why is diluted Fe(NO3)3 in HNO3 is used as the blank reagent ?

Question

In an spectrophotometric determination of equilibrium constant for a reaction,
why is diluted 0.20M Fe(NO3)3 in 0.1M HNO3 is used as the blank reagent instead of distilled water which has zero absorbance?

Answer 1

Often a reagent blank is used instead of a water blank because it will give better measurements at the lower absorbance end of the standard curve.

This is especially true if the absorbance of the specimens would read too high if read against water (in other words, if the absorbance of the reagent itself is high).

This will often depend upon the wavelength of light being measured in the spectrophotometer.

Answer 2

High solubility and low deviation.