Hydroxylamine In Aqueous Solution Research Articles

Dual Electrode Detection in LC‐EC Analysis of Sanger‐tagged Amino Acids: Electrochemical Reduction of Aromatic Nitro Groups

AbstractAnalysis of amino acids is crucial to protein structure elucidation. Amino acids, amenable to separation by liquid chromatography (LC), can be detected by sensitive detectors used in liquid chromatograph including absorbance, fluorescence or electrochemical detector when derivatized. Derivatization of amino acids using suitable reagents enhances the separation and detection of amino acids using liquid chromatography. Sanger's reagent (1‐fluoro‐2,4‐dinitrobenzene, DNFB) makes amino acids suitable for absorbance detection. However, little has been done in terms of liquid chromatography with electrochemical detection (LC‐EC) of the DNFB derivatized amino acids. Furthermore, sensor development based on electrochemistry of nitroaromatics has increased dramatically for explosive detection. Since nitroaromatics are electrochemically active, it is essential to determine the reduction pathway. Cyclic voltammetry (CV), rotating disk electrode (RDE) and rotating ring disk electrode (RRDE) experiments have been performed on nitrobenzene (NB) and N‐methyl‐2,4‐dinitroaniline (NMDNA). The results showed that the nitro group was reduced to hydroxylamine in aqueous solutions by addition of four electrons and four protons per nitro group and the hydroxylamine can be reversibly oxidized into a nitroso by removal of two electrons and two protons. Electrochemical detection of the nitro groups is appropriate for dual electrode detection of DNFB labeled amino acids in which reduction can occur on an upstream electrode and corresponding oxidation of the reduction product occurs on the downstream electrode.

Conversion Mechanisms of Nitroxyl Radical (TEMPO), Oxoammonium Cation, and Hydroxylamine in Aqueous Solutions: Two-Dimensional Correlation Ultraviolet-Visible Spectroscopy.

Oxidation reactions of alcohols have been of interest due to their broad applications in different fields. Oxoammonium cation (TEMPO+) of 2,2,6,6-tetramethyl piperidine-1-oxyl (TEMPO) is a high-potential oxidant for the selective oxidation of primary alcohols, with hydroxylamine (TEMPOH) as a side product. TEMPO or TEMPO+ has been widely applied for various reactions. However, the conversion mechanisms among TEMPO, TEMPO+, and TEMPOH are not well understood and remain controversial, due to complications in the direct observation of the reactions. In this work, two-dimensional correlation (2D-COS) UV-visible (UV-Vis) spectroscopy is applied to examine the correlations between the characteristic bands of each species, to obtain insights into the complete reaction mechanisms. Series of dynamic UV-Vis spectra of solutions under different external perturbations (as a function of reaction time) were recorded and used in the generation of 2D-COS synchronous and asynchronous maps. The key UV-Vis band assignments are as follows: 250 nm and 400 nm for TEMPO, 290 nm and 480 nm for TEMPO+, and 200 nm and 315 nm for TEMPOH. The results indicate that the conversion between TEMPO and TEMPOH in acidic solution is a reversible process, which reaches an equilibrium state after two hours. However, the reaction becomes irreversible after three hours, due to a higher degree of irreversible protonation of TEMPOH to form TEMPOH-H+. Fast conversion from TEMPO to TEMPO+ is observed when sodium hypochlorite co-oxidant is added. The synproportionation-disproportionation also reaches an equilibrium. However, there is no evidence of the conversion from TEMPOH to TEMPO+ under the reaction conditions. At high reaction temperature, the formation of TEMPOH occurs first from TEMPO+ decomposition, followed by TEMPO decomposition. These detailed mechanisms are beneficial in designing the optimum process conditions for the oxidation of specific alcohols.

