Mmol Dm Research Articles

Liquid Junction Potential Between Hydrophobic Ionic Liquid and Aqueous Electrolyte Solution

The liquid junction potential (LJP) across the two phases is very important for the basic science and real application of electrochemistry. In the case of the reference electrode the KCl salt-bridges are widely used, because the diffusion potential can be neglected in some cases by the almost same mobility of potassium ion and chloride ion in aqueous solution. However, if the difference of the concentration of solute between the two phases is great the LJP becomes several ten mV. And in the KCl salt-bridge KCl solution flow from reference electrode to the sample solution is essentially inevitable. To overcome these shortcomings of KCl salt bridge Kakiuchi et al. introduced the ionic liquid salt bridge (ILSB) concept and found that LJP for ILSB is always constant and independent on the concentration of electrolyte in aqueous phase. The LJP for ILSB is distribution of cation an anion of IL to aqueous solution.[1] In the present study we have measured the electric conductivity of lithium bis(pentafluoroehtanesulfonyl)amide (LiC2C2N) and bis(pentafluoroethanesulfonyl)amide acid(HC2C2N) in aqueous solution. C2C2N anion is the ionic liquid component of ILSB and the cation is tributyl(2-methoxyethyl)phosphonium (TBMOEP). Form the conductivity measurements we have found that the molar conductivity at infinite dilution of C2C2N anion is 28.1 ± 0.7 S cm2 mol-1 and mobility is 2.91 × 10-4 cm2 V-1 s-1. Sakaida et al. reported the solubility of TBMOEP cation in aqueous solution is 2.64 x 10-4 cm2 V-1 s-1.[2] From these results we can estimate the diffusion potential across TBMOEPC2C2N | HCl solution using the Henderson equation. Even in the case of 0.5 mmol dm-3 the magnitude of the diffusion potential is 60 microvolt and the we can conclude that the diffusion potential is negligible in the TBMOEPC2C2N ILSB. We are now investigating the coupled Nernst-Plank and Poisson-Boltzmann equation to see the detail of the potential and the concentration profile. From the conductivity measurements we also found that HC2C2N is strong acid and C2C2N anion in aqueous phase is completely dissociated.

Functionalization of Cellulose with Cysteamine: Synthesis, Characterization, and Adsorption

The cellulose matrix was chemically modified with cysteamine (CelCTA) and characterized by elemental analysis, and subsequently applied to the removal of copper ion in an aqueous media. The experimental adsorption isotherm obtained for the studied system was adjusted to linear models: Langmuir, Freundlich, and Temkin. Among the proposed models, the Langmuir model was the one with the best linear adjustment to data obtainedexperimentally, with an adsorption capacity of the adsorbent for 0.3292 mmol dm-3.

Treatment by glyphosate-based herbicide alters life history parameters of the rose-grain aphid Metopolophium dirhodum.

Glyphosate is the number one herbicide in the world. We investigated the sub-lethal effects of this herbicide on the aphid Metopolophium dirhodum (Walker), using an age-stage, two-sex life table approach. Three concentrations of the herbicide (low - 33.5, medium - 66.9 and high - 133.8 mmol dm−3 of active ingredient) and distilled water as the control were used. The LC50 of the IPA salt of glyphosate on M. dirhodum was equivalent to 174.9 mmol dm−3 of the active ingredient (CI95: 153.0, 199.0). The population parameters were significantly negatively affected by herbicide application, and this negative effect was progressive with the increasing concentration of the herbicide. A difference of two orders of magnitude existed in the predicted population development of M. dirhodum between the high concentration of the herbicide and the control. This is the first study that comprehensively documents such a negative effect on the population of an herbivorous insect.

Synthesis by ligand exchange and electrochemistry of ruthenocenyl-containing β-diketonato complexes of titanocene. Structure of [TiCp2(RcCOCHCOCH3)][ClO4

