cleaning phosphate residues

[biotechdude]

Little question...

Swix has a flask used for the production of HI using KI and H3PO4.  It has annoying phosphate residue stains and it would be nice to remove them...

Swix has tried all the usual hot solvent/acid cleaning methods, as well as refluxing in phosphoric (H3PO4) to no avail.  He is also aware of all TFSE techniques eg. pirahna but he is more inclined to believe oxidative heat from H2O2 and H2SO4/HCl would be the best method. 

In short, have anybees removed old phosphate residues and by what method?

[gsus]

or superficially etched glass? this may bee something that won't go away. sounds like you had some hot, concentrated phosphoric in there for a while.

[methyl_ethyl]

First off I have absolutely no theoretical chemistry background. 

That being said, try this; charge the flask with some .1N AgNO₃.  If there was some residual phosphate in the flask would it not form the phosphate salt of silver? Ag₃PO₄  From there you could take up the  precipitate with some 2N HNO3.  Would this work?  I am not sure.  I have been on a three day bender now and I feel as if I have the answer to everything.     I probably have no idea of what I am talking about however.  I will learn chemistry at some point, I hope.  

regards,

methyl_ethyl

[Osmium]

Hot phosphoric acid attacks glass.
Most phosphates are soluble anyway and should readily dissolve.

[biotechdude]

residue? or superficially etched glass? this may bee something that won't go away. sounds like you had some hot, concentrated phosphoric in there for a while.

Its more of a thin cloudy white residue.  The flask was only semi-cleaned at the time (of the reaction), and now the phosphoric is near impossible to remove.  The bottom half is clean, but the upper half is stained and difficult to get at with scrubbing brushes etc.

Superficially etched glass??  Do u mean glass that has been etched or shaved back from refluxing abrasive solutions eg concentrated NaOH(aq)

Hot phosphoric acid attacks glass.
Most phosphates are soluble anyway and should readily dissolve.

Thats what swix thought.  So he refluxed in 86% phosphoric for half an hour and it made no difference.  He then tried 'vaporising' and/or melting the residue off over a flame and that did nothing either.  The shitty thing is that the residue only 'appears' when the flask is dry, so its hard to see the effectiveness of cleaning whilst wet.

Oh well, any other ideas?  Or just live with it?   ...

[Scottydog]

Thanks for the info, looks like Swim is plagued with the same glass etching problem from his H3PO3 rxns. It is starting to make sense, considering that in the end some of it decomposes to phosphoric acid.

Post 523011

(Scottydog: "Cleaning Eudragit Tainted Glassware?", Chemicals & Equipment)

As was previously mentioned, the white salt-like stain is transparent when wet and reappears when dry! Phosphate etching... To think that Swim believed it to bee the result of some kind of eudragit contamination.

No solvent or intense amount of scrubbing will remove this residue.

"Hot phosphoric acid attacks glass."

Over time, will the phosphates stress or weaken the glass? I imagine bees that dream of producing HI from scratch, dedicate one specific flask for that purpose. Just look for the stain and a bee knows which one to use.