Thermal decomposition of hydroxylamine in aqueous solutions in the presence of NaCl, KCl or Na2SO4 in the temperature range 120 °C–140 °C

The thermal decomposition of hydroxylamine aqueous solutions in two different concentrations and three temperatures was studied in the presence of NaCl, KCl and Na2SO4 using isoperibolic calorimetry. The purpose of this study was to generate data for comparison of the thermal decomposition of different nitrogen containing compounds towards the ultimate, long-term goal to reveal the decomposition pathways of nitrogen containing species and to pinpoint the factors affecting these paths. It was found that Na2SO4 was substantially reducing hydroxylamine decomposition rate, while the other two inorganic salts had a non-measurable effect on hydroxylamine decomposition rate. When NaCl and Na2SO4 were used together the effect of the latter predominated. In all cases the non-condensable gas generation was not practically affected by the presence of any of the additives. The results were compared with the effect that KCl and Na2SO4 had on ammonium nitrate thermal-decomposition rate and similarities and differences are discussed. It is believed that rate deceleration effects of Na2SO4 are owing to the alkaline character of SO42− anion.

Determination of hydroxylamine using a carbon paste electrode modified with graphene oxide nano sheets

A carbon paste electrode that was chemically modified with 3-(4'-amino-3'-hydroxy-biphenyl-4-yl)-acrylic acid (3,4-AA) was used as a selective electrochemical sensor for the detection of hydroxylamine. Cyclic voltammetry (CV), choronoamperometry (CHA) and square wave voltammetry (SWV) were used to investigate oxidation of hydroxylamine in aqueous solution. Under optimized concentration the electrocatalytic oxidation current peak for hydroxylamine increased linearly with concentration in the range of 0.025–10.0 μM. The detection limits for hydroxylamine was 0.012 μM. Finally, the modified electrode was applied to detection hydroxylamine in water samples.

A sensitive electrochemical sensor for hydroxylamine determination: Using multiwall carbon nanotube paste electrode (MWCNTPE) and promazine hydrochloride as homogenous mediator

A sensitive electrochemical sensor for determination of hydroxylamine has been introduced. This sensor profits by promazine hydrochloride (PHC) as homogenous mediator and multiwall carbon nanotubes to improve the surface of carbon paste electrode as working electrode. Cyclic voltammetry (CV), linear sweep voltammetry (LSV), chronoamperometry, and differential pulse voltammetry (DPV) are used to study of the suitability of promazine hydrochloride, at the surface of multiwall carbon nanotube paste electrode (MWCNTPE), as a homogenous mediator for the electrocatalytic oxidation of hydroxylamine in aqueous solutions. Hydroxylamine hardly oxidizes at about +1.1V at the carbon paste electrode (CPE), whereas catalytically oxidizes in presence of PHC as mediator at the MWCNTPE. The electrocatalytic peak current of hydroxylamine oxidation shows a linear dependence on hydroxylamine concentration in the range of 0.008–0.100μM, by DPV in optimized conditions at pH 9.0. In addition, using LSV and in same conditions calibration plot was obtained at hydroxylamine concentration in the range of 0.17–10.0mM. The detection limit (3σ) was obtained 1.4nM and 4.2μM for DPV and LSV, respectively. Also, sensitivity in the DPV measurement was 476μA/μM. Kinetic parameters such as electron transfer coefficient, α and diffusion coefficient, D, are also determined. The proposed method was successfully applied as a highly sensitive, selective, simple, and precise method for hydroxylamine determination in the real samples.

Hydroxylamine as an oxygen nucleophile: substitution of sulfonamide by a hydroxyl group in benzothiazole-2-sulfonamides

Benzothiazole-2-sulfonamides react with an excess of hydroxylamine in aqueous solutions to form 2-hydroxybenzothiazole, sulfur dioxide, and the corresponding amine. Mechanistic studies that employ a combination of structure-reactivity relationships, oxygen labeling experiments, and (in)direct detection of intermediates and products reveal that the reaction proceeds via oxygen attack, and that oxygen incorporated in the 2-hydroxybenzothiazole product derives from hydroxylamine. The reaction, which is performed under mild conditions, can be used as a deprotection method for cleavage of benzothiazole-2-sulfonyl-protected amino acids.