A new synthetic route involving ligand exchange towards a series of ruthenocenyl-containing β-diketonato titanocene complexes of the type [TiIV(C5H5)2(RcCOCHCOR)][ClO4] with Rc = RuII(C5H5)(C5H4) and R = C10F21 (1), CF3 (2), C6F5 (3), C10H21 (4), CH3 (5), Rc (6) and Fc = FeII(C5H5)(C5H4) (7) are described. The complex [TiIV(C5H5)2(RcCOCHCOCH3)][ClO4], 5, (Z = 4) crystallised in the monoclinic space group P21/n. A cyclic voltammetric electrochemical study in CH2Cl2/0.1 M [NBu4][B(C6F5)4] showed the one-electron transfer redox processes of the TiIII/TiIV and Rc/Rc+ couples exhibited irreversible electrochemical behaviour. In CH3CN/0.1 M [N(nBu)4][PF6], the ruthenocenyl couple switched to an electrochemically and chemically irreversible two-electron transfer redox couple, Rc/Rc2+, involving the RuIV redox centre. The redox potentials of the TiIII/TiIV, Rc/Rc+ and Rc/Rc2+ couples were in the range −642 < Eo′ < −1479 mV (both solvents), 1016 mV < Eo′ < 1034 mV (CH2Cl2) and 609 mV < Eo′ < 832 mV (CH3CN) vs. the free ferrocene couple, FcH/FcH+, respectively. The Fc/Fc+ redox process associated with 7 was observed at Eo′ = 0.259 (CH3CN) and 0.235 V (CH2Cl2). The cytotoxicity of [TiIV(C5H5)2(RcCOCHCOCF3)][ClO4], 2, against CoLo DM320 and HeLa cells, IC50 = 10.2 and 9.7 μmol dm−3 respectively, was found to be in the same order of magnitude as those of the free ligand, RcCOCH2COCF3.

The effect of petroleum jelly, light-cured varnish and different storage media on the flexural strength of glass ionomer dental cements

This study determined the influence of coating with either petroleum jelly or light-cured varnish and storage medium on the flexural strength of glass-ionomer cements (GIC). The flexural strength of two glass-ionomer cements (Fuji Equia Fil and Ketac Molar Aplicap) was measured. Specimens (2 × 2 × 25 mm) were prepared in three groups: uncoated, coated with petroleum jelly, or coated with light-cured varnish (EquiaCoat) cured for 20 s using a cure lamp (Bluephase G2, Ivoclar Vivadent, Schaan, Liechtenstein). Specimens were stored for 1 week at 37 °C in water, artificial saliva or 20 mmol dm−3 lactic acid, then flexural strength was determined in 3-pont bend. Data were analyzed by ANOVA and Tukey HSD post hoc test (p < 0.05). In addition, the mold was filled with water and the temperature change caused by the cure lamp was measured with a thermocouple. For both materials, storage in water gave the lowest flexural strength. It was slightly higher in either saliva or lactic acid, and was improved by coating in petroleum jelly. Specimens coated with light-cured varnish, that also involved heating with a cure lamp, gave the highest flexural strength. The heating effect of the lamp was demonstrated by the temperature rise in the water in the mold after light exposure from 21.9 (± 1.0) °C to 26.8 (± 1.0) °C. hence, sealing of GIC from aqueous media improves flexural strength. The cure lamp emitted heat, which may enhance the flexural strength of specimens coated with light-cured varnish.

An investigation of tebuconazole degradation using a gold electrode

Tebuconazole, a pesticide that presents risks for ecosystems, groundwater and human health, was degraded on a gold electrode as the catalyst in 0.05 M NaHCO 3 using cyclic voltammetry (CV). First, tebuconazole was characterized on the gold electrode using CV and square wave voltammetry (SWV). The gold electrode, being highly sensitive, provided linear relationships (currents vs . concentrations) in the range: (0.076–0.76 μmol dm –3 ). Based on the CV measurements, the investigated process is irreversible and diffusion-controlled. The observed catalytic role of the Au electrode in the oxidation of tebuconazole and the data obtained from CV and SWV provided the experimental conditions for the degradation of tebuconazole. The degradation was performed by continuous cycling followed by gas chromatography–mass spectroscopy (GC–MS) analysis. This enabled the catalytic elimination of tebuconazole for 60 min, which promoted the use of a gold electrode as the catalyst for the degradation of environmental pollutants. The scheme of the possible mechanism of tebuconazole degradation is given.

