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71.
Simultaneous electrochemical determination of nitrate and nitrite in aqueous solution using Ag-doped zeolite-expanded graphite-epoxy electrode 总被引:2,自引:0,他引:2
In this work a new electrochemical sensor based on an Ag-doped zeolite-expanded graphite-epoxy composite electrode (AgZEGE) was evaluated as a novel alternative for the simultaneous quantitative determination of nitrate and nitrite in aqueous solutions. Cyclic voltammetry was used to characterize the electrochemical behavior of the electrode in the presence of individual or mixtures of nitrate and nitrite anions in 0.1 M Na2SO4 supporting electrolyte. Linear dependences of current versus nitrate and nitrite concentrations were obtained for the concentration ranges of 1-10 mM for nitrate and 0.1-1 mM for nitrite using cyclic voltammetry (CV), chronoamperometry (CA), and multiple-pulsed amperometry (MPA) procedures. The comparative assessment of the electrochemical behavior of the individual anions and mixtures of anions on this modified electrode allowed determining the working conditions for the simultaneous detection of the nitrite and nitrate anions. Applying MPA allowed enhancement of the sensitivity for direct and indirect nitrate detection and also for nitrite detection. The proposed sensor was applied in tap water samples spiked with known nitrate and nitrite concentrations and the results were in agreement with those obtained by a comparative spectrophotometric method. This work demonstrates that using multiple-pulse amperometry with the Ag-doped zeolite-expanded graphite-epoxy composite electrode provides a real opportunity for the simultaneous detection of nitrite and nitrate in aqueous solutions. 相似文献
72.
Balogh E Mato-Iglesias M Platas-Iglesias C Tóth E Djanashvili K Peters JA de Blas A Rodríguez-Blas T 《Inorganic chemistry》2006,45(21):8719-8728
Two novel ligands containing pyridine units and phosphonate pendant arms, with ethane-1,2-diamine (L2) or cyclohexane-1,2-diamine (L3) backbones, have been synthesized for Ln complexation. The hydration numbers obtained from luminescence lifetime measurements in aqueous solutions of the Eu(III) and Tb(III) complexes are q = 0.6 (EuL2), 0.7 (TbL2), 0.8 (EuL3), and 0.4 (TbL3). To further assess the hydration equilibrium, we have performed a variable-temperature and -pressure UV-vis spectrophotometric study on the Eu(III) complexes. The reaction enthalpy, entropy, and volume for the hydration equilibrium EuL <--> EuL(H2O) were calculated to be DeltaH degrees = -(11.6 +/- 2) kJ mol(-1), DeltaS degrees = -(34.2 +/- 5) J mol(-1) K(-1), and = 1.8 +/- 0.3 for EuL2 and DeltaH degrees = -(13.5 +/- 1) kJ mol(-1), DeltaS degrees = -(41 +/- 4) J mol(-1) K(-1), and = 1.7 +/- 0.3 for EuL3, respectively. We have carried out variable-temperature 17O NMR and nuclear magnetic relaxation dispersion (NMRD) measurements on the GdL2(H2O)q and GdL3(H2O)q systems. Given the presence of phosphonate groups in the ligand backbone, a second-sphere relaxation mechanism has been included for the analysis of the longitudinal (17)O and (1)H NMR relaxation rates. The water exchange rate on GdL2(H2O)q, = (7.0 +/- 0.8) x 10(8) s(-1), is extremely high and comparable to that on the Gd(III) aqua ion, while it is slightly reduced for GdL3(H2O)q, = (1.5 +/- 0.1) x 10(8) s(-1). This fast exchange can be rationalized in terms of a very flexible inner coordination sphere, which is slightly rigidified for L3 by the introduction of the cyclohexyl group on the amine backbone. The water exchange proceeds via a dissociative interchange mechanism, evidenced by the positive activation volumes obtained from variable-pressure 17O NMR for both GdL2(H2O)q and GdL3(H2O)q (DeltaV = +8.3 +/- 1.0 and 8.7 +/- 1.0 cm(3) mol(-1), respectively). 相似文献
73.
J. Mijnheer 《Journal of Mathematical Sciences》1995,76(2):2283-2287
Let {X(t): 0≤t<∞) be a stable subordinator with α∈(0,1). For increasing sequences tk we give normalizing constants ak such thatliminf
k→∞ a
k
−1
X(tk) is a.s constant. We also derive a.s. upper bounds.
Proceedings of the XVI Seminar on Stability Problems for Stochastic Models Part I, Eger, Hungary, 1994. 相似文献
74.
