Nitrate Sensor Research Articles

An accurate and stable nitrate-selective electrode for the in situ determination of nitrate in agricultural drainage waters.

A field evaluation of a novel nitrate-ion selective electrode (ISE) was undertaken by continuous immersion over a period of 5 months in agricultural drainage weirs. The nitrate sensor N,N,N-triallyl leucine betaine was covalently attached to polystyrene-block-polybutadiene-block-polystyrene (SBS) using a free radical initiated co-polymerisation, to produce a rubbery membrane which was incorporated into a commercially available electrode body. A measurement unit was constructed comprising the nitrate-ISEs, a reference electrode and a temperature probe connected through a pre-amplifier to a data-logger and battery supply. A temperature correction algorithm was developed to accomodate the temperature changes encountered in the drainage weirs. The nitrate results obtained with the ISEs at hourly intervals compared very favourably (R2 = 0.99) with those obtained with laboratory automated chemical determinations made on contemporaneous samples of drainage in a concentration range 0.47-16 ppm nitrate-N. The ISEs did not require re-calibration and no deterioration in performance or fouling of the membrane surface was observed over four months of deployment.

Theoretical and experimental studies of metallated phenanthroline derivatives as carriers for the optimization of the nitrate sensor

Experimental and theoretical investigations were carried out on a series of metallated phenanthroline derivatives in order to elucidate the role of both the metal center and the ligand employed, to fully optimize the analytical characteristics of the nitrate ISE. The organic ligands of the ionophores used are based on the phenanthroline moiety, metallated with metals in the 2+ oxidation state. A series of sensors were constructed based on these ionophores in combination with different plasticizers and ionic additives. The potentiometric data combined with the information obtained from ab initio theoretical calculations of the electron density and the electronegativity of the metals were used to optimize the overall analytical characteristics of the sensor towards the anion of nitrate. It is concluded that there is a direct coordination of the metal center with the nitrate and other basic anions, since both the electronegativity of the transition metal and the acidity of the phenanthroline derivative affect both the binding constant and the selectivity of the ionophore towards the nitrate anion. Finally, Ni(II) 5-phenyl-1,10-phenanthroline dissolved in the plasticizer tris(2-ethylhexyl) phosphate (TEHP) and adsorbed onto a porous carbon transducer produces a nitrate sensor with very good selectivity and exceptional signal stability.

FEASIBILITY OF DETECTING SOIL NITRATE CONTENT USING A MID–INFRARED TECHNIQUE

The long–range goal of this study was to develop a portable, soil nitrate sensor to determine soil nitrate content in–situ. The immediate objective of this study was to develop a rapid technique to determine soil nitrate content using diffuse reflectance spectroscopy in the mid–infrared (MIR) range. A Fourier Transform Infrared (FTIR) spectrophotometer was used to determine the MIR response of various concentrations of calcium nitrate solution in water. The results clearly showed the existence of a strong nitrate absorption peak at 7194 nm (1390 cm –1 ). For KBr–diluted soil samples, the ratio of the area under the nitrate absorbance peak (1360–1390 cm –1 ) to the water absorbance peak (1640–1660 cm –1 ) was proportional to nitrate concentration. Mid–infrared diffuse reflectance spectroscopy detected soil nitrate content at low concentrations in KBr–diluted soil samples. However, when undiluted soil samples containing varying amounts of nitrate content were tested in the MIR range, it was necessary to further condition the data with noise filtering techniques. Three noise removal techniques were compared to improve the spectral quality: (1) the Savitzky and Golay, (2) FFT filtering, and (3) wavelet denoising. The wavelet denoising technique resulted in the highest coefficient of determination (R 2 = 0.856) for nitrate prediction. These noise removal techniques could not remove specular reflection effects and band overlap. To overcome these problems, a continuous wavelet transform, which could remove specular reflection as well as minimize noise, was used to deconvolute the soil spectral data, resulting in a high correlation (R 2 = 0.878) between soil nitrate content and MIR diffuse spectral reflectance.