Drivers of summer oxygen depletion in the central North Sea

Abstract. In stratified shelf seas, oxygen depletion beneath the thermocline is a result of a greater rate of biological oxygen demand than the rate of supply of oxygenated water. Suitably equipped gliders are uniquely placed to observe both the supply through the thermocline and the consumption of oxygen in the bottom layers. A Seaglider was deployed in the shallow (≈ 100 m) stratified North Sea in a region of known low oxygen during August 2011 to investigate the processes regulating supply and consumption of dissolved oxygen below the pycnocline. The first deployment of such a device in this area, it provided extremely high-resolution observations, 316 profiles (every 16 min, vertical resolution of 1 m) of conductivity, temperature, and depth (CTD), dissolved oxygen concentrations, backscatter, and fluorescence during a 3-day deployment.The high temporal resolution observations revealed occasional small-scale events (&lt; 200 m or 6 h) that supply oxygenated water to the bottom layer at a rate of 2 ± 1 µmol dm−3 day−1. Benthic and pelagic oxygen sinks, quantified through glider observations and past studies, indicate more gradual background consumption rates of 2.5 ± 1 µmol dm−3 day−1. This budget revealed that the balance of oxygen supply and demand is in agreement with previous studies of the North Sea. However, the glider data show a net oxygen consumption rate of 2.8 ± 0.3 µmol dm−3 day−1, indicating a localized or short-lived (&lt; 200 m or 6 h) increase in oxygen consumption rates. This high rate of oxygen consumption is indicative of an unidentified oxygen sink. We propose that this elevated oxygen consumption is linked to localized depocentres and rapid remineralization of resuspended organic matter.The glider proved to be an excellent tool for monitoring shelf sea processes despite challenges to glider flight posed by high tidal velocities, shallow bathymetry, and very strong density gradients. The direct observation of these processes allows more up to date rates to be used in the development of ecosystem models.

Understanding the bioavailability and sequestration of different metal cations in the presence of a biodegradable chelant S,S-EDDS in biological fluids and natural waters

Ethylenediamine-N,N'-disuccinic acid is a biodegradable alternative to EDTA, therefore its use for the sequestration of Ca2+, Sn2+, Cu2+, Zn2+ and Fe3+ is analyzed. New data on its binding ability towards these cations were obtained with potentiometric, voltammetric and calorimetric measurements at different ionic strengths and at T = 298.15 K. Real multi-component fluids, namely fresh water, urine, sea water, saliva and blood plasma were chosen as case studies to evaluate the sequestering ability of EDDS in comparison with EDTA. Speciation diagrams were drawn in selected conditions, considering all interactions among the “natural” components of the fluid and those studied in this work, EDDS and EDTA (cL = 1 mmol dm−3) as sequestering agents and the cited metal cations (cM ∼ 10−5 mol dm−3). The comparison of the sequestering ability of EDDS and EDTA is done using pM and pL0.5. In blood plasma the plasma mobilizing index was adopted. It was found that EDDS is a good alternative to EDTA, which tends to bind Ca2+ and Mg2+ more than EDDS. In particular, EDTA cannot be used as a sequestrant for Sn2+ when cCa > cEDTA. EDDS is more efficient than EDTA at pH < 8, particularly in urine, where carbonate is absent. In sea water, the sequestering ability of EDDS towards Fe3+ is higher than that of EDTA. In blood plasma, the PMI of EDDS towards Cu2+ is higher than that of EDTA. Thermodynamic information, in terms of ΔH and ΔS, for the protonation and metal complex formation reactions are reported.

Switching central trinucleotide sequences of DNA heptamer regulates adsorption on mercury electrode

The article aims to present an electrochemical investigation of DNA heptamers d(GCNNNGC) with a different central trinucleotide sequence (NNN = GAA, AAA, CCC, and GGG, where A, C, and G is adenine, cytosine, and guanine, respectively). In phosphate-acetate buffer at pH 5.8 all heptamers were adsorbed on a hanging mercury electrode, and these adsorbed heptamers underwent reduction and oxidation. The voltammetric reduction peaks of A and C moiety and oxidation peaks of G moiety at different concentrations (0.25–10 µmol dm−3) were analyzed. It was found that the changes in the central trinucleotide sequence of the heptamers result in altered adsorption ability at a charged phase interface. An electron transfer process in an adsorptive state was evidenced by the elimination voltammetric procedure. For the evaluation of adsorption efficiency of the heptamers the Langmuir adsorption model was applied and the adsorption coefficient b was calculated.

