[tchristides]

I have been trying to make calibration curves for a single beam UV Spec using ferric chloride and ferrous sulphate.
My  calibration concentration range is 10-100,000 micromolar, and I have adjusted the pH of my solutions to 7 using NaOH (or tried to anyway).
The problem I am having is that at concentrations of ferrous sulphate greater than 100 micromolar I am getting percipitation of the iron as pH increases - and I don't even think its ferrous iron anymore as it changes color to orange/red - ferric iron?
If the expected levels of Fe++&Fe+++ in my unknowns are 10-100 micromolar, can my calibration curves also be in that range?
Also, I'm assuming that absorbtion will change for a given valence of iron as pH changes?
Any insights would be very much appreciated.