medium.