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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
http://www.dandrake.com/