[Mitch]

I want to quantitate the two species of Iron using an EDTA titration. I'm a bit worried that the complexation with EDTA might shift the equilibrium of these two species in my samples. Is this a valid or concern, or does that process not happen readily?

The iron concentrations are going to be in the 10s-100s parts per billion regime, so I was planning to use ICP-MS.

[Borek]

Not sure if I understand what you wrote. ppb concentrations are way too low for titration.

Then, if there is no other redox pair, EDTA should not change Fe³⁺/Fe²⁺ ratio. At least I can't think of a way.

[Fleaker]

Either ICP or AA is what should be used; I think AA would be faster/easier/cheaper.

You can use a titration, but that would require (a lot) concentration of your analyte and probably waste a good amount of time.

[enahs]

ICP-MS Is only going to tell you total Iron, not oxidation state.
AA is not good for anything less then 1ppm due to the boltzman distribution (do not let any equipment manufacture tell you otherwise, it is shady statistics to get less then that).

As Borek said, ppb is to low for classical titration.

The easiest way to determine the ratio of Fe3+ to Fe2+ would probably be voltammetry. And by combining that with total iron from ICP-MS, you know what you need.

[Mitch]

I was just going to use EDTA as a complexing agent for an IC before the ICP-MS.