[CrimpJiggler]

I did an experiment where we determined the concentration of iron by complexing it with phenanthroline then doing UV-Vis spectroscopy on the orange complex formed. I've been searching for about half an hour now trying to find some information on WHY a pH of 3.5 is ideal for this experiment but I can't find any. Why is a pH of 3.5 optimal for this experiment? Why not higher pHs?

[Borek]

Iron doesn't like high pH, phenantroline doesn't like low pH.

[CrimpJiggler]

Why are high pHs a problem for iron? I know that at low pH phenthroline will be protonated and exist as the phenanthrolium ion and therefore it no longer has its electron pairs to use to chelate iron but I don't understand how that is beneficial for the formation of ferroin.

[Borek]

Too low pH is not beneficial - you want the pH to be as high as possible so that phenantroline is not protonated in substantial amounts.

What do you know about solubility of Fe(OH)₃?