Redox enzymes in the plant plasma membrane and their possible roles

ABSTRACTPurified plasma membrane (PM) vesicles from higher plants contain redox proteins with low‐molecular‐mass prosthetic groups such as flavins (both FMN and FAD), hemes, metals (Cu, Fe and Mn), thiol groups and possibly naphthoquinone (vitamin K1), all of which are likely to participate in redox processes. A few enzymes have already been identified: Monodehydroascorbate reductase (EC 1.6.5.4) is firmly bound to the cytosolic surface of the PM where it might be involved in keeping both cytosolic and, together with a b‐type cytochrome, apoplastic ascorbate reduced. A malate dehydrogenase (EC 1.1.1.37) is localized on the inner side of the PM. Several NAD(P)H‐quinone oxidoreductases have been purified from the cytocolic surface of the PM, but their function is still unknown. Different forms of nitrate reductase (EC 1.6.6.1–3) are found attached to, as well as anchored in, the PM where they may act as a nitrate sensor and/or contribute to blue‐light perception, although both functions are speculative. Ferric‐chelate‐reducing enzymes (EC 1.6.99.13) are localized and partially characterized on the inner surface of the PM but they may participate only in the reduction of ferric‐chelates in the cytosol. Very recently a ferric‐chelate‐reducing enzyme containing binding sites for FAD, NADPH and hemes has been identified and suggested to be a trans‐PM protein. This enzyme is involved in the reduction of apoplastic iron prior to uptake of Fe2+ and is induced by iron deficiency. The presence of an NADPH oxidase, similar to the so‐called respiratory burst oxidase in mammals, is still an open question. An auxin‐stimulated and cyanide‐insensitive NADH oxidase (possibly a protein disulphide reductase) has been characterized but its identity is still awaiting independent confirmation. Finally, the only trans‐PM redox protein which has been partially purified from plant PM so far is a high‐potential and ascorbate‐reducible b‐type cytochrome. In co‐operation with vitamin K1 and an NAD(P)H‐quinone oxidoreductase, it may participate in trans‐PM electron transport.

Kinetics of nirS expression (cytochrome cd1 nitrite reductase) in Pseudomonas stutzeri during the transition from aerobic respiration to denitrification: evidence for a denitrification-specific nitrate- and nitrite-responsive regulatory system.

After shifting an oxygen-respiring culture of Pseudomonas stutzeri to nitrate or nitrite respiration, we directly monitored the expression of the nirS gene by mRNA analysis. nirS encodes the 62-kDa subunit of the homodimeric cytochrome cd1 nitrite reductase involved in denitrification. Information was sought about the requirements for gene activation, potential regulators of such activation, and signal transduction pathways triggered by the alternative respiratory substrates. We found that nirS, together with nirT and nirB (which encode tetra- and diheme cytochromes, respectively), is part of a 3.4-kb operon. In addition, we found a 2-kb monocistronic transcript. The half-life of each of these messages was approximately 13 min in denitrifying cells with a doubling time of around 2.5 h. When the culture was subjected to a low oxygen tension, we observed a transient expression of nirS lasting for about 30 min. The continued transcription of the nirS operon required the presence of nitrate or nitrite. This anaerobically manifested N-oxide response was maintained in nitrate sensor (NarX) and response regulator (NarL) knockout strains. Similar mRNA stability and transition kinetics were observed for the norCB operon, encoding the NO reductase complex, and the nosZ gene, encoding nitrous oxide reductase. Our results suggest that a nitrate- and nitrite-responsive regulatory circuit independent of NarXL is necessary for the activation of denitrification genes.

A submersible battery‐powered flowinjection (FI) sensor for the determinationof nitrate in estuarine and coastal waters

The design, construction and performance of a remotely deployed submersible flow injection-based nutrient (total oxidized nitrogen) sensor are described. The sensor featured a custom-built microcomputer and a solid-state, flow-through spectrophotometric detector, and the derivatization chemistry was based on in-line coppercadmium reduction of nitrate to nitrite, and diazotization with N1NED and sulphanilamide. The limit of detection was 0.0014 mg l-1 NO3-N and the linear range was 0.0014- 0.77 mg l-1 with a 260 μl sample volume and a 20 mm path length flow cell. Results from submersed deployments in the Tamar estuary and North Sea are also reported.

Development of a sensitive nitrate-selective electrode for on-site use in fresh waters

Twenty seven membranes hot pressed from mixtures of a variety of quaternary ammonium salt sensor molecules and mediators in Krynac polymer have been evaluated as nitrate-selective electrode membranes suitable for use in fresh waters. The best performance was obtained from a covalently bound triallyldecylammonium nitrate sensor having a Nernstian slope of –57.7 mV decade–1 over a nitrate range of 1 × 10–1 to 1.2 × 10–5 mol dm–3 NO3– with a limit of detection of 8.8 × 10–6 mol dm–3 NO3–.Membrane performance remained unchanged for at least six months use. Interfering ions, temperature and pH dependency studies are reported . Selectivity towards chloride at the 10–2 mol dm–3 level was measured as kpotNO3–,Cl– 1 × 10–4.

