[Monochrome]

Greetings, dear fellow chemists! I have an issue with one analytical problem and ask you to help me solve it if you have time and will to do so. Here's the point: there is a 50.00 ml solution containing Fe(II) and Fe(III) ions. When the pH of this solution is equal to 2.0, it takes 13.7 mL of EDTA solution to titrate it. When the pH of the initial solution is 6.0, it takes 29.6 mL of EDTA solution to titrate it. The EDTA solution concentration in both cases is 0.012 mol/L. The task is to find the concentration of both Fe(II) and Fe(III) ions.

I think that the equilibrium in both cases looks like that:
Fe²⁺ + Y^4- = FeY^2-

Fe³⁺ + Y^4- = FeY^2-

...but I don't know where to go from it. Any help is much appreciated!

Thanks for your attention, looking forward to your suggestions!

[Borek]

Your reactions are OK. Apparently at one pH only one ion gets complexed, while at the other both reacts with EDTA. Start by checking complex stability constants and remember that amount of non-protonated Y^4- (which is the reacting form) depends on the pH.

[Monochrome]

Hi Borek,

Many thanks for your reply! It really helped me out a lot. I was able to calculate the required concentrations in a matter of several minutes.