Understanding and eliminating iron interference in colorimetric nitrate and nitrite analysis

Abstract

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. © Springer Science + Business Media B.V. 2009.