External-source contamination during extraction—distillation in isotope-ratio analysis of soil inorganic nitrogen

The sources of contamination introduced during the extraction, distillation and drying phases of isotope-ratio analysis of soil inorganic nitrogen (ammonium and nitrite + nitrate) were identified, and the individual amounts of contaminants were quantified by isotope dilution. The procedure involves addition of internal standard solutions of ¹⁵N-labelled ammonium and nitrite to reagent blanks which are carried through each stage of the analysis at the same time as the test samples. Potassium chloride extractants, filter-papers, distillation reagents and atmospheric ammonia all contributed to dilution of the sample ¹⁵N. Some materials tested contained sufficient contaminants to cause large errors in the determination of sample ¹⁵N abundance. Both the amount and isotopic composition of contaminants can be determined by the isotope-dilution procedure, which permits the measured sample ¹⁵N abundance to be corrected for contamination.     

A suite of sensitive chemical methods to determine the δ15N of ammonium, nitrate and total dissolved N in soil extracts

Natural ¹⁵N abundances (δ¹⁵N values) of different soil nitrogen pools deliver crucial information on the soil N cycle for the analysis of biogeochemical processes. Here we report on a complete suite of methods for sensitive δ¹⁵N analysis in soil extracts. A combined chemical reaction of vanadium(III) chloride (VCl₃) and sodium azide under acidic conditions is used to convert nitrate into N₂O, which is subsequently analyzed by purge‐and‐trap isotope ratio mass spectrometry (PTIRMS) with a cryo‐focusing unit. Coupled with preparation steps (microdiffusion for collection of ammonium, alkaline persulfate oxidation to convert total dissolved N (TDN) or ammonium into nitrate) this allows the determination of the δ¹⁵N values of nitrate, ammonium and total dissolved N (dissolved organic N, microbial biomass N) in soil extracts with the same basic protocol. The limits of quantification for δ¹⁵N analysis with a precision of 0.5‰ were 12.4 µM for ammonium, 23.7 µM for TDN, 16.5 µM for nitrate and 22.7 µM for nitrite. Copyright © 2010 John Wiley & Sons, Ltd.     

High-performance ion chromatography assessment of inorganic and organic nitrogen fractions in potatoes

A new high-performance ion chromatography assay for organic and inorganic nitrogen analysis has been proposed and examined. In the devised protocol, inorganic sample constituents were measured after ultrasonically assisted water extraction. The amine and amide nitrogen content was assessed after modified Kjeldahl digestion and determined as NH₄⁺, and the total nitrogen content was quantified as NO₃⁻ after microwave-facilitated digestion. Finally, the nitro, azo, azoxy nitrogen was calculated by comparison of the total nitrogen content and all measured nitrogen species. The detection limits of the measured ions were 2.0, 0.82 and 0.17 mg L⁻¹ for nitrate, nitrite and ammonium, respectively. For samples of potatoes, the average shares of the nitrogen species found in the total nitrogen content were: 0.83% of nitrate nitrogen, <0.03% of nitrite nitrogen, 2.1% of ammonium nitrogen, 71% of nitro, azo, azoxy nitrogen, and 26% of amine, amide nitrogen. We expect the method to be applicable to different vegetable samples. The quality of the results obtained was verified by analyzing certified reference material and comparing to another analytical method.     

Molecular emission cavity analysis: Part 23. Determination of Nitrite and Nitrate after Conversion to Nitrogen Monoxide

Molecular emission cavity analysis is applied to the determination of nitrite and nitrate after their reduction to nitrogen monoxide by iodide or zinc. The white emission stimulated from nitrogen monoxide in an oxy-cavity placed in a hydrogen—nitrogen diffusion flame is measured at 526 nm. Calibration graphs are linear up to 300 μg N ml^-1; the detection limit is 0.5 μg N ml^-1 for nitrite and 2 μg N ml^-1 for nitrate. There are few interferences. Procedures for the determination of nitrite and nitrate in admixture are described.     

