Simple flow-injection system for the simultaneous determination of nitrite and nitrate in water samples

A novel spectrophotometric reaction system was developed for the determination of nitrite as well as nitrate in water samples, and was applied to a flow-injection analysis (FIA). The spectrophotometric flow-injection system coupled with a copperised cadmium reductor column was proposed. The detection was based on the nitrosation reaction between nitrite ion and phloroglucinol (1,3,5-trihydroxybenzene), a commercially available phenolic compound. Sample injected into a carrier stream was split into two streams at the Y-shaped connector. One of the streams merged directly and reacted with the reagent stream: nitrite ion in the samples was detected. The other stream was passed through the copperised cadmium reductor column, where the reduction of nitrate to nitrite occurred, and the sample zone was then mixed with the reagent stream and passed through the detector: the sum of nitrate and nitrite was detected. The optimised conditions allow a linear calibration range of 0.03–0.30 μg NO₂⁻-N ml⁻¹ and 0.10–1.00 μg NO₃⁻-N ml⁻¹. The detection limits for nitrite and nitrate, defined as three times the standard deviation of measured blanks are 2.9 ng NO₂⁻-N ml⁻¹ and 2.3 ng NO₃⁻-N ml⁻¹, respectively. Up to 20 samples can be analyzed per hour with a relative standard deviation of less than 1.5%. The proposed method could be applied successfully to the simultaneous determination of nitrite and nitrate in water samples.     

Spectrophotometric determination of nitrate and nitrite in natural water and sea-water

Nitrate and nitrite in natural waters are determined spectrophotometrically by passage through an amalgamated zinc reductor at pH 3.4 into iron(III)-Ferrozine solution. Interference by high levels of nitrite is eliminated by treatment with azide. Levels as low as 0.2 mug ml (expressed as nitrogen) can be determined with a precision of +/- 3%.     

Simultaneous spectrophotometric determination of nitrite and nitrate by flow-injection analysis

An automatic direct spectrophotometric method for the simultaneous determination of nitrite and nitrate by flow-injection analysis has been developed. Nitrite reacts with 3-nitroaniline in the presence of hydrochloric acid (0.96-1.8 M HCl or pH 0.5-0.7) to form a diazonium cation, which is subsequently coupled with N-(1-naphthyl)-ethylenediamine dihydrochloride to form a stable purple azo dye, the absorbance of which is measured at 535 nm. Nitrate is reduced on-line to nitrite in a copper-coated cadmium column which is then treated with azo dye reagent and the absorbance due to the sum of nitrite and nitrate is measured; nitrate is determined from the difference in absorbance values. A copper column incorporated into the reaction manifold before the copperised cadmium column not only improves the long-term accuracy, but also extends the life time of the copperised cadmium column. Various analytical parameters, such as effect of acidity (pH), flow rate, sample size, dispersion coefficient, time, temperature, reagent concentration and interfering species, were studied. The calibration graphs were rectilinear for 0.1-3.5 mug ml(-1) of NO(3) and 10 ng ml(-1)-2.2mug ml(-1) of NO(2). The method is successfully applied to some food samples (meat, flour and cheese), environmental waters (inland and surface), beer and soil samples. Up to 30 samples can be analysed per hour with a relative precision of approximately 0.1-2%.     

Photo-induced flow-injection determination of nitrate in water

A sensitive flow-injection method for the chemiluminescent determination of ultra-low concentration of nitrate in water is presented. Nitrate is on-line photolytically converted to peroxynitrite by absorption of UV light inside of 60 mm long quartz capillary (i.d. 530 µm, o.d. 720 µm). Peroxynitrite is subsequently determined by the chemiluminescent reaction with luminol. The detection limit of nitrate is 7 × 10^?10 M (S/N = 3). The linear range of the method is 2 × 10^?9–1 × 10^?5 M nitrate. The interference of nitrite is eliminated by its conversion to nitrogen after mixing of sample with a solution of sulfamidic acid. Other common anions do not interfere. The interference of cations is eliminated by passing the sample through a cation-exchange column. The FIA procedure allows analysing of 15 samples per hour. The method was applied to the determination of nitrate in various real water samples. The results are in good agreement with a reference ion chromatographic method.     

Determination of nitrate in natural waters by flow-injection analysis

Summary Nitrate was determined in natural water samples by flow-injection spectrophotometry. It was reduced to nitrite with copperized cadmium and the nitrite thus produced reacted with p-aminoacetophenone and m-phenylenediamine. The limit of detection was about 1.5 g l^–1 for sample injections of 650 l. The sampling rate was about 40 samples h^–1 and the relative standard deviation was above 1% for 0.1–0.3 mg l^–1 nitratenitrogen. Nitrite present in the sample was determined separately and subtracted.

