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Iron: a factoid and two questions

A small contribution to the dialogue on monitoring iron levels when using
a supplement such as PMDD:

One common chelating agent is EDTA 
(That's _Mister_ EthyleneDiamineTetraacetic Acid to you, buddy). Ferric
sodium EDTA is easily available, if anyone is crazy enough to want to
make up a trace element solution from scratch.  However, a technician at
Hach tells me that no ordinary test kit can hope to measure it; you need
something like an acid digestion, heating the sample with successive
additions of nitric and hydrochloric acids.  Not the routine aquarium
sampling technique you want to use.

BTW I have experimented with this.  The LaMotte kit won't detect iron in
a sample of FeNaEDTA at 1:10,000, which is about 10 ppm iron, or 100
times what you want in the tank.

Question 1:

EDTA, however, is not what the commercial chelated trace mixes seem to
use.  Does anyone know for sure what they do use?  (And, if it's a
4-letter abbreviation, what the _name_ is?)  (And how does the strength
of the complex compare with EDTA?)

Question 2:

Meanwhile, everyone knows that plants don't absorb iron in the +3
(ferric) oxidation state, but only in the +2 (ferrous).  I assume this is
true. But does anyone _know_ what kind of evidence there is for it?  

This is not all intellectual curiosity: it may not be safe to draw
certain inferences (as in the debate on ferrous gluconate a while ago)
unless one knows this.  For instance, is the conclusion based on the fact
that Fe+++ is extremely insoluble in water at pH 7, and the plants
are getting iron somehow, so it must be as Fe++?  (That's question
2b -- or not 2b, if the answer to 2 already answers it.)  If so, it would
say nothing about what plants do when the iron is present in complexed
form, whether ferrous gluconate or ferric whatever EDTA.

Dan Drake
dd at dandrake_com