Effect of Ammonium Ions on the Radiation Degradation of Potassium Nitrate

The formation and thermal conversion of paramagnetic centers in -irradiated single crystals of potassium nitrate with an ammonium nitrate impurity was studied by EPR spectroscopy. It was found that the presence of ammonium nitrate decreased the radiation-chemical yield of nitrite ions, which are one of the final radiolysis products of nitrate-containing compounds. An analysis of the results of a study on NO₂ orientation in the KNO₃ crystal lattice with an impurity of NH⁺ ₄ allowed us to propose a mechanism for selective N–O bond rupture in the radiation-stimulated dissociation of the nitrate ion in a potassium nitrate matrix.     

The reaction of ammonium salts in molten alkali metal nitrites

The thermal decomposition of ammonium chloride and sulphate in molten alkali metal nitrite eutectics cannot be represented by a simple stoichiometry. Nitrous oxide and nitric oxide are produced as well as nitrogen and water.Thermogravimetry is complicated by loss of solid material when the extremely rapid reaction commences at temperatures just below the melting points of the nitrite eutectics and by volatilisation of unreacted ammonium compounds, largely chloride and nitrite/nitrate from reactant solutions of ammonium chloride and ammonium sulphate, respectively.     

Spectrophotometric simultaneous determination of nitrite, nitrate and ammonium in soils by flow injection analysis

A new rapid flow injection procedure for the simultaneous determination of nitrate, nitrite and ammonium in single flow injection analysis system is proposed. The procedure combines on-line reduction of nitrate to nitrite and oxidation of ammonium to nitrite with spectrophotometric detection of nitrite by using the Griess-llosvay reaction. The formed azo dye was measured at 543 nm. The influence of reagent concentration and manifold parameters were studied. Nitrite, nitrate and ammonium can be determined within the range of 0.02–1.60 μg mL⁻¹, 0.02–1.60 μg mL⁻¹ and 0.05–1.40 μg mL⁻¹, respectively. R.S.D. values (n = 10) were 2.66; 1.41 and 3.58 for nitrate, nitrite and ammonium, respectively. This procedure allows the determination and speciation of inorganic nitrogen species in soils with a single injection in a simple way, and high sampling rate (18 h⁻¹). Detection limits of 0.013, 0.046 and 0.047 μg mL⁻¹were achieved for nitrate, nitrite and ammonium, respectively. In comparison with others methods, the proposed one is more simple, it uses as single chromogenic reagent less injection volume (250 mL in stead of 350 mL) and it has a higher sampling rate.     

A new mathematical approach for calculating the contribution of anammox, denitrification and atmosphere to an N2 mixture based on a 15N tracer technique

Denitrification and anaerobic ammonium oxidation (anammox) have been identified as biotic key processes of N2 formation during global nitrogen cycling. Based on the principle of a 15N tracer technique, new analytical expressions have been derived for a calculation of the fractions of N2 simultaneously released by anammox and denitrification. An omnipresent contamination with atmospheric N2 is also taken into account and is furthermore calculable in terms of a fraction. Two different mathematical approaches are presented which permit a precise calculation of the contribution of anammox, denitrification, and atmosphere to a combined N2 mixture. The calculation is based on a single isotopic analysis of a sampled N2 mixture and the determination of the 15N abundance of nitrite and nitrate (simplified approach) or of ammonium, nitrite, and nitrate (comprehensive approach). Calculations are even processable under conditions where all basal educts of anammox and denitrification (ammonium, nitrite, and nitrate) are differently enriched in 15N. An additional determination of concentrations of dissolved N compounds is unnecessary. Finally, the presented approach is transferable to studies focused on terrestrial environments where N2 is formed by denitrification and simultaneously by codenitrification or chemodenitrification.     

