Understanding and eliminating iron interference in colorimetric nitrate and nitrite analysis

Across many environments, nitrate (NO₃^-) is an important form of N available for microorganisms and photosynthetic organisms. Accurate NO₃^- measurements are important for examining N cycling and retention in terrestrial and aquatic ecosystems, but a common method of NO₃^- analysis can underestimate NO₃ ^- concentrations when soluble iron is present (iron > 10 mg L ^-1). The basic method is robust, using copperized cadmium to reduce NO₃^- and then diazotizing the resulting NO ₃^- in a two-step process to form an easily measured colored product. We show that iron interference is unique to using an NH ₄Cl 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 NH₄Cl/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.