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1.
Although the establishment of a pH scale and the determination of the pH in water is not problematic, it is not a straightforward task in non-aqueous solvents. As capillary zone electrophoresis (CZE) in organic solvents has gained increasing interest, it seems to be valuable to re-discuss the concept of the pH in such media, especially pointing to those aspects, which make pH measurement uncertain in non-aqueous solvents. In this review, the relevant aspects when dealing with primary standard (PS) and secondary standard (SS) as recommended by the International Union of Pure and Applied Chemistry (IUPAC), and the usage of the operational pH are discussed with special emphasis to non-aqueous solvents. Here, different liquid junction potentials, incomplete dissociation of the electrolytes (especially in solvents with low or moderate relative permittivity) and the occurrence of homo- and heteroconjugation must be taken into account. Problems arising in capillary zone electrophoresis practice are addressed, e.g. when the background electrolyte (BGE) consists of organic solvents, but the measuring electrode (normally the glass electrode) is calibrated with aqueous buffers, and the liquid junction potentials between the solvents do not cancel each other. The alternative concept of establishing a certain pH is described, using mixtures of reference acids or bases with known pKa in the organic solvent, and their respective salts, at a certain concentration ratio, relying to the Henderson-Hasselbalch equation. Special discussion is directed to those organic solvents most common in capillary zone electrophoresis, methanol (MeOH) and acetonitrile (ACN), but other solvents are included as well. The potential significance of small amounts of water present in the organic solvent on changes in pKa values, and thus on the pH of the buffering components is pointed out. 相似文献
2.
B. Sop Tamo G. Kamgang-Youbi E. Acayanka L. Medjo Simo A. Tiya-Djowe D. Kuete-Saa S. Laminsi L. Tchadjie 《Plasma Chemistry and Plasma Processing》2016,36(6):1449-1469
A Cameroonian kaolinite powder was treated with gliding arc plasma in order to increase the amount of hydroxyl functional groups present on its external surfaces. The functional changes that occurred were monitored by Fourier transform infrared spectroscopy. The crystalline changes were followed by the X-ray diffraction. The ionisation effect, acid effect, and water solubility of the treated samples were also evaluated. Results showed that there is breaking of the bonds in the Si–O–Si and Si–O–Al groups, followed by the formation of new aluminol (Al–OH) and silanol (Si–OH) groups at the external surface of kaolinite after exposing the clay to the gliding arc plasma. The increase in hydroxyl groups on the surface of kaolinite leads to the increase of its hydrophilicity. Moreover, new charges appear on its surfaces and no significant change in crystallinity has occurred. This study shows that clays in powder form being can effectively be functionalised by gliding arc plasma in spatial post discharge processing mode. Knowing that the treatment in spatial post discharge offers the possibility to process large amounts of clay, this work is of great interest to the industry. 相似文献
3.
Dr. Joffrey Pijeat Léo Chaussy Roxanne Simoës Dr. Jacopo Isopi Prof. Dr. Jean-Sébastien Lauret Prof. Dr. Francesco Paolucci Prof. Dr. Massimo Marcaccio Dr. Stéphane Campidelli 《ChemistryOpen》2021,10(10):997-1003
The synthesis of π-extended porphyrins containing anthracenyl moieties still represents an important challenge. Here, we report on the synthesis of a series of unsubstituted naphthyl-, pyrenyl- and anthracenyl-fused zinc porphyrin derivatives. To this aim, meso-substitued porphyrins are synthesized and the fusion of the PAHs (Polycyclic Aromatic Hydrocarbon) on the β-positions are performed through thermally induced dehydro-aromatization. The fused zinc-porphyrin derivatives are fully characterized and their optical absorption and photoluminescence properties are reported. We also demonstrate that zinc can be removed from the porphyrin core, giving rise to pure C, H, N materials. This work constitutes the first step towards the synthesis of the fully-fused tetra-anthracenylporphyrin. 相似文献
4.
Peak dispersion effects in nonaqueous capillary electrophoretic separations of aromatic anionic analytes were investigated in a propanolic background electrolyte solution. Poly(glycidylmethacrylate-co-N-vinylpyrrolidone) coating was applied to the capillary to suppress the electroosmotic flow and to improve the repeatability of the migration times. Electrical field strengths up to 2000 Vcm(-1) were applied in separations and the separation efficiencies were compared with theoretical values calculated on the basis of plate height theory. The contributions to the total plate height were calculated for injection plug length, diffusion, Joule heating, electromigration dispersion, analyte adsorption to the capillary wall, and detector slit aperture length. Analyte diffusion coefficients were measured by Taylor dispersion method, while distribution constants were measured chromatographically. Agreement between the calculated and empirical results was fairly good even though some approximations were required. In most cases the longitudinal diffusion contribution governed the total plate height, while the contribution of Joule heating was insignificant even at exceptionally high field strengths used. The relatively long detection slit aperture was found to influence the separation efficiency strongly, while the other dispersion sources that were investigated were of minor importance, except for adsorption in the case of one analyte. With all analytes, the dispersive effect of longitudinal diffusion was reduced as the field strength was increased, leading to enhanced migration velocities and faster separations. 相似文献
5.