Stability of hydroxylamine isomers in aqueous solution: Ab initio study using continuum, cluster-continuum and Shells Theory of Solvation

The relative stability of the NH2OH and NH3O isomers of hydroxylamine in aqueous solution was investigated at ab initio CCSD(T)/TZVPP+diff level for gas phase energy and Shells Theory of Solvation (STS) method for solvation contribution. Our calculations point out that the zwitterionic form (NH3O) is only 3.5kcalmol−1 less stable than the normal isomer (NH2OH). The solvation contribution obtained by the SM8 and SM8AD models are in close agreement with the STS calculations, whereas the PCM (or CPCM) and cluster-continuum methods need small cavities for predicting the correct stability value.

A theoretical analysis of the acid–base equilibria of hydroxylamine in aqueous solution

Ammonia oxide ( +NH 3O −), a zwitterionic tautomer of hydroxylamine (NH 2OH), has been proposed to explain the high reactivity of NH 2OH with phosphate esters. The key parameter for kinetically significant reactions through +NH 3O − is the value of the tautomeric constant K T = [ +NH 3O −]/[NH 2OH], which it has so far proved elusive to measure experimentally. Thermodynamic cycles using CBS-QB3 and CPCM computed data have been used to estimate microscopic and macroscopic acid–base equilibria involving NH 2OH, the latter in close agreement to the experimental ones, but those involving anions. Calculated K T is 2.6 × 10 −2, which indicates that although molecular NH 2OH is favoured, there should be enough +NH 3O − in aqueous solution to be a kinetically active species.

Kinetics and mechanism of the reduction of monochloramine by hydroxylamine and hydroxylammonium ion

AbstractThe kinetics and mechanism by which monochloramine is reduced by hydroxylamine in aqueous solution over the pH range of 5–8 are reported. The reaction proceeds via two different mechanisms depending upon whether the hydroxylamine is protonated or unprotonated. When the hydroxylamine is protonated, the reaction stoichiometry is 1:1. The reaction stoichiometry becomes 3:1 (hydroxylamine:monochloramine) when the hydroxylamine is unprotonated. The principle products under both conditions are Cl–, NH+4, and N2O. The rate law is given by −[d[NH2Cl]/dt] = k+[NH3OH+][NH2Cl] + k0[NH2OH][NH2Cl]. At an ionic strength of 1.2 M, at 25°C, and under pseudo‐first‐order conditions, k+= (1.03 ± 0.06) ×103 L · mol−1 · s−1 and k0=91 ± 15 L · mol−1 · s−1. Isotopic studies demonstrate that both nitrogen atoms in the N2O come from the NH2OH/NH3OH+. Activation parameters for the reaction determined at pH 5.1 and 8.0 at an ionic strength of 1.2 M were found to be ΔH‡ = 36 ± 3 kJ · mol–1 and Δ S‡ = −66 ± 9 J · K−1 · mol−1, and Δ H‡ = 12 ± 2 kJ · mol−1 and Δ S‡ = −168 ± 6 J · K−1 · mol−1, respectively, and confirm that the transition states are significantly different for the two reaction pathways. © 2005 Wiley Periodicals, Inc. Int J Chem Kinet 38: 124–135, 2006

The aqueous reference for ESR oximetry

The interaction of molecular oxygen with derivatives of nitroxide EPR spin labels has been investigated using nuclear spin-relaxation spectroscopy in aqueous and nonaqueous solvents. The proton spin–lattice relaxation rate induced by oxygen provides a measure of the local concentration of oxygen, which we find is dependent on solvent. In water, the hydrophobic effect increases the local concentration of oxygen in the nonpolar portions of solute molecules. For nitroxides reduced to the hydroxylamine in aqueous solutions, we find that the local concentration of oxygen is approximately twice that associated with a free diffusion hard sphere limit, while in octane, this effect is absent. These results show that nitroxide based ESR oximetry may suffer a reference concentration shift of order a factor of two if the aqueous nitroxide spectrum or relaxation is used as the reference.

