Understanding and eliminating iron interference in colorimetric nitrate and nitrite analysis

Across many environments, nitrate (NO_3(-)) is an important form of N available for microorganisms and photosynthetic organisms. Accurate NO_3(-) measurements are important for examining N cycling and retention in terrestrial and aquatic ecosystems, but a common method of NO_3(-)) analysis can underestimate NO_3(-)) concentrations when soluble iron is present (iron > 10 mg L(-1)). The basic method is robust, using copperized cadmium to reduce NO_3(-)) and then diazotizing the resulting NO_2(-)) in a two-step process to form an easily measured colored product. We show that iron interference is unique to using an NH(4)Cl and ethylenediaminetetraacetic acid (EDTA) buffer. We hypothesize that interference is through iron-catalyzed reduction of the intermediate color product, a diazonium ion. We examine three historical buffers as alternatives to NH(4)Cl/EDTA and recommend replacement of EDTA with diethylenetriaminepentaacetic acid, which chelates metals much like EDTA, but unlike EDTA, it does not cause interference in the presence of iron.