[TheForge] Iron Pickle Vol 75, Issue 24

[lee robbins]

Hi Mike,
i asked the question to my former biochemistry professor Peter Weber in Florida, His answer:

 FePO4 or ferric phosphate is most likely the compound that will eventually form in a phosphoric acid solution of iron as the initial ferrous form is slowly oxidized in a container exposed to air to the ferric form which ismuch less soluble in water than the ferrous phosphate. Whitish crystals are indeed the form in which the ferrous phosphate crystallizes. 

 
  Lee 

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Message: 5
Date: Fri, 23 Apr 2010 03:41:04 -0300
From: mspencer at tallships.ca (Mike Spencer)
Subject: [TheForge] Ping Bruce (or any other chemists): Phosphate
    deposits    in pickle
To: theforge at mailman.qth.net
Message-ID: <201004230641.o3N6f4u16297 at nudel.nodomain.nowhere>


When I leave a tray of used phosphoric acid pickle sit around for a
prolonged time, I get a bunch hard, brittle, rocky deposit adhering to
the tray.  Stuff is white when rinsed off, insoluble or only very
slightly soluble in cold water and HCl.  What is it?

Only stuff in the mix should be phosphoric acid, water and, of course,
iron.  So is this iron phosphate?  The HBoC&P doesn't mention acid
solubility of iron phosphate but intuitively, I would expect it to be
HCl soluble.

It doesn't form in my container of new, undiluted phosphoric acid but
does form in a stored gas can of used acid.

Brain dump on chemistry of H2PO4, Fe, H2O welcome.

- Mike

-- 
Michael Spencer                  Nova Scotia, Canada       .~. 
                                                           /V\ 
mspencer at tallships.ca                                     /( )\
http://home.tallships.ca/mspencer/                        ^^-^^