Rapid analysis of fertilizers by the direct-reading thermometric method

The authors have developed rapid methods for the determination of the main components of fertilizers, namely phosphate, potassium and nitrogen fixed in various forms. In the absence of magnesium ions phosphate is precipitated with magnesia mixture; in the presence of magnesium ions ammonium phosphomolybdate is precipitated and the excess of molybdate is reacted with hydrogen peroxide. Potassium is determined by precipitation with silico-fluoride. For nitrogen fixed as ammonium salts the ammonium ions are condensed in a basic solution with formalin to hexamethylenetetramine; for nitrogen fixed as carbamide the latter is decomposed with sodium nitrite; for nitrogen fixed as nitrate the latter is reduced with titanium(III). In each case the temperature change of the test solution is measured. Practically all essential components of fertilizers may be determined by direct-reading thermometry; with this method and special apparatus the time of analysis is reduced to at most about 15 min for any determination.     

Determination of trace amounts of nitrite by single-sweep polarography

A single-sweep polarographic determination of nitrite in 0.2 M sulphuric acid medium containing nickel(II) sulphate and ammonium thiocyanate is described. The ternary complex (NiSCNNO)⁺ which is formed in the solution is strongly adsorbed on the surface of the mercury electrode and an adsorptive polarographic wave at ?0.57 V (vs. SCE) is related to the concentration of nitrite in the range 2.0 × 10^?8-1.0 × 10^?6 M. The detection limit is 8 × 10^?9 M. The relative standard deviation is 1.5% and the regression coefficient is 0.998. Most common anions and cations do not interfere. The mechanism of the electrode process was studied by several electrochemical methods. The polarographic wave is attributed to the reduction of nitrogen monoxide in the adsorbed (NiSCNNO)⁺ complex to hydroxylamine. The procedure was applied to the determination of trace amounts of nitrite in sausage, water and nitrate.     

Sequential injection system for on-line analysis of total nitrogen with UV-mineralization

An automatic method for the determination of total nitrogen in wastewater by sequential injection analysis and mineralization with UV radiation has been developed. The method is based on the mineralization of the samples with sodium persulphate in basic medium under UV radiation. Small volumes of sample and reagents are firstly aspirated into a single channel and then propelled by flow reversal to the UV reactor and then to the detector. The organic and inorganic nitrogen compounds are oxidized to nitrate that is then measured at 226 nm. The sequential injection procedure has been optimized and the factors affecting the efficiency of the oxidation have been studied with a number of test substances with different chemical structures and properties. Solutions in the concentration range 1-56 g l⁻¹ of nitrogen can be analyzed with the described procedure. The sample rate is of 30-40 samples h⁻¹. The LOD is 0.6 mg l⁻¹ N and the reproducibility is 1.8% (28 mg l⁻¹ N). Organic carbon in the form of glucose was added to a number of test solutions to study the potential interference of organic matter.The method was compared with the Kjeldahl digestion method by analyzing 15 wastewater samples with both methods. The nitrate and nitrite content of the non-oxidized samples were subtracted from the corresponding nitrogen content determined after photo-oxidation and the value compared with the Kjeldahl nitrogen content.     

Chemical effects induced by γ-irradiated Na2SO4 in aqueous ammonium nitrate

It is well known that paramagnetic centers are formed when Na₂SO₄ crystals are exposed to -radiation. The dissolution of such crystals in aqueous ammonium nitrate results in reduction of nitrate to nitrite. Various factors which influence the yield of nitrite are investigated. The yield of nitrite is found to vary with the amount of irradiated Na₂SO₄ added, the dose absorbed by Na₂SO₄ crystals, the storing period of the irradiated salt, photoannealing time, concentration of aqueous ammonium nitrate and particle size of the Na₂SO₄ crystals. The G(NO ₂ ^– ) value under optimum conditions of the conversion of nitrate to nitrite by irradiated Na₂SO₄ in aqueous ammonium nitrate is 0.009. The efficiency of energy transfer is 1.5%. The mechanism of reduction is based on the reactions of paramagnetic centers with nitrate ions.     

