Fe2+ and Fe3+ ions


>	Can aquatic plants really not cope with FeIII in the substrate?
>The chelated iron we add is a _very_ tightly bound FeIII complex with
>EDTA, and they use that, it seems.  Why can they not use FeIII in solid
>material in the substrate, provided the roots can reach it?  _Must_ the
>substrate go anoxic?

According to Gesting (Berti Gesting-"Nature and Aquarium-Advanced Aquarium
management-aquarium plants")

"Some ions can, of course, be present in a different form, eg Iron as Fe3+ 
of Fe2+ and manganese as Mn4+ instead of Mn2+, but it means that they are 
longer plant accessible. These ions are then often bound onto natural 
(organic compounds) which plants can absorb in order to obtain the 
nutrients they

"Iron exists as nutrient-iron Fe2+ only in waters with oxygen levels of 
less than 1mg/l.
Because it is in this form water soluble, those waters can contain up to 
several mg/l
of nutrient iron. A typical example of this are groundwater streams: 
because they are
poor in oxygen and rich in water soluble mineral nutrients, they are 
frequently plant
paradises. Once the water has been exposed to the air it has absorbed so 
much oxygen,
that the Fe2+ ions change to Fe3+ ions and are no longer water soluble."

I dont see why they cannot. As long as the oxygen content in the substrate 
is less than
1 mg/l, there will be Fe2+ ions generated as the material "rusts". Normal 
oxidisation of Fe results in the generation of Fe(lll)2O3, which is the 
common stuff we see on rusting
car bodies, the other compound Fe(ll)O is a black substance that is soluble 
in water, but
breaks down very quickly to Fe(lll)2O3 in the presence of disolved O.

The only other question then is if a substrate _can_ be maintained with an 
content of less than 1mg/l, is the rate of Fe2+ ions being generated from a 
solid piece of
Fe. This is primarily determined by the surface area. A powder for 
instance, would have a
much higher surface area than any solid object (such as a nail :->) and so 
would achieve
the highest generation of Fe2+ ions.

On another rusty topic:

Something that I cant get out of my mind was a talk given, where the
units of ppm and mg/l were tauted as being approximately the same.

This has been bothering me for some time, as the Dupla Fe test kit
measures Fe in mg/l and the Aquasonic test kit I have, measures
Fe in ppm.

According to my periodic table, Fe has an atomic weight of 55.847. So
therefore one Mole of Fe ions would weigh approximately 55.8 grams, 
for the fact that the Fe ion has lost electons in becoming an ion.

Avogadro's Number, 6.022E23 using scientific notation, is the number of
molecules that would be present in 1 mole of any substance. As Fe ions are
present as single atoms in solution, 1 mole of Fe ions would weigh 55.8g 
there would be 6.022E23 atoms.

Now water, H2O has an atomic weight of:
	H  atomic weight 1
	O atomic weight 15.9994
H2O then has an atomic weight of 17.9994 or about 18
So one mole of pure H20 has a weight of 18grams. Therefore one litre of 
(which weighs 1kg) contains 55.56 moles or (55.56 * 6.022E23) or 3.35E25
molecules of H2O.

To create a solution then, that contained 1mg/l of Fe ions, we would have 
to add
(6.022E23 / 55.8) or 1.08E22 ions of Fe to our 1 litre of water.

We would than have 1.08E22 ions of Fe in  3.35E25 molecules of H20. Or to 
it another way,

 Interested Quantity                            Fe ions
------------------------------ *1E6   = 
  ---------------------------------------    *  1E6  =  322.4ppm
  Total Quantity                         H2O molecules + Fe ions

322.4 ions of Fe for every 1millon parts of Fe ions and water molecules.

ie:  1mg/l = 322.4 ppm.

If my analysis is correct :-?, these quantities are not similar at all, or 
have I missed the
bucket completely and covered myself with its contents (10 litres of water 
or 335E25 molecules of H20).    :-)

marque crozman     ANGFA - NSW