Zh��n Y��ng Ashley S. Harvey Anna Infortuna Joop Schoonman Ludwig J. Gauckler 《Journal of Solid State Electrochemistry》2011,15(2):277-284
Bulk $ {\hbox{B}}{{\hbox{a}}_x}{\hbox{S}}{{\hbox{r}}_{{1} - x}}{\hbox{C}}{{\hbox{o}}_y}{\hbox{F}}{{\hbox{e}}_{{1} - y}}{{\hbox{O}}_{{3} - \delta }} $ compositions (BSCF) were synthesized by the solid-state reaction method. The electrical conductivity of ceramic bars was measured using a dc four-probe method as a function of temperature in air up to 970?°C. All compositions showed thermally activated p-type semi-conductivity up to ~450?°C and then a transition to metal-like conductivity. The small-polaron hopping p-type semi-conductivity depends on the oxygen nonstoichiometry, which increases with increasing temperature. Metal-like conductivity is attributed to the overlap of the transition metal d-electron orbitals with the oxygen p-orbitals. Strontium-rich compositions show higher conductivity. The Co/Fe ratio does not influence much the p-type semi-conduction. Iron-rich compositions revealed more metal-like conduction behavior. The degree of overlap between transition metal d-orbitals and oxygen p-orbitals depends on the Ba/Sr as well as on the Co/Fe ratios. 相似文献
75.
76.
Jan K. Eweg Franz Müer Dick Bebelaar Joop D. W. van Voorst 《Photochemistry and photobiology》1980,31(5):435-443
Abstract— Spectral properties of alloxazine and isoalloxazine derivatives were determined at high temperatures in the vapour phase. In comparison with spectra observed in solution, vapour phase spectrometry gives primary photophysical properties of the isolated chromophore. Fluorescence lifetimes in the vapour phase range from <0.5 to 18 ns. There is some indication that an intramolecular complex can exist between the isoalloxazine moiety and its aliphatic side chain carrying a polar (hydroxyl) group. Direct photodissociation of the molecule in its first electronically excited singlet state is not very probable. 相似文献
77.
Prudêncio M Rohovec J Peters JA Tocheva E Boulanger MJ Murphy ME Hupkes HJ Kosters W Impagliazzo A Ubbink M 《Chemistry (Weinheim an der Bergstrasse, Germany)》2004,10(13):3252-3260
A lanthanide complex, named CLaNP (caged lanthanide NMR probe) has been developed for the characterisation of proteins by paramagnetic NMR spectroscopy. The probe consists of a lanthanide chelated by a derivative of DTPA (diethylenetriaminepentaacetic acid) with two thiol reactive functional groups. The CLaNP molecule is attached to a protein by two engineered, surface-exposed, Cys residues in a bidentate manner. This drastically limits the dynamics of the metal relative to the protein and enables measurements of pseudocontact shifts. NMR spectroscopy experiments on a diamagnetic control and the crystal structure of the probe-protein complex demonstrate that the protein structure is not affected by probe attachment. The probe is able to induce pseudocontact shifts to at least 40 A from the metal and causes residual dipolar couplings due to alignment at a high magnetic field. The molecule exists in several isomeric forms with different paramagnetic tensors; this provides a fast way to obtain long-range distance restraints. 相似文献
78.
In this work, thin films of zinc oxide (ZnO) for gas-sensor applications were deposited on platinum coated alumina substrate, using electrostatic spray deposition (ESD) technique. As precursor solution zinc acetate in ethanol was used. Scanning electron microscopy (SEM) evaluation showed a porous and homogeneous film morphology and the energy dispersive X-ray analysis (EDX) confirmed the composition of the films with no presence of other impurities. The microstructure studied with X-ray diffraction (XRD) and Raman spectroscopy indicated that the ZnO oxide films are crystallized in a hexagonal wurtzite phase. The films showed good sensitivity to 1 ppm nitrogen dioxide (NO2) at 300 °C while a much lower sensitivity to 12 ppm hydrogen sulphide (H2S). 相似文献
79.
Stroink T Schravendijk P Wiese G Teeuwsen J Lingeman H Waterval JC Bult A de Jong GJ Underberg WJ 《Electrophoresis》2003,24(6):1126-1134
Since biologically active peptides usually exhibit their effects in low concentrations, the development of sensitive analytical methods has become a challenge. In this paper, a multidimensional system is presented, consisting of a size-exclusion chromatographic (SEC) separation followed by a trapping procedure on a 4 mm x 3 mm ID reversed-phase C18 (RP18) column with subsequent elution of the trapped fraction and separation by capillary zone electrophoresis (CZE). The system has been tested with mixtures of six enkephalins and albumin, mimicking biological matrices such as plasma and cerebrospinal fluid. After separation of albumin from the enkephalins in the SEC dimension a heart-cut of 200 micro L, containing the enkephalin peak, is taken, concentrated on the RP18 microcolumn and, after elution with a 20 micro L plug of 80% acetonitrile, electrokinetically injected into the CZE system, where stacking and separation is achieved. While validation shows generally good linearity and reproducibility, the quantitation limit with UV detection is acceptable (2.5 micro g/ mL with an injection volume of 50 micro L). 相似文献
80.
Kooistra FB Knol J Kastenberg F Popescu LM Verhees WJ Kroon JM Hummelen JC 《Organic letters》2007,9(4):551-554
We report the synthesis, characterization, and electrochemical properties of ten new fullerene derivatives for usage in organic solar cells. The phenyl ring of PCBM was substituted with electron-donating and electron-withdrawing substituents to study their influence on the LUMO level of the parent fullerene. We varied the LUMO level over a range of 86 mV and show a small but significant change of the open circuit voltage upon application in MDMO-PPV:methanofullerene bulk-heterojunction photovoltaic cells. [structure: see text]. 相似文献