Voltammetry of Ion Transfer at a Water–Toluene Micro‐ Interface

AbstractVoltammograms of ion transfer at the water (W)|toluene interface were recorded for the first time by using a micro‐interface electrolytic cell and bis(triphenylphosphoranylidene)ammonium tetrakis[3,5‐bis(trifluoromethyl)phenyl]borate (BTPPA+TFPB−). The high solubility of BTPPA+TFPB− in toluene at room temperature enabled the electrochemical measurements. The potential window at the W|toluene interface is about 1.09 V, which is 0.30 V larger than that at the W|1,2‐dichloroethane interface when using 10 mmol dm−3 NaCl as the supporting electrolyte in W. The transfer reactions of quaternary ammonium ions across the W|toluene interface were found to be reversible. Formal potentials for ion transfer from W to toluene were evaluated, and linear correlations were observed between these formal potentials and the standard potentials reported at the W|nitrobenzene, W|1,2‐dichloroethane, and W|2‐nitrophenyl octyl ether interfaces.

Discovery of a Bacterial Glycoside Hydrolase Family 3 (GH3) β-Glucosidase with Myrosinase Activity from a Citrobacter Strain Isolated from Soil.

A Citrobacter strain (WYE1) was isolated from a UK soil by enrichment using the glucosinolate sinigrin as sole carbon source. The enzyme myrosinase was purified using a combination of ion exchange and gel filtration to give a pure protein of approximately 66 kDa. The N-terminal amino acid and internal peptide sequence of the purified protein were determined and used to identify the gene, which, based on InterPro sequence analysis, belongs to the family GH3, contains a signal peptide, and is a periplasmic protein with a predicted molecular mass of 71.8 kDa. A preliminary characterization was carried out using protein extracts from cell-free preparations. The apparent KM and Vmax were 0.46 mM and 4.91 mmol dm(-3) min(-1) mg(-1), respectively, with sinigrin as substrate. The optimum temperature and pH for enzyme activity were 25 °C and 6.0, respectively. The enzyme was marginally activated with ascorbate by a factor of 1.67.

A Flow Method for Chemiluminescence Determination of Antimony(III) and Antimony(V) Using a Rhodamine B-Cetyltrimethylammonium Chloride Reversed Micelle System Following On-Line Extraction

A rapid and sensitive flow method, based on the combination of on-line solvent extraction with reversed micellar mediated chemiluminescence (CL) detection using rhodamine B (RB), was developed for the determination of antimony(III) and antimony(V) in aqueous samples. The on-line extraction procedure involved ion-pair formation of the antimony(V) chloro-complex anion with the protonated RBH(+) ion and its extraction from an aqueous hydrochloric acid solution into toluene, followed by phase separation using a microporous membrane. When in a flow cell of a detector, the ion-pair in the extract driven was mixed with the reversed micellar solution of cetyltrimethylammonium chloride in 1-hexanol-cyclohexane/water (0.60 mol dm(-3) H2SO4) containing cerium(IV), its uptake by the reversed micelles and the subsequent CL oxidation of RB with Ce(IV) occurred easily, then the produced CL signal was measured. Using the proposed flow method under the optimized experimental conditions, a detection limit (DL) of 0.35 μmol dm(-3) and a linear calibration graph with a dynamic range from DL to 16 μmol dm(-3) were obtained for Sb(V) with a precision of 1.4% relative standard deviation (n = 5) at the Sb(V) concentration of 8.2 μmol dm(-3). The present method was successfully applied to the determination of Sb(V) in water samples and to the differential determination of Sb(III) and Sb(V) in copper electrolyte industrial samples, where total antimony Sb(III) + Sb(V) was determined after oxidation of Sb(III) to Sb(V) with Ce(IV) and Sb(III) was calculated by difference, for which the DL was almost the same as that for Sb(V).

High performance aliphatic-heterocyclic benzyl-quaternary ammonium radiation-grafted anion-exchange membranes

High performance benzylpyrrolidinium anion-exchange membrane.

Alkali Metal Ion Complexes with Phosphates, Nucleotides, Amino Acids, and Related Ligands of Biological Relevance. Their Properties in Solution.

