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Re: EDTA chelation
> From: Greg Morin <greg at seachem_com>
> Subject: Re: iron gluconate
>
> Well, we're both right. I listed the pKa's for the carboxylic acids only.
> The 6.0 and 10.1 pKa values are for the amines. However, I think you got
> the pH importance backwards ;-) As the pH goes below 6 the interaction of
> one of the amines becomes less and less important (as it becomes
> increasingly ionized). Going above 6 decreases the ionization of the first
> amine resulting in a stronger interaction of the amine with whatever metal
> is chelated. You would need to go above a pH of 10 for maximum chelation
> (so that both amines are unionized) but there is really no need to do that.
> (So I guess I just talked you out of trying a ferrous gluconate product!
> ;-)
EDTA complexes generally have both amine nitrogens and three of the
carboxylates attached to the metal atom. The sixth co-ordination position
on the metal atom is associated with a water molecule. The pH does _not_
have to be very high for that. Quite strongly basic amines co-ordinate
well with transition metal ions at moderate pH, because the bond formed
between the nitrogen lone pair and the metal atom is strong.
--
Paul Sears Ottawa, Canada