Understanding and eliminating iron interference in colorimetric nitrate and nitrite analysis

Abstract

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.

Potential of Wheat Straw, Spruce Sawdust, and Lignin as High Organic Carbon Soil Amendments to Improve Agricultural Nitrogen Retention Capacity: An Incubation Study

“…The remaining KCl extract was dried at 65°C, re-dissolved in 3 ml of 1 M NaOH, and mixed with 37 ml acetone for 30 s. After centrifugation at 3,000 rpm for 20 min, the acetone supernatant containing most of the NO 3 - was transferred into glass beakers, dried at 30–40°C, re-dissolved in 5 ml of deionized water, freeze-dried, and finally transferred into Sn capsules for EA-IRMS analysis ( Zhu et al, 2013 ). Nitrite was monitored as secondary parameter, using an improved colorimetric method according to Colman (2010) and Homyak et al (2015) . A DU-800 spectrophotometer (Beckman Coulter, Fullerton, United States) at a wavelength of 560 nm was used for measurement.…”

Aridity and plant uptake interact to make dryland soils hotspots for nitric oxide (NO) emissions

“…Although this level of contamination is possible, it is inconsistent with the patterns observed when we added 1 atom% [ 15 , we substituted ethylenediaminetetraacetic acid with diethylenetriaminepentaacetic acid to minimize interferences produced by iron (67). Water extracts for WEON were analyzed on a total organic carbon analyzer (TOC-V CSN; Shimadzu Scientific Instruments) using combustion catalytic oxidation/nondispersive infrared method with an ASI-V autosampler and TNM-1 total nitrogen module.…”

Chemical Composition of High Organic Carbon Soil Amendments Affects Fertilizer-Derived N2O Emission and Nitrogen Immobilization in an Oxic Sandy Loam

“…In detail, 1 M potassium chloride (KCl, VWR, Germany) solution was mixed with freeze-dried soil at a ratio of 1:5 (w/v) for 2 h. The mixture was centrifuged at 3500 rpm for 20 min, then the suspension was measured with a pH meter (multi 340i, WTW GmbH, Germany). Soil NO − 2 concentration was determined according to an improved colorimetric method (Colman, 2010;Homyak et al, 2015): 5 g of fresh soil was mixed with 25 ml of 0.012 M NaOH solution for 2 h to extract NO 2 − ; after filtered through 0.45 µm, NO 2 − in the extract was reacted with sulfanilamide and N-(1napthyl)-ethylenediamine in diethylenetriaminepentaacetic acid buffer; finally the colored product was subsequently measured with a spectrophotometer (DU-800, Beckman Coulter, Fullerton, United States) at 560 nm. Dissolved organic carbon (DOC) and dissolved total nitrogen (DTN) were extracted with 0.01 M CaCl 2 solution (Gonet and Debska, 2006), while soil moisture was measured with the classical thermo-gravimetric technique (Lekshmi et al, 2014).…”