For homogeneous isotropic turbulence study, the acquisition of band-pass filtered velocity increments (FVI) in a non-forced turbulent box is still a challenge both experimentally and numerically. Turbulence and associated physical processes, at a given instant, are permanently contaminated by a forcing process which can seldom be universal. The situation tends to be the origin of intermittency and the non-Gaussian probability density distribution for acceleration and velocity gradients. To reveal implied mechanism, grid turbulence is adapted to observe non-perturbed homogeneous isotropic turbulence. The velocity increments (VI) can be obtained following Comte-Bellot and Corrsin (GCBC) by means of two point-two time shifted velocity measurements. It is difficult to obtain decaying turbulence (DT) at large turbulent Reynolds number without pollution coming from walls. Nevertheless it is also significant to investigate DT in low Reynolds number regimes to determine non-polluted tendencies. The similarity of DT between particle image velocimetry (PIV) and hot wire anemometry measurements by GCBC are presented. Here we focus our tendency on VI and FVI probability density function (PDF) shapes in this letter. In conclusion, the tendency to Gaussian shape in inertial zone wavenumbers, demonstrates that there will be no intermittency if turbulent cascade is not perturbed. 相似文献
6.
In the present paper, we obtain a criterion for the stabilization of the Cauchy problem for an integro-differential equation
in the class of functions of polynomial growth γ ≥ 0.
Published in Ukrains’kyi Matematychnyi Zhurnal, Vol. 57, No. 11, pp. 1571–1576, November, 2005. 相似文献
7.
Communicated by P.Holmes 相似文献
8.
The 13C NMR spectra of phloropyron BB, desaspidin BB, albaspidin BB and margaspidin BB were recorded and the structures of the compounds investigated with the aid of chemical shifts and CH coupling constants. The filicinic acid ring of the first three compounds appeared to have a monoketonic structure with the carbonyl group in position 2 (acyl group in position 3). The pyronone ring of phloropyron BB also has a monoketonic structure, with the carbonyl function adjacent to the ring oxygen. The two rings of the first three compounds attain a conformation where a stabilizing hydrogen bond(s) is (are) formed between the two rings, as shown by the observed CH couplings to some of the hydroxyl protons. The spectrum of margaspidin BB, whichconsists of two aromatic acyl phloroglucinol rings, indicates less inter-ring hydrogen bonding interactions. 相似文献
9.
Pyropheophytin a, which is an unsymmetric porphyrin, has been titrated with trifluoroacetic acid (TFA) in tetrahydrofuran, the protonation reaction being followed by 13C NMR spectroscopy. TFA was added in small increments to a 0.28 M solution of pyropheophytin a in tetrahydrofuran, and the chemical shift changes of the macrocyclic carbons were determined as a function of the TFA increments. On the addition of TFA the signals of the α-carbons of ring II experienced a large upfield change, whereas the signals of all other macrocyclic carbons moved only slightly downfield or remained constant. These observations were interpreted as indicating the formation of a monocation in which the proton is attached to the nitrogen of ring II. The 13C protonation shifts of pyropheophytin a were compared with those previously reported for symmetric porphyrins. On the basis of this comparison, the basicity of the macrocyclic nitrogen atoms, the N–H tautomerism and the electron delocalization in structurally different porphyrin macrocycles are discussed. 相似文献
10.
The 13C NMR spectra of C-10 epimeric chlorophylls a and a′, pheophytins a and a′, pyrochlorophyll a and pyropheophytin a have been recorded and assigned by chemical shift comparison, by long-range selective 1H decoupling experiments and by the examination of the fully coupled spectra. Various factors influencing the 13C chemical shifts of the chlorophyll derivatives, e.g. the coordination of magnesium to the chlorin nucleus, the effect of solvent and the steric strain at the periphery of the macrocycle, have been examined. The 13C NMR spectra of chlorophyll a measured in acetone-d6 and tetrahydrofuran-d8 (THF) were compared, and remarkable solvent effects on the 13C chemical shifts were observed. These effects were interpreted mostly in terms of specific chlorophyll-solvent interactions. Different electron donor and steric properties of acetone and THF were considered to cause conformational alterations in the macrocycle, induced by the ligation of the solvent molecule(s) to the axial position(s) of the central magnesium atom of chlorophyll a. These results show that 13C NMR spectroscopy is a method of high information value for investigations of the unique electron donor acceptor (EDA) properties of the chlorophylls. The structural differences between the C-10 epimeric chlorophylls and pheophytins were examined in terms of the substituent chemical shift (SCS) parameters for the C-10 methoxycarbonyl group. The analysis showed that the change from the (10R) to the 10(S) configuration induces conformational alterations in the whole macrocycle which are, however, most prominent in rings IV and V. Owing to the increased steric interaction (repulsion) between the bulky substituents at C-7 and C-10, the peripheral strain is larger in the (10S) form, and is relieved by more pronounced deviations of rings IV and V from the macrocyclic plane compared with the (10R) form. The examination of the SCS parameters also showed that the peripheral steric strain is dissipated to a larger extent over the entire macrocycle in the Mg-free derivatives. These results confirm the previous conclusions based on 1H NMR and CD data. The possible function of chlorophyll a′ in photosynthesis is briefly discussed. 相似文献