Iron solubility

There have been some comments on Iron solubility that I thaught I would 
post some information on.

FeCl3 has a solubility of 74g/100ml (ie Fe3+)
FeCl2                     64.4g/100ml (ie Fe2+)

The solubility of the phosphate forms of these ions is very limited but 
at the common concentrations in aquariums should not precipitate as 
phoshates. The sulphates are listed as sparingly soluble in water, but 
soluble in acids. This data is from the CRC Handbook of chemistry and 
physics. It is written for people trying to make solutions considerably 
more concentrated than those that should be encountered in the aquarium. 
Another thing is that it is a long way from being a handbook, it has a 
great deal of very interesting data in it.

So to round off the discussion on Iron I would like to put my two cents 
worth in. The solubility of both 2+ and 3+ Iron should not be a problem 
under most situations, especially if in a well planted tank that the 
phosphate concentration is kept low relataive to the iron and also 
chelated with EDTA. If peopel would direct me to some papers that 
actually detail the unavailability/unusability of Fe2+ over 3+ I would be 
grateful. The chemist in me thinks that this would be highly unlikely, 
the cytosol of eukaryotic cells is reductive and so the Fe3+ would be 
reduced there to Fe2+. I have a great deal of difficulty in accepting 
that the valence state of the ions makes that much difference. Perhaps 
what is really being talked about are the variety of Iron oxides that 
really are very insoluble and as such unavailable for use ? A word of 
caution here to, something that has not appeared in an externally 
reviewed scientific paper is often pretty dodgy in terms of its 
reproducibility and rigor. I know that it seems pedantic, but it is 
usually the unrefereed journals that we have problems with.

Hoping that this stirs some comments

Peter Hughes     ANGFA(ACT)