The question is that Iron can be determined spectrophotometrically after reaction with a ligand that results in a highly colored complex. In one determination, iron is reduced with hydroquinone to ensure it is all in Fe2+ oxidation state. Then, o-phenanthroline is added to complex it with the Fe2+. The reagents necessary to perform this experiment include a solution of hydroquinone, citric acid buffer, a solution of o-phenanthroline, and an Fe(NH4)2(SO)4 x 6H2O solution. The concentrations of which of these solutions must be known precisely in order to make an accurate determination of iron in an unknown sample.