Alkali metal ions play very important roles in all biological systems, some of them are essential for life. Their concentration depends on several physiological factors and is very variable. For example, sodium concentrations in human fluids vary from quite low (e.g., 8.2 mmol dm(-3) in mature maternal milk) to high values (0.14 mol dm(-3) in blood plasma). While many data on the concentration of Na(+) and K(+) in various fluids are available, the information on other alkali metal cations is scarce. Since many vital functions depend on the network of interactions occurring in various biofluids, this chapter reviews their complex formation with phosphates, nucleotides, amino acids, and related ligands of biological relevance. Literature data on this topic are quite rare if compared to other cations. Generally, the stability of alkali metal ion complexes of organic and inorganic ligands is rather low (usually log K < 2) and depends on the charge of the ligand, owing to the ionic nature of the interactions. At the same time, the size of the cation is an important factor that influences the stability: very often, but not always (e.g., for sulfate), it follows the trend Li(+) > Na(+) > K(+) > Rb(+) > Cs(+). For example, for citrate it is: log K ML = 0.88, 0.80, 0.48, 0.38, and 0.13 at 25 °C and infinite dilution. Some considerations are made on the main aspects related to the difficulties in the determination of weak complexes. The importance of the alkali metal ion complexes was also studied in the light of modelling natural fluids and in the use of these cations as probes for different processes. Some empirical relationships are proposed for the dependence of the stability constants of Na(+) complexes on the ligand charge, as well as for correlations among log K values of NaL, KL or LiL species (L = generic ligand).

Electrochemical properties of α-tocopherol in aqueous electrolytes after its previous extraction into the glassy carbon paste from aqueous-acetonic mixture

Study on electrochemical behavior of lipophilic α-tocopherol, which is recognized as the most active form of vitamin E, was performed using extraction voltammetry in solely aqueous supporting electrolytes. In the first step, α-tocopherol was extracted into the silicone oil, acting as a binder of glassy carbon paste electrode, from aqueous-organic mixture. Generally, the extraction into the electrode material was carried out from solution containing 50 µmol dm−3 α-tocopherol and 60 % acetone for period of 240 s and corresponding stirring rate of 400 rpm. Electrochemical study was undertaken subsequently by cyclic voltammetry after placing the electrode in aqueous supporting electrolyte. Due to the accumulation effect of extraction, voltammetric records exhibited intensive current signals in acidic media when compared to those obtained at solid glassy carbon electrode. Investigations revealed a promising application of carbon paste electrodes for sensitive analysis of α-tocopherol or other electroactive lipophilic species after their extractive preconcentration.

Differential pulse voltammetric determination of 4-nitroaniline using a glassy carbon electrode: comparative study between cathodic and anodic quantification

Differential pulse voltammetry with a glassy carbon electrode was used for the determination of trace amounts of 4-nitroaniline in Britton-Robinson buffer at pH 2.0 as an optimal base electrolyte. 4-Nitroaniline can be determined by differential pulse voltammetry directly using either its electrochemical oxidation or reduction. The calibration curve was measured for both reactions in the same concentration range from 2 to 100 μmol dm−3. The calculated limits of detection and quantification are 0.2 and 0.7 μmol dm−3 (RSD 6.62 % and R 2 = 0.9906 for n = 5) for cathodic reduction and 0.3 and 1.0 μmol dm−3 (RSD 7.62 % and R 2 = 0.9995 for n = 5) for anodic oxidation.

Polarographic and voltammetric determination of genotoxic 4-nitroindane at mercury electrodes

Determination and electrochemical investigation of genotoxic nitrated polycyclic aromatic hydrocarbon 4-nitroindane (4-NI) were carried out using direct current tast polarography (DCTP) and differential pulse polarography (DPP) at a classical dropping mercury electrode (DME) and using direct current voltammetry (DCV), differential pulse voltammetry (DPV), and cyclic voltammetry (CV) at a hanging mercury drop minielectrode (HMDmE). The following optimal media for the determination of 4-NI were found: methanol—Britton-Robinson (BR) buffer of pH 12.0 (1:1) for DCTP and DPP, methanol—BR buffer of pH 9.0 (1:9) for DCV, and methanol—BR buffer of pH 8.0 (1:9) for DPV. For CV investigation, mixtures of methanol—BR buffer of pH 3.0, 8.0, and 12.0 (1:1) were used. The limits of quantification were: 0.65 µmol dm−3 (DCTP at the DME), 0.10 µmol dm−3 (DPP at the DME), 0.075 µmol dm−3 (DCV at the HMDmE), and 0.070 µmol dm−3 (DPV at the HMDmE). An attempt to increase the determination sensitivity using adsorptive accumulation of 4-NI on the HMDmE surface was not successful. The newly developed polarographic and voltammetric methods were successfully applied for the determination of 4-NI in model samples of drinking and river water.

