Re: Calcium etc.
Elizabeth Worobel writes:
>I have never read that high pH seriously reduces the chelating capacity
>and stability of EDTA ...
I'm not quite sure who you are responding to, but whoever said that has it
backwards. EDTA complex formation is pH dependent, but higher pH _favors_
complex formation. This is why EDTA titrations of metals which form relatively
weak EDTA complexes, like Ca & Mg, are performed at high pH. EDTA titrations
for Ca & Mg are typically done in a pH 10 buffer.
>doubt that Ca could displace Fe since the FeEDTA complex has a formation
>constant 15 orders of magnitude greater than the CaEDTA complex ... thats
>a 1 with 15 zeros behind it.
Quite right. Both the Fe(II) and Fe(III) complexes are more stable than the
Ca complex. I'd add one small caveat, however. Based on the spread you have
given (10^15) you are probably looking at formation constants for pH > 12. So,
while they are indicitave of the properties of metal-EDTA complexes, they
cannot be directly applied to EDTA in an aquarium without accounting for the pH
difference. It doesn't really make much difference for your statement, but at
aquarium pH levels the formation constants will be lower.