Determination of Nitrate and Nitrite by Ion-Selective Electrodes in Relation to Gypsum Formation on Some Travertine Buildings in Polluted Atmosphere of Ankara

Abstract

Gypsum formation on calcareous building stones in the polluted atmosphere of cities is thought to be accelerated by the presence of nitrogen-containing compounds. A reliable method for nitrate and nitrite determinations is proposed and the presence of nitrogen compounds is discussed in relation to their possible influence on gypsum formation. Investigations on nitrate and nitrite determinations with ionselective electrodes have been carried out along with gypsum determinations on the samples taken from altered surfaces of some travertine buildings in Ankara. Interfering effects of the ions which may be present in the matrix studied have been examined and the proper masking solutions for their removal have been introduced. Investigations show that nitrate and nitrite determinations with ion-selective electrodes are reliable, rapid and inexpensive in stone extracts. The sulphate to nitrate ratios in this study vary between 32 and 393. These data are compared with earlier results and the possible effects of nitrogen-containing compounds in gypsum formation are discussed.     

Development of a simple method for the determination of nitrite and nitrate in groundwater by high-resolution continuum source electrothermal molecular absorption spectrometry

In this work, it was developed a method for the determination of nitrite and nitrate in groundwater by high-resolution continuum source electrothermal molecular absorption spectrometry of NO produced by thermal decomposition of nitrate in a graphite furnace. The NO line at 215.360 nm was used for all analytical measurements and the signal obtained by integrated absorbance of three pixels. A volume of 20 μL of standard solution or groundwater sample was injected into graphite furnace and 5 μL of a 1% (m/v) Ca solution was co-injected as chemical modifier. The pyrolisis and vaporization temperatures established were of 150 and 1300 °C, respectively. Under these conditions, it was observed a difference of thermal stability among the two nitrogen species in the presence of hydrochloric acid co-injected. While that the nitrite signal was totally suppressed, nitrate signal remained nearly stable. This way, nitrogen can be quantified only as nitrate. The addition of hydrogen peroxide provided the oxidation of nitrite to nitrate, which allowed the total quantification of the species and nitrite obtained by difference. A volume of 5 μL of 0.3% (v/v) hydrochloric acid was co-injected for the elimination of nitrite, whereas that hydrogen peroxide in the concentration of 0.75% (v/v) was added to samples or standards for the oxidation of nitrite to nitrate. Analytical curve was established using standard solution of nitrate. The method described has limits of detection and quantification of 0.10 and 0.33 μg mL⁻¹ of nitrogen, respectively. The precision, estimated as relative standard deviation (RSD), was of 7.5 and 3.8% (n = 10) for groundwater samples containing nitrate–N concentrations of 1.9 and 15.2 μg mL⁻¹, respectively. The proposed method was applied to the analysis of 10 groundwater samples and the results were compared with those obtained by ion chromatography method. In all samples analyzed, the concentration of nitrite–N was always below of the limit of quantification of both the methods. The concentrations of nitrate–N varied from 0.58 to 15.5 μg mL⁻¹. No significant difference it was observed between the results obtained by both methods for nitrate–N, at the 95% confidence level.     

Sequential atomic absorption spectrometric determination of nitrate and nitrite in meats by liquid-liquid extraction in a flow-injection system

Sequential determinations of nitrate and nitrite based on continuous liquid-liquid extraction, and suitable for their routine determinations in meats, are reported. Nitrate reacts with bis(2,9-dimethyl-1,10-phenanthrolinato)copper(I) to form an ion-pair which is extrated into 4-methyl-2-pentanone in a flow-injection manifold. In one aliquot of sample, nitrite is oxidized by cerium(IV), so that total nitrate is determined. In another, nitrite is converted to nitrogen with sulfamic acid, so that only the original nitrate is determined. By measuring the atomic absorption signal of copper in the organic phase, mixtures of these anions can be determined at μg ml^?1 levels for nitrate/nitrite ratios from 10:1 to 1:10, with a sampling frequency of ca. 20 h^–1.     