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Nitratbestimmung in natürlichen Wässern mit Hilfe der Fließinjektions-Analyse

Zusammenfassung Nitrat wird bei dieser Methode mit Hilfe einer Cu/Cd-Reduktionssäule zu Nitrit reduziert, das mit p-Aminoacetophenon und m-Phenylendiamin zur Reaktion gebracht wird. Die gebildete Verbindung wird spektral-photometrisch gemessen. Die Nachweisgrenze beträgt etwa 1,5 g/l bei injizierten Probevolumina von 650 l. Der Probendurchsatz beträgt 40/h. Die relative Standardabweichung liegt über 1% bei 0,1–0,3 mg/l Nitrat-Stickstoff. Vorhandenes Nitrit wird gesondert bestimmt und abgezogen.

     

Automated simultaneous determination of nitrate and nitrite by pre-valve reduction of nitrate in a flow-injection system

Nitrate is reduced to nitrite by using the pre-valve in-valve reduction technique prior to the sampling system. One loop of a two-position sampling valve is replaced by a copperised cadmium column. Nitrite from the samples as well as nitrite formed in the reduction procedure is sampled by a second valve and introduced into the flow system. The two sampling valves are synchronised in such a way that two peaks are obtained, one corresponding to the nitrate plus nitrite and the other to the nitrite only. The method is suitable for the simultaneous determination of nitrate and nitrite at a sampling rate of up to 72 determinations per hour with coefficients of variation better than 1.96% for nitrate and 0.83% for nitrite.     

A green analytical procedure for flow-injection determination of nitrate in natural waters

Nitrate determination in waters is generally carried out with cadmium filings and carcinogenic reagents or by reaction with phenolic compounds in highly concentrated sulfuric acid medium. In this work, it was developed a green analytical procedure for nitrate determination in natural waters based on direct spectrophotometric measurements in ultraviolet, using a flow-injection system with an anion-exchange column for separation of nitrate from interfering species. The proposed method employs only one reagent (HClO₄) in a minimum amount (equivalent to 18 μL concentrated acid per determination), and allowed nitrate determination within 0.50-25.0 mg L⁻¹, without interference of up to 200.0 mg L⁻¹ humic acid; 1.0 mg L⁻¹ NO₂⁻; 200.0 mg L⁻¹ PO₄³⁻; 75.0 mg L⁻¹ Cl⁻; 50.0 mg L⁻¹ SO₄²⁻ and 15.0 mg L⁻¹ Fe³⁺. The detection limit (99.7% confidence level) and the coefficient of variation (n = 20) were estimated as 0.1 mg L⁻¹ and 0.7%, respectively. The results obtained for natural water samples were in agreement with those achieved by the reference method based on nitrate reduction with copperized cadmium at the 95% confidence level.     

Flow-injection analysis method for the determination of nitrite and nitrate in natural water samples using a chemiluminescence NOx monitor.

A flow-injection analysis method for the determination of nitrite and nitrate in natural water samples has been developed that consists of two systems based on their reduction to NO with hydrazine and/or ascorbic acid, followed by chemiluminescence detection. The procedure of sweeping the generated NO into an NOx monitor, by means of a gas-liquid separating coil consisting of microporous polytetrafluoroethylene (PTFE) tubing, offers practical advantages. The adjustment of the carrier gas-flow rates could yield the same calibration graphs for the two measurement systems, and the accumulation sweeping mode provides a higher sensitivity. Chemiluminescence detection allows a wide linear calibration range of 5 x 10(-8) to 5 x 10(-5) M. The detection limits for nitrate and nitrite, defined as three-times the standard deviation of measurement blanks, are 2 x 10(-8) M and 1 x 10(-8) M, respectively, and the average precision was 3.2% at ambient natural concentration levels. Recovery tests were between 94% and 106% for a variety of natural water samples. The method is relatively free from interferences from the substances normally found in natural water, and only ferric ion has an effect for the nitrite determination.     

Determination of nitrate and nitrite by continuous liquid-liquid extraction with a flow-injection atomic-absorption detection system

Summary A flow injection system coupled on-line with a continuous liquid-liquid extractor is used for the indirect determination of nitrate and nitrite with an atomic-absorption detection system. These anions form ion-pairs with the copper(I)-neocuproine chelate which are extracted into methyl-isobutyl-ketone, the atomic-absorption signal of copper from the organic phase being proportional to the nitrate or nitrite concentration. The methods proposed herein are suitable for determining nitrate or nitrite at the g ml^–1 level with a sampling frequency of 35 ± 5 h^–1. The methods compare favourably with their batch counterparts with regard to sensitivity, selectivity, sample volume and sampling frequency.

