Eliminating interference from iron(III) for ultraviolet absorbance measurements of dissolved organic matter

The presence of iron(III) has long been recognized as a difficulty when ultraviolet absorbance measurements of dissolved organic matter are desired. This interference was studied in water samples of diverse origins and properties, and a procedure is discussed which uses hydroxylamine to reduce up to 10 mg L−1 (0.18 mM) of Fe(III) to non-interfering Fe(II). This procedure eliminated the effect of Fe(III) in all samples from about 220 to 400 nm, and removed interference down to 200 nm in most samples.