A low power ultra violet spectrophotometer for measurement of nitrate in seawater: introduction, calibration and initial sea trials

A low power UV spectroabsorptiometer has been developed to measure the concentration of dissolved nitrate in seawater, in situ, with a rapid 1 Hz response. Measurements at a number of wavelengths in the region of 220 nm determine the concentrations of dissolved nitrate, sea salt and, possibly, dissolved organics, whilst a third channel at 300 nm acts as a reference channel to correct for light intensity changes related to lamp output and scattering by particles in seawater. This paper describes the first prototype UV nitrate sensor designed to evaluate and compare options for the light source, spectral filter and detector. The choice of components was based on a compromise between performance, cost, power consumption and convenience of use. The eventual instrument uses a xenon flashlamp light source, fused silica windows and lenses, a sample cavity, a grating spectrometer and UV enhanced silicon photodiode detectors. The power consumption of the sensor was typically from 3 to 4 W, depending on the selected repetition rate of the flashlamp source. Initial laboratory trials have shown the sensor to be unresponsive to changes in pressure allowing for continuous use to depths of up to 5000 m. Initial responses to changes in temperature have been eliminated by the adjustment of the sensor’s power supply and lamp flash rate. Calibration of the sensor has been performed under laboratory conditions and resulted in a second order linear fit ( r 2>0.99, p<0.05) with respect to nitrate concentration. Calibrations performed to assess the response of the instrument to changes in salinity also resulted in second order linear fits ( r 2>0.99, p<0.05). Salinity calibrations indicated that a change of 1 psu resulted in a change in absorbance equivalent to 0.52 μmol l −1 NO 3. The detection limit of the sensor is a function of the signal/noise ratio. The minimum nitrate concentration change resolved by the sensor was found to be >0.21 μmol l −1 NO 3. Measurements made in situ were compared to concentrations of nitrate determined in water samples using an AAII type autoanalyser. These demonstrated the capability of the instrument to precisely determine nitrate concentrations in seawater.

Electrochemical Preparation and Characterization of an Anion-Permselective Composite Membrane for Sensor Technology

We describe the construction and characterization of an ultrathin-film composite membrane (UTFCM) prepared by electropolymerization of a thin coating of poly[1-methyl-3(pyrrole-1-ylmethyl) pyridinium] (poly-MPP), across the surface of a microporous support membrane. The ion-exchange properties of the UTFCM were investigated by testing the permeability of the membrane for electroactive anions and cations. Both polycarbonate and alumina microporous membranes were employed as support membranes. The alumina supports proved superior in that thinner anion-permselective poly-MPP coatings could be obtained. These thinner coatings allowed for fast permeation of anions while still blocking the passage of cations. Alumina–poly-MPP composite membranes were used to construct a nitrate sensor in which the UTFCM separates the analyte solution from an internal sensing solution which contained the enzyme nitrate reductase and an electrocatalyst (methyl viologen).

Experience with the upgrading of 14 treatment plants to N &amp; P removal in the municipality of Aarhus

The experience with the upgrading of 14 treatment plants to N and P removal is presented. For all plants it has been possible to fulfil the effluent standards. Through process optimization it has been possible to achieve a stable biological P-removal. The introduction of control systems based on nn-line sensors for ammonia, nitrate and phosphate has resulted in a more stable effluent quality and an increased capacity. The experience also shows that it can be very difficult to predict and control the changes in the industrial load. From this it is concluded that instead of spending a lot of time on fine-tuning the design criteria in the design phase, it may be better to incorporate a considerable flexibility into the design, allowing the processes to be adjusted to the changing operational conditions during time.

Biocatalyst electrode modified with whole-cells of P. denitrificans for the determination of nitrate

Intact bacterial cells of Paracoccus denitrificans were immobilized behind a dialysis membrane on the surface of a graphite electrode. Durohydroquinone (DQH 2) was used as an electron transfer mediator between the electrode and respiratory chain of the bacterial cell. The catalytic reduction current based on denitrification reductase activity observed with the electrode was proportional to the concentration of both nitrate and nitrite. In this report, characteristics of the electrode as a nitrate sensor are described. The coefficient of variation of the current measured at concentration of 5 μM nitrate was 3.8% ( n=10). The method has a detection limit of 1 μM nitrate and was applied to determination of nitrate in human urine. No interference of glucose, ascorbic acid, and acetone was observed. The response for nitrite of the electrode decreased in the presence of an inhibitor such as antimycin. When hexacyanoferrate (II), Fe(CN) 6 4−, was used as a mediator, the electrode exhibited nitrite selectivity.