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Bestimmung von nitrat und nitrit durch kontinuierliche flüssig-flüssig-extraction mit detektion durch ein fließinjektions-AAS-system

Zusammenfassung Ein Fließinjektionssytem, das on-line mit einem Flüssig-Flüssig-Extraktor gekoppelt ist, wird zur indirekten AAS-Bestimmung von Nitrat und Nitrit benutzt. Diese Anionen bilden mit Kupfer(I)-neocuproinchelat Ionenpaare, die mit Methylisobutylketon extrahiert werden. Das AAS-Signal des Kupfers aus der organischen Phase ist der Nitrat- bzw. Nitritkonzentration proportinal. Die vorgeschlagenen Verfahren eignen sich zur Nitrat- oder Nitritbestimmung im g/ml-Bereich mit einer Probenfrequenz von 35 ± 5 je Stunde. Diese Methoden sind den Batch-Verfahren in bezug auf Empfindlichkeit, Selektivität sowie Probevolumen und -frequenz überlegen.

     

Determination of sulphate in natural water by flow-injection analysis

Summary Sulphate was determined in natural water samples by flow-injection analysis using dimethylsulphonazo-III as reagent. The interference by Ca was eliminated by a cation-exchanger column inserted directly after the sample injection valve. In order to ensure high sensitivity and reproducibility it was necessary to saturate the carrier solution with barium sulphate and to fill the reaction coil with ethanol-water (11) when not in use. Standard deviations were 0.94–1.2% for 6–10mg/l sulphate. The limit of detection was about 0.2mg/l. The calibration graph was linear up to 14mg/l Mg²⁺, NH ₄ ⁺ , Na⁺, K⁺, Cl^–, NO ₃ ^– , PO ₄ ^3– , HCO ₃ ^– and SiO ₃ ^2– did not interfere in the normally occurring concentrations.

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Sulfatbestimmung in natürlichem Wasser durch Flow-Injection-Analyse

Zusammenfassung Als Reagens dient Dimethylsulfonazo-III. Die Störung durch Calcium bei dieser Bestimmung wird durch eine Kationenaustauschersäule eliminiert, die direkt nach dem Probeinjektions-Ventil angeordnet ist. Zur Erhöhung der Empfindlichkeit und Reproduzierbarkeit ist es notwendig, die Trägerlösung mit Bariumsulfat zu sättigen sowie bei Nichtbenutzung des Systems den Reaktionsteil mit Ethanol-Wasser (11) zu füllen. Mit dem so modifizierten Verfahren ergaben sich Standardabweichungen von 0,94–1,2% für 6–10 mg/l Sulfat, die Nachweisgrenze liegt bei 0,2 mg/l. Die Eichkurve ist bis 14 mg/l linear. Mg²⁺, NH ₄ ⁺ , Na⁺, K⁺, Cl^–, NO ₃ ^– , PO ₄ ^3– , HCO ₃ ^– und SiO ₃ ^2– stören in den normalerweise vorkommenden Konzentrationen nicht.

     

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.     

Portable flow-injection analyzer with liquid-core waveguide based fluorescence, luminescence, and long path length absorbance detector

We describe a multifunctional flow analysis instrument that is portable ( cm, 2.3 kg) for facile field deployment. Using a 50 cm long Teflon^® AF tubing as final reaction and optical measurement conduit, we combine a liquid-core waveguide (LCW) based fluorescence detector that is transversely illuminated by an addressable light emitting diode array, a chemiluminescence (CL) detector and an absorbance detector with a solid-state broadband (400-700 nm) source. Several illustrative experiments have been carried out to test the performance of the instrument in different detection modes. A S/N=3 limit of detection (LOD) of 0.25 μg l⁻¹ for chromium(VI) was established using the diphenylcarbazide chemistry, and an LOD of 5 μg l⁻¹ was similarly established for Al(III), using Pyrocatechol Violet (PCV) as the chelating chromogenic dye, in both cases using long path absorption detection. The LOD for aqueous hydrogen peroxide was 16 nM using a fluorescence method based on the formation of thiochrome from thiamine and 4 nM by a luminol chemiluminescence method. With a Nafion membrane diffusion scrubber (DS), the LOD was 8.0 pptv for gaseous hydrogen peroxide by the fluorescence method.     

