The solid that is obtained from an aqueous solution of ferric and oxalate
ions varies in its colour (from yellow to green), composition and
solubility, depending on the parameters of the method of its preparation,
especially the pH, temperature, time and relative concentrations of the
components. Richard Sullivan has evidently struck on a method of obtaining
a 'user-friendly' variety which has escaped the "major chemical houses",
and good luck to him. I once had a research student prepare ferric oxalate
by three different published methods, and he came up with three different
substances.
The variable amount of water in the formula of solid ferric oxalate
reflects the fact that the ferric iron is to some, variable, extent
hydrolysed (linked to other ferric ions via hydroxy bridges) and therefore
polymerised, which affects the readiness of its solubility in water. The
variability and poor crystallinity of solid ferric oxalate mean that its
molecular structure (which is undoubtedly complex) has not yet been
determined, to the best of my knowledge, and chemists still know very
little about it. Nor are they, (with one or two notable exceptions!) very
interested.
When Luis Nadeau says:-
>Of all the chemicals they supply ferric oxalate is one of the least
>reliable. There is practically no need for it these days for anything. In
>the UK, Mike Ware told me that it is simply not available, period.
-it was true at the time. I now note from recent Catalogues (Aldrich '93
and Alfa/Johnson Matthey '95) that it has been re-introduced,-at a cost!
The fact that the price of ferric oxalate they now quote is about one
hundred times the price of ferrous oxalate, next in the catalogue, is a
pretty strong indicator that it's hard to make, and not much use to anyone
;-). This also makes it more expensive than the palladium salt!
If you are prepared to make up your own solution of 'ferric oxalate' by one
of the several traditional methods, then it's not really necessary to try
to isolate a solid. Otherwise, my inclination would be to stick with the
B&S product. It is our peculiar misfortune, as alternative-process workers,
that one of our principal chemicals should turn out to be such a maverick.
It is for this reason that I never use ferric oxalate. All my processes are
based for preference on ammonium ferrioxalate (aka ammonium iron(III)
oxalate, or ammonium trisoxalatoferrate, which is obtainable as a pure,
crystalline, well-characterised solid, highly soluble in water), or
ammonium ferricitrate (not so well characterised, but cheap and reliable).
Mike