A Fluorescence-Based Sensor for Ammonium and Nitrate

Measurement of nitrite and nitrate in rain samples was performed using a fluorescence based sensor. Nitrite and nitrate were reduced to ammonia using Devarda's alloy, and the gas was then passed through a membrane and reacted with o-phthalaldehyde. A linear relationship between the nitrate concentration and the luminescence intensity was observed over the concentration range of 1 – 5 mg/1, with a relative standard deviation (RSD) of 2.3% at a nitrate concentration of 1 mg/1 (n = 5). The system did not show any response toward sulfate or chloride. Results obtained from the measurement of river water samples using the sensor showed good agreement with those obtained using a conventional method.

Experience with the upgrading of 14 treatment plants to N &amp; P removal in the municipality of aarhus

The experience with the upgrading of 14 treatment plants to N and P removal is presented. For all plants it has been possible to fulfil the effluent standards. Through process optimization it has been possible to achieve a stable biological P-removal. The introduction of control systems based on nn-line sensors for ammonia, nitrate and phosphate has resulted in a more stable effluent quality and an increased capacity. The experience also shows that it can be very difficult to predict and control the changes in the industrial load. From this it is concluded that instead of spending a lot of time on fine-tuning the design criteria in the design phase, it may be better to incorporate a considerable flexibility into the design, allowing the processes to be adjusted to the changing operational conditions during time.

In Situ Measurement of the Rate of NO-3 Release from Controlled-release Fertilizers

Controlled-Released Fertilizer (CRF) has a great potential for applications in the nursery container industry. However, the specific mechanisms of the control are proprietary. The longevity claimed by manufacturers are unclear. The longevity of one CRF is claimed to be 2 to 3 months at 80 °F, resulting in a deviation of 30%. Thus, the actual release rate will have a 30% deviation from the claimed longevity. A preliminary study was conducted to test the longevity of two types of RCFs. 1.00 g (7.7% NO-3-N, fast release) and 1.30 g (5.9% NO-3-N slow release) of CRF was added to 500 ml distilled water in separate flasks and stirred continuously at a low speed during measurement period. A nitrate electrode and a reference electrode were set in the solution. The nitrate electrode responded to the increase in nitrate concentration caused by nitrate release from he CRFs. The response analog signal from the nitrate sensor was input to a 16-bit analog/digital converter with 1-minute interval for each measurement. The results indicated that 9% of the nitrate from the fast CRF (2- to 3-month longevity) was released in 10 hours. About 11.5% of the nitrate from the slow CRF (8- to 9-month longevity) was released in 260 hours. Based on the observed release rates, a 2- to 3-month longevity CRF will last about 111 hours in the stirred distilled water at room temperature. A CRF with 8 to 9 month longevity will last about 94.2 days. Even though field conditions are different from the experimental conditions, the real longevity of CRF in the fields may have to be further investigated. In the tropical southern Florida climate, the release rates of nutrients from CRFs are likely to be enhanced.

Nitrate ion‐selective sensor based on electrochemically prepared conducting polypyrrole films

AbstractNitrate‐doped polypyrrole (PPy) films on a glassy carbon substrate have been prepared electrochemically in aqueous, acetonitrile, and propylene carbonate solutions for use as nitrate sensors. Lithium nitrate, sodium nitrate, nitric acid, tetraethylammonium p‐toluene sulfonate (TS), and tetradodecylammonium nitrate (TDN) were employed as electrolytes. The effect of dibutylphthalate (DBP) as a plasticizer on the sensitivity and lifetime of PPy film sensors was also investigated. A Nernstian behavior with a slope of 56.9 m V/decade over 0.1–7.4 × 10−5 M NO and a detection limit of 4.7 × 10−5 M were observed for the polymer sensor prepared in acetonitrile solution containing lithium nitrate and 15% plasticizer (DBP). A lifetime of more than 6 months for this PPy film electrode was obtained.