Free radical reactions of monochloramine and hydroxylamine in aqueous solution

The use of Advanced Oxidation Technologies to destroy organic contaminants in drinking water may be impacted by the presence of disinfection chemicals such as monochloramine (NH 2Cl). To allow a quantitative evaluation of the effect of NH 2Cl on the destruction of organics in water rate constants for its reaction with the hydrated electron, the hydroxyl radical and the hydrogen atom were determined in this study. The corresponding values of (2.2±0.2)×10 10, (2.8±0.2)×10 9, and (1.2±0.1)×10 9 M −1 s −1, respectively, were incorporated into a kinetic computer model whose predictions were in good agreement with experimental chloramine removal under large scale, steady-state electron-beam irradiation conditions. Rate constants were also determined for the reaction of the hydroxyl radical and hydrogen atom with the chloramine hydration product hydroxylamine to supplement established literature data. Hydroxyl radical rate constants for the basic (NH 2OH) and acidic (NH 3OH +) forms were determined as (8.5±0.4)×10 9 and ⩽5×10 7 M −1 s −1, respectively, while for hydrogen atom reaction, corresponding rate constants of (4.5±0.1)×10 7 and (3.6±1.5)×10 5 M −1 s −1 were found.

Removal of hydroxylamine by processes generating OH [rad] radicals in aqueous solution

The oxidation of hydroxylamine in aqueous solution ([NH 2OH] o = 1 mM, pH = 7) by OH radicals was studied using H 2O 2/UV and γ-irradiation as radicals generation processes. Nitrites and nitrates were identified as the major by-products in the aqueous phase. Their concentration depends on the experimental conditions, i.e. the presence or the absence of oxygen, and the concentration of hydrogen peroxide(H 2O 2). For the smallest initial concentrations of H 2O 2 studied (below 10 mM), hydroxylamine was oxidized into gases of low solubility. It is assumed that these gases are mainly constituted of N 2 and N 2O. Subsequently to the H-abstraction from the molecule of hydroxylamine by OH radicals, NHOH radical recombinations are favored in the absence of oxygen and H 2O 2 and would lead to N 2. In the presence of oxygen, NOH can be produced and can dimerise into N 2O or react with NH 2OH to form N 2. Reactions involving O 2 and H 2O 2, or O 2 − can account for nitrites then nitrates.

Determination of hydroxylamine in aqueous solutions of pyridinium aldoximes by high-performance liquid chromatography with UV and fluorometric detection

A high-performance liquid chromatographic assay using UV and fluorescence detection was developed that monitored hydroxylamine as the vanillin oxime derivative while simultaneously monitoring concentrations of a complex mixture of a pyridinium aldoxime and its degradation products. The technique should be useful for measuring hydroxylamine levels in other aqueous systems.

Kinetics and mechanism of the equilibration reaction between (2,2′,2″-nitrilotriethoxy)nitrosylvanadate(1–) and cyanide. Crystal structures of sodium (2,2′,2″-nitrilotriethoxy)nitrosylvanadate(I)–sodium perchlorate tetrahydrate and of barium cyano(2,2′,2″-nitrilotriethoxy)nitrosylvanadate(I) pentahydrate