Efficiency of energy transfer from γ-irradiated ammonium halides in aqueous iodide and nitrate solutions

It is well known that ammonium halide (NH₄X) crystals, on -exposure, store energy in the form of primary and secondary radiolytic products. Such crystals on dissolution in aqueous iodide and nitrate solutions result in oxidation of iodide and reduction of nitrate, respectively. The yields of iodine and nitrite are determined by chemical methods under varying conditions of the amount, dose and particle size of the irradiated ammonium halide salts. The maximum values of the efficiency of energy transfer for oxidation and reduction processes for ammonium halide salts correspond to 40% and 10%, respectively. At low doses, an empirical relation proposed between the percent efficiency of energy transfer and the absorbed dose is valid. The concentrations of inherent oxidizing and reducing species initially present are 7.0×10¹⁸ and 1.0×10¹⁸ per mol of ammonium halide, respectively.     

Two rapid and sensitive automated methods for the determination of nitrite and nitrate in soil samples

Spectrophotometric flow injection methods were developed for the individual determination of nitrite or nitrate, and for the simultaneous determination of nitrite and nitrate, in soil samples. Nitrite was determined directly using a modified version of the Griess-Ilosvay diazo-coupling reaction, measuring at 543 nm the absorbance of the azo-dye complex formed. Simultaneous nitrite and nitrate determinations were based on on-line nitrate reduction in a micro column containing copperised cadmium. A single chromogenic reagent containing all the necessary reactants was used in both methods. For determinations, the chemical and instrumental variables were optimised by univariate analysis and simplex chemometric method. The optimised conditions gave a linear calibration range between 0.05 and 1.6 µg m L^− 1 for N-NO₂⁻ and between 0.05 and 7.0 µg m L^− 1 for N-NO₃⁻. The detection limits for nitrite and nitrate were 22 µg L^− 1 and 44 µg L^− 1 respectively. The proposed methods allowed up to 35-40 samples per hour to be analysed with good precision. The simultaneous method was successfully used for the determination of nitrite and nitrate in soil samples (the results obtained were validated against those obtained by reference methods). The proposed methods are simpler and faster than conventional methods and could be routinely used in environmental monitoring laboratories.     

Synthesis,chemical characterization,X-ray crystal structure and magnetic properties of oxalato-bridged copper(II) binuclear complexes with 2,2′-bipyridine and diethylenetriamine as peripheral ligands

Two new μ-oxalato binuclear copper(II) complexes, [{Cu(NO₃)(H₂O)(bipy)}₂(ox)] (1) and [{Cu(dien)}₂(ox)](NO₃)₂ (2), with ox=oxalate, dien=diethylenetriamine and bipy=2,2′-bipyridine, have been synthesized and their crystal and molecular structures have been determined by single-crystal X-ray diffraction methods. The crystal structure of 1 consists of centrosymmetric neutral dimers where the copper atoms lie in a strongly elongated octahedral environment, surrounded by two nitrogen atoms of a bipy molecule and two oxygen atoms of the bridging oxalato group in the equatorial plane and oxygen atoms of water molecules and nitrate ions in the axial positions. Crystal structure of 2 is made up of non-coordinated nitrate anions and asymmetric binuclear cations in which copper atoms are in a distorted square–pyramidal coordination with three atoms of a diethylenetriamine ligand and an oxygen atom of the asymmetrically coordinated oxalato bridge building the basal plane and the other oxygen atom of the oxalato ligand filling the apical position. Both compounds have been also characterized by Fourier transform infrared (FT-IR) and electron spin resonance (ESR) spectroscopies, thermal analysis and variable temperature magnetic susceptibility measurements. The two compounds exhibit antiferromagnetic exchange with a singlet–triplet separation of −382 and −6.5 cm⁻¹ for 1 and 2, respectively. Magnetic and ESR results are discussed with respect to the crystal structure of the compounds.     