Application of potential-sensitive fluorescent dyes in anion and cation-sensitive polymer membranes

The applicability of two potential-sensitive dyes (PSDs) for optical sensing of ions is reported. In particular, nitrate and nitrite-responsive as well as potassium and mercury-sensitive polymer membranes have been developed. In general, membranes are composed of a plasticized polymer, an ion carrier and a fluorescent dye which optically transduces the extraction of the analyte ion in the polymer matrix. The nitrate sensor membrane is composed of rhodamine B octadecylester and the anion-exchange catalyst is tridodecylmethylammonium chloride. Both are dissolved in plasticized PVC. The nitrite sensor is based on the same dye and the same polymer matrix but with benzylbis (triphenylphosphine) palladium(II) chloride acting as the nitrite-selective carrier. The potassium sensor membrane consists of the carbocyanine dye DiOC16(3), valinomycin and a lipophilic borate salt. The mercury ‘sensor’ is based on the irreversible decomposition of borate by mercury ions and is composed of DiOC16(3) and borate only. All sensor membranes have been investigated in terms of signal change, sensitivity, stability, limits of detection and the selectivity for the analyte over interferent ions. The mechanism of the sensor membranes is discussed on the basis of changes of the microenvironment of solvatochromic dyes, which result in analyte-dependent signal changes.

Effects of the polymer matrix on an optical nitrate sensor based on a polarity-sensitive dye

A membrane responsive to nitrate has been developed; it is composed of a polymer matrix, an anion exchanger (tridodecylmethyl-ammonium chloride), and a polarity-sensitive dye (rhodamine B octadecyl ester perchlorate). On exposure to nitrate, its fluorescence intensity increases, while the wavelength of the excitation and emission maxima remain unchanged. The effect of the polymer matrix on the relative signal change, sensitivity and selectivity has been investigated. It is shown that the sensitivity and selectivity for nitrate over other anions strongly depends on the lipophilicity and polarity of the polymer: increasing lipophilicity results in both higher sensitivity and selectivity. The limit of detection (LOD) in the case of lipophilic matrices is of the order of 0.1 ppm nitrate, and highest signal changes are observed between 1 and 100 ppm. Hydrophilic matrices have an LOD of 1 ppm and are most sensitive to nitrate at levels between 10 and 1000 ppm. The selectivity factor of lipophilic matrices for nitrate over chloride is 200, whereas it is around 10 only in hydrophilic matrices. The optical signal of the sensor material is pH dependent and this dependence again is affected by the lipophilicity of the matrix. in general, lower lipophilicity increases the cross-sensitivity of the nitrate sensor to pH. As a result, the response of the sensor may be fine-tuned by the appropriate choice of the polymer material.

Adaptive control of external carbon flow rate in an activated sludge process

In this study an adaptive carbon flow rate controller is applied to a pre-denitrifying pilot-scale plant. The goal of the control strategy is to maintain a low nitrate concentration in the anoxic zone by controlling the external flow rate of the carbon source. Results from practical experiments show that the control strategy works well and the nitrate concentration could be kept close to a desired set-point (reference value). The used nitrate sensor needed, however, frequent maintenance. During the experiment the carbon flow rate was automatically varied to keep the nitrate concentration at a constant low level. We conclude that a low effluent nitrate concentration without an excessive carbon flow rate is hard to achieve without an automatic control strategy.

A serial array of ISEs for use in a portable battery‐powered flow injection analyzer

AbstractPortable flow injection potentiometry (FIP) is described with a flow‐through multi‐ion sensor array system based on a planar design. Potassium, sodium and nitrate sensors are used as examples for simultaneous multi‐ion determinations based on the availability of a planar sensor design for these ions. The miniaturized flow cell developed with the three sensors in series in the FIP system is used to demonstrate the reliability of this portable design when using manual injection of the sample solution. The system is characterized by its small size, light weight and low power requirements, and makes use of a notebook computer for real‐time display of data and data acquisition. Rapid FIP response characteristics are shown after optimization of flow carrier conditions, and are successfully applied to the analysis of mineral water samples.

Adaptive control of external carbon flow rate in an activated sludge process

In this study an adaptive carbon flow rate controller is applied to a pre-denitrifying pilot-scale plant. The goal of the control strategy is to maintain a low nitrate concentration in the anoxic zone by controlling the external flow rate of the carbon source. Results from practical experiments show that the control strategy works well and the nitrate concentration could be kept close to a desired set-point (reference value). The used nitrate sensor needed, however, frequent maintenance. During the experiment the carbon flow rate was automatically varied to keep the nitrate concentration at a constant low level. We conclude that a low effluent nitrate concentration without an excessive carbon flow rate is hard to achieve without an automatic control strategy.