The reaction of triethanolamine, ammonium vanadate(V), and hydroxylamine in aqueous solution (pH 10) affords quantitatively a red nitrosyl complex (reductive nitrosylation), Na[V(NO){N-(C2H4O)3}]·NaClO4·4H2O, (1), which upon addition of cyanide forms a blue complex, Ba[V(NO)-(CN){N(C2H4O)3}]·5H2O, (2). The structures of these two compounds have been determined by single-crystal X-ray studies: (1), monoclinic, space group P21/a, with a= 12.234(3), b= 11.405(3), c= 12.478(4)A, β= 93.56(3)°, and Z= 4, R= 0.089 from 2 104 reflections; (2), orthorhombic, space group P21ab, with a= 10.918(1), b= 11.604(3), c= 12.327(3)A, and Z= 4, R= 0.047 for 3 285 reflections. The complex anion of (1) exhibits distorted trigonal-bipyramidal co-ordination (O3N2) about the vanadium atoms with a linear {V–N–O}4 moiety. In contrast, the anion in (2) has distorted octahedral co-ordination (O3N2C) about the vanadium, involving the quadridentate ligand, a linear V–N–O group, and a cyanide. The rather long N–O distances in complexes (1) and (2)(average 1.253 A) are in accord with low ν(N–O) stretching frequencies at 1 490 and 1 450 cm–1. The {V–NO}4 moiety of (1) is susceptible to electrophilic attack by protons, generating most probably a hydroxylamidovanadium(V) species. This process is reversible; in alkaline solutions (1) is regenerated. The kinetics of the equilibration reaction of complex (1) with cyanide have been measured by stopped-flow spectrophotometry at pH 11 [I= 2.0 mol dm–3(NaClO4)]. A rate constant, kf= 1.12 dm3 mol–1 s–1, for the forward step and one, kr= 0.61 s–1, for the reverse have been determined at 20 °C.

A possible re-interpretation of the mechanisms of oximation of carbonyl compounds

Differential temperature—time curves obtained during the reactions of propanal and cyclohexanone with excess of hydroxylamine in aqueous solutions over a wide range of pH values show that addition of hydroxylamine does not become rate-determining except in very strongly acidic solutions (pH < 0.4), which contradicts the mechanism that is generally accepted. An alternative mechanism is proposed; it involves (a) the equilibrium between hydroxylamine and hydroxylammonium ion, (b) a reversible reaction of the carbonyl compound with hydroxylamine, yielding a carbinolamine adduct, (c) equilibria among the neutral carbinolamine and its mono- and diprotonated forms, (d) dehydration of the monoprotonated and neutral forms of the carbinolamine, and (e) equilibrium between the protonated and neutral forms of the oxime. Values were obtained for enthalpy changes accompanying all of the elementary steps; for the equilibrium constants and entropy changes for all of the steps save those involving dehydration, which appear to be complete under the conditions studied; and for the rate constants for all of the steps save those involving proton-transfer, which appear to be instantaneous under the conditions studied. So many of these values appear reasonable that further inquiry into such a mechanism seems justified, but the nature of step (b) could not be fully elucidated.

Photolysis of hydroxylamine in aqueous solution

Photolysis of hydroxylamine in aqueous solution

Metal ion-catalysed condensation of acetic acid with hydroxylamine in aqueous solution

The first page of this article is displayed as the abstract.

Studies in the biological fixation of nitrogen: I. Inhibition in Azotobacter vinelandii by hydroxylamine

A method is described whereby the rate of increase in cell content of a particular material may be measured isotopically. The method is free from sampling errors, and gives accurate values in short experiments. It has been applied to the study of the effect of hydroxylamine on fixation of nitrogen and on respiration in Azotobacter vinelandii. Respiration and fixation are inhibited in series. The results have been interpreted in terms of a linked enzyme system, and the linkage is believed to occur through the medium of a high-energy product of respiration, possibly hydrogen peroxide acting as a source of hydroxyl radicals. In the course of preliminary investigations, the rate of decomposition of hydroxylamine in aqueous solution and in medium (both sterile and inoculated with A. vinelandii) were measured. The reaction is first order over a wide range of concentration; nitrite and ammonia appear among the decomposition products. Nitrogen does not interchange with nitrite or hydroxylamine. Hydroxylamine is not utilised by the organism.