A mechanism for the photochemical transformation of nitrate in snow

Photochemical reactions of trace compounds in snow have important implications for the composition of the atmospheric boundary layer in snow-covered regions and for the interpretation of concentration profiles in snow and ice regarding the composition of the past atmosphere. One of the prominent reactions is the photolysis of nitrate, which leads to the formation of OH radicals in the snow and to the release of reactive nitrogen compounds, like nitrogen oxides (NO and NO₂) and nitrous acid (HONO) to the atmosphere. We performed photolysis experiments using artificial snow, containing variable initial concentrations of nitrate and nitrite, to investigate the reaction mechanism responsible for the formation of the reactive nitrogen compounds. Increasing the initial nitrite concentrations resulted in the formation of significant amounts of nitrate in the snow. A possible precursor of nitrate is NO₂, which can be transformed into nitrate either by the attack of a hydroxy radical or the hydrolysis of the dimer (N₂O₄). A mechanism for the transformation of the nitrogen-containing compounds in snow was developed, assuming that all reactions took place in a quasi-liquid layer (QLL) at the surface of the ice crystals. The unknown photolysis rates of nitrate and nitrite and the rates of NO and NO₂ transfer from the snow to the gas phase, respectively, were adjusted to give an optimum fit of the calculated time series of nitrate, nitrite, and gas phase NO_x with respect to the experimental data. Best agreement was obtained with a ∼25 times faster photolysis rate of nitrite compared to nitrate. The formation of NO₂ is probably the dominant channel for the nitrate photolysis. We used the reaction mechanism further to investigate the release of NO_x and HONO under natural conditions. We found that NO_x emissions are by far dominated by the release of NO₂. The release of HONO to the gas phase depends on the pH of the snow and the HONO transfer rate to the gas phase. However, due to the small amounts of nitrite produced under natural conditions, the formation of HONO in the QLL is probably negligible. We suggest that observed emissions of HONO from the surface snow are dominated by the heterogeneous formation of HONO in the firn air. The reaction of NO₂ on the surfaces of the ice crystals is the most likely HONO source to the gas phase.     

Axenic Cultures of Nitrosomonas europaea and Nitrobacter winogradskyi in Autotrophic Conditions: a New Protocol for Kinetic Studies

As a part of a natural biological N-cycle, nitrification is one of the steps included in the conception of artificial ecosystems designed for extraterrestrial life support systems (LSS) such as Micro-Ecological Life Support System Alternative (MELiSSA) project, which is the LSS project of the European Space Agency. Nitrification in aerobic environments is carried out by two groups of bacteria in a two-step process. The ammonia-oxidizing bacteria (Nitrosomonas europaea) realize the oxidation of ammonia to nitrite, and the nitrite-oxidizing bacteria (Nitrobacter winogradskyi), the oxidation of nitrite to nitrate. In both cases, the bacteria achieve these oxidations to obtain an energy and reductant source for their growth and maintenance. Furthermore, both groups also use CO₂ predominantly as their carbon source. They are typically found together in ecosystems, and consequently, nitrite accumulation is rare. Due to the necessity of modeling accurately conversion yields and transformation rates to achieve a complete modeling of MELiSSA, the present study focuses on the experimental determination of nitrogen to biomass conversion yields. Kinetic and mass balance studies for axenic cultures of Nitrosomonas europaea and Nitrobacter winogradskyi in autotrophic conditions are performed. The follow-up of these cultures is done using flow cytometry for assessing biomass concentrations and ionic chromatography for ammonium, nitrite, and nitrate concentrations. A linear correlation is observed between cell count and optical density (OD) measurement (within a 10?% accuracy) validating OD measurements for an on-line estimation of biomass quantity even at very low biomass concentrations. The conversion between cell count and biomass concentration has been determined: 7.1?×?10¹² cells g dry matter (DM)^?1 for Nitrobacter and 6.3?×?10¹² cells g DM^?1 for Nitrosomonas. Nitrogen substrates and products are assessed redundantly showing excellent agreement for mass balance purposes and conversion yields determination. Although the dominant phenomena are the oxidation of NH ₄ ⁺ into nitrite (0.95?mol?mol N^?1 for Nitrosomonas europaea within an accuracy of 3?%) and nitrite into nitrate (0.975?mol?mol N^?1 for Nitrobacter winogradskyi within an accuracy of 2?%), the Nitrosomonas europaea conversion yield is estimated to be 0.42?g DM mol?N^?1, and Nitrobacter winogradskyi conversion yield is estimated to be 0.27?g DM mol?N^?1. The growth rates of both strains appear to be dominated by the oxygen transfer into the experimental setups.