Understanding and eliminating iron interference in colorimetric nitrate and nitrite analysis

摘要

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 copper-ized 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_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 NH_4Cl/EDTA and recommend replacement of EDTA with di-ethylenetriaminepentaacetic acid, which chelates metals much like EDTA, but unlike EDTA, it does not cause interference in the presence of iron.