Design of a field flow system for the on-line spectrophotometric determination of phosphate, nitrite and nitrate in natural water and wastewater

This study describes the design and optimisation of a field flow system for the in-situ collection and on-line determination of phosphate, nitrate and nitrite by flow injection analysis-spectrophotometry. The method is based on the initial determination of phosphate as its phosphoantimonylmolybdenum blue complex which is then oxidized on-line by nitrite and the decrease in absorbance is monitored at 880 nm. Nitrate is determined as the difference between total and initial nitrite content in a separate flow after reduction to nitrite in a cadmium reductive column. The calibration curves were linear in the range 0–2.00 mg L⁻¹ P-phosphate, 0–10.00 mg L⁻¹ nitrite and 0–7.00 mg L⁻¹ nitrate with correlation coefficients of 0.9979, 0.9993 and 0.9995, respectively. The detection limits, calculated as 3S/N, were 0.15 mg L⁻¹ for P-phosphate, 0.17 mg L⁻¹ for nitrite and 0.09 mg L⁻¹ for nitrate. The reproducibility was below 3.0% (n = 7). Method validation in the analysis of natural water and wastewater samples revealed that it can efficiently be applied to the determination of the target analytes, with recoveries in the range of 92–108%. Correspondence: Athanasios G. Vlessidis, Laboratory of Analytical Chemistry, Department of Chemistry, University of Ioannina, Ioannina 45110, Greece     

Electrochemical detection of nitrate,nitrite and ammonium for on-site water quality monitoring

This review examines the most recent electrochemical developments for nitrate, nitrite and ammonium detection for on-site water monitoring. There remains a high demand for effective field-based detection of the dissolved inorganic nitrogen (DIN) analytes to aid in mitigating nitrogen loading. Electrochemical approaches show increasing potential to fill this role as advancements in nanotechnology continually improve analytical performance and on-site applicability. However, translating these improvements into the field still faces the resonating challenges of reaching analytical proficiency (selectivity, sensitivity, robustness, stability), practical end-user functionality, minimal matrix interferences and cost effectiveness. Herein, we elaborate on these challenges via a critical evaluation of current studies and examine how realistic the prospects of on-site nitrate, nitrite and ammonium are. We also present recommendations in addressing these gaps to conclude the review.     

Configuration with internally coupled valves to overcome shortcomings in the simultaneous determination of nitrite and nitrate by flow-injection analysis

A configuration with internally coupled valves and a reductant column located in the loop of the secondary valve is proposed for the simultaneous determination of nitrite and nitrate. Depending on the column characteristics, a washing stream flowing in the opposite direction to the sample may or may not be required. The washing step may conveniently be performed by use of the proposed configuration.     

Fluorometric determination of nitrite in natural waters with 3-aminonaphthalene-1,5-disulphonic acid by flow-injection analysis

Nitrite in river and sea-water was determined fluorometrically by flow-injection analysis. In acidic medium, nitrite was reacted with 3-aminonaphthalene-1,5-disulphonic acid (C-acid) to form the diazonium salt, which was converted into the fluorescent azoic acid salt in an alkaline medium. The carrier stream, into which the sample solution was injected, was distilled water. The reagent solution stream, which contained C-acid, EDTA and hydrochloric acid, was mixed with the carrier stream in a 13-m length of Teflon tubing (bore 0.5 mm) maintained at 90 degrees in a thermostatic bath. After passing through the mixing coil, the stream was mixed with an alkaline solution. The fluorescence intensity (excited at 365 nm) was measured at 470 nm. The detection limit (S N = 3) was 1 x 10(-9)M (14 ng 1 . nitrite-nitrogen) and the RSD of 10 injections of 10(-6)M nitrite was 0.4%. Analyses can be done at a rate of up to 45 hr .     

Determination of traces of nitrite and nitrate in water by solid phase spectrophotometry

A simple and sensitive method for the determination of nitrite and nitrate in water using solid phase spectrophotometry is described. The method utilizes the quantitative and rapid sorption of the dye formed from nitrite, using the Griess reaction, into a thin layer of polyurethane foam (PUF) where a preconcentration factor of >140 has been achieved. Nitrate is pre-reduced using a cadmium reductor before applying the Griess reaction. The direct spectrophotometric measurement of the dye enriched in the solid foam phase has allowed the detection of as little as 5 and 40 ng ml⁻¹ nitrite and nitrate, respectively. Optimization of the parameters affecting the quantitative formation and sorption of the dye into PUF has been considered. Analysis of natural water samples has been performed.     

Effect of surfactants on the determination of nitrite and nitrate in water samples

Summary The effect of four surfactants on the determination of nitrite and nitrate has been examined. The method which has been tested for nitrite is based on the formation of an azodye. The results show that cationic and non-ionic surfactants do not interfere with the determination of nitrite while anionic surfactants cause significant interferences, which could be eliminated by treating the water samples with a cationic surfactant.Two methods have been tested for the determination of nitrate in the presence of surfactants. One method is based on the nitration of salicyclic acid, while the other is based on the reduction of nitrate to nitrite. Results for the first method show that the non-ionic surfactant Triton-X causes significant interferences. Cationic and anionic surfactants do not interfere, when their concentration is relatively low. For higher concentrations an increasing interference is observed. Results for the second method show effects similar to those obtained for nitrite.

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Wirkung oberflächenaktiver Substanzen auf die Bestimmung von Nitrit und Nitrat in Wasserproben