On March 4, 1998 you wrote:
> Sean,
> You're right on. Phosphoric acid is the main ingredient in "Naval
> Jelly", a popular and effective rust converter. The phosphoric acid
> combines with rust to form iron phosphate and water. The iron
> phosphate takes the form of a grey powder that washes away pretty
> easily. I'll leave it to the chemists on the list to put up the
> exact chemical reaction--I can't remember if phosphoric acid is H2PO4
> or H3PO4.
> Scott
Scott:
Time for the chemists to reply. For the most part you are
correct, the phosphoric acid in the Naval Jelly combines with the
Iron(III) oxide (rust) to form some Iron(III) phosphate species. I am
a bit skeptical, however, of the simple answer of the sole formation of
Iron(III)
phosphate and water , a possible better answer is that a mixture of
products form--- [Fe(PO4)3)]6- and [Fe(HPO4)3]3-, plus water. In
addition, these salts might be hydrated, i.e., there are water
molecules associated with this complex, but they are not bonded to
the Iron(III) center. Most likely they are "attracted" to the
Iron(III) center, through the oxygen atoms. This is due to the
polarizability of the O-H bond in the water molecule, oxygen has a
high affinity for electrons and will "pull" electron density from
this bond towards itself, and therefore acquire a partial negative
charge. This will in turn be attracted to the Iron(III) center. The
location of these hydrated species will most likely be slightly
outside the Iron(III)phosphate, in what's known as the "outer
coordination sphere."
I do not know the exact coordination
enviroment around the Iron (what's bonded to what) but Iron(III)
shows a marked preference for oxygen so I'm assuming that the
phosphates are bonded to the Iron(III) through their oxygens. I believe
that the grey color of the Iron(III) phosphate is correct, because a
similar reaction of Iron(III)chloride with phosphoric acid results in
a colorless compound. I have searched other references for more info
on color, but haven't found much. Other Iron(III) complexes are
colored though, with a variety of shades of yellow and pale-reds.
BTW, phosphoric acid has the formula of H3PO4. It is not a
strong acid (like sulfuric and nitric) nor is it overly weak (like
acetic). It's somewhere in-between. It's choice for use in Naval
Jelly most likely has to do with several factors, most importantly
being the reactivy with Iron(III)oxide(obviously). Other
considerations of solubility, safety, cost and other will also be a
consideration.
Sorry for so much information, just trying to explain (and
promote discussion) things to all those non-chemists (and some chemists too)
out there.
Regards,
Matt Milkevitch
Matthew Milkevitch
Graduate Student, Dept of Chemistry
Dr. Karen J. Brewer, Advisor
Va. Tech
Blacksburg, VA 24061
(540) 231-4708
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