Voltammetric determination of homovanillic acid and vanillylmandelic acid on a disposable electrochemical measuring cell system with integrated carbon composite film electrodes

An array of carbon composite electrodes embedded in a 96-well microtitration plate was applied to differential pulse voltammetry determination of standards of tumor biomarkers 2-(4-hydroxy-3-methoxy-phenyl) acetic acid (homovanillic acid, HVA) and (RS)-hydroxy(4-hydroxy-3-methoxy-phenyl) acetic acid (vanillylmandelic acid, VMA). For the preparation of composite electrodes, graphitic conductive microparticles and nonconductive polystyrene binder were used. For the measurement, a buffer of pH 2 was selected as the optimum medium for both analytes. In this medium, concentration dependences of stand-alone standards were measured; calculated quantification limits were 0.3 µmol dm−3 for HVA and 0.8 µmol dm−3 for VMA. Sorption of analytes on employed working electrode was examined in order to verify, whether the current is influenced by the time period between cell filling and measurement. The sorption was not observed and the current was stable even after 10 min. Developed method was verified by the determination of a mixture of standards of both analytes. Mixtures of analytes of various proportions were measured and the obtained calibration curves showed good agreement with standard values. Furthermore, the results showed that the behavior of the mixture of analytes is not different from the behavior of the actual standards, so that it is possible to calculate the concentration of both analytes side by side.

Microbial processes and genesis of methane gas jets in the coastal areas of the Crimean Peninsula

Hydroasoustic techniques were used for detection and mapping of gas jet areas in the coastal regions of the Crimean peninsula. Gas seep areas in the bays Laspi, Khersones, and Kazach'ya were chosen for detailed microbiological investigation. The first type of gas jets, observed in the Laspi Bay, was probably associated with discarge of deep thermogenic methane along the faults. Methane isotopic composition was char- acterized by Δ13C of -35.3 degrees. While elevated rates of aerobic methane oxidation were revealed in the sandy sediments adjacent to the methane release site, no evidence of bacterial mats was found. The second type of gas emission, observed in the Khersones Bay, was accompanied by formation of bacterial biofilms of the "Thiodendron" microbial community type, predominated by filamentous, spirochete-like organisms, in the areas of gas seepage. The isotopic composition of methane was there considerably lower (-60.4 degrees), indicating a considerable contribution of modern microbial methane to the gas bubbles discharged in this bay. Activity of the third type of gas emission, the seeps of the Kazach'ya Bay, probably depended directly on modern microbial processes of organic matter degradation in the upper sediment layers. The rates of sulfate reduction and methanogenesis were 260 and 34 μmol dm(-3) day(-1), respectively. Our results indicate different mechanisms responsible for formation of methane jets in the Laspi Bay and in the coastal areas of the Heracles Peninsula, where the bays Kazach'ya and Khersones are located.

Electrochemical study of 5-nitroquinoline using carbon film electrode and its determination in model samples of drinking and river water

This work is focused on the determination of submicromolar concentrations of 5-nitroquinoline (5-NQ) using differential pulse voltammetry (DPV) and direct current voltammetry (DCV) at a novel type of carbon film electrode (CFE). The advantages of CFE are its wide potential window in both cathodic and anodic regions (ca 3 V span), high sensitivity, and low noise of measurements. The other advantages of CFE are the possibility to quickly and easily renew the electrode surface and also non-toxicity for environment compared to mercury electrodes. Calibration dependences in deionized water are linear from 0.4 to 100 µmol dm−3, with limit of quantification (LOQ) 1 µmol dm−3 using DCV, and from 0.2 to 100 µmol dm−3, with LOQ 0.4 µmol dm−3 using DPV. The DPV method was verified for model samples of drinking and river water. The calibration dependences were linear in the concentration ranges from 0.2 to 10 µmol dm−3 for both matrices, with LOQ 0.2 µmol dm−3 for drinking water and LOQ 0.6 µmol dm−3 for river water, respectively. This work has proved the usability of CFE for submicromolar determination of 5-NQ based on cathodic reduction of the present nitro group.