A new variant of thin-layer chromatography (TLC), based on a gradual change of mobile phase acidity during elution, is proposed.
The pH change occurs in the mobile phase moving along the TLC plate as a result of its contact with an acidic or a basic gas
phase that replaces the initial mobile phase vapor in the TLC chamber. The potential of this approach has been demonstrated
by using carbon dioxide and ammonia gases to improve the resolution of benzoic acids and aromatic amines on polyamide TLC
plates. 相似文献
Summary The strongly enhanced acidity of the bay hydroxyl group as compared to the respectiveperi hydroxyl groups of fringelite D, hypericin, and stentorin could be rationalized on the basis of a vinylogous carboxylic acid and was nicely corroborated by semiempirical calculations of the AM1 type. Experimental data obtained from several independent experimental methods, like polarized absorption spectroscopy, hole burning, and isotope effects, as well as from semiempirical AM1 and 6–31G levelab initio calculations conclusively pointed to dissymmetrical hydrogen bonding systems in both theperi andbay regions of the correspondingbay phenolate ions.
Zu Acidität und Wasserstoffbrückenbindung von Hydroxyphenanthroperylenchinonen wie Fringelit D, Hypericin und Stentorin
Zusammenfassung Die stark erhöhte Acidität der bay-Hydroxylgruppen gegenüber jener derperi-Hydroxylgruppen von Fringelit D, Hypericin und Stentorin läßt sich auf der Basis einer vinylogen Carbonsäure verstehen und wurde auch durch semiempirische Rechnungen vom AM1-Typ erhärtet. Daten aus unabhängigen Experimenten wie Polarisationsspektroskopie, Lochbrennen und Isotopeneffekte sowie semiempirische AM1- undab initio-Rechnungen auf 6–31G Niveau belegen ein unsymmetrisches Wasserstoffbrückenbindungssystem sowohl für denperi-als auch denbay-Bereich der entsprechendenbay-Phenolationen.
Aluminosilicates can present different structures such as crystalline true zeolite molecular sieves or amorphous silica–aluminas. With a large surface area available, both can be involved as catalysts, adsorbents or catalyst supports, and the determination of their surface acidic properties is an important parameter in the study of such materials.
The number, strength and strength distribution of the acidic sites were determined using microcalorimetry linked to a volumetric line. Ammonia was used as a basic probe molecule. The adsorption temperatures ranged from 353 K up to 473 K. The samples consisted of two amorphous silica–aluminas (Si/Al ≈ 6.5) and three microporous zeolites H-β, H-ZSM-5 and H-MCM-22 with similar Si/Al ratios (Si/Al ≈ 13).
The differential heats of ammonia adsorption versus coverage and the corresponding isotherms are given. The H-ZSM-5, H-MCM-22, H-β samples display a plateau of constant adsorption heats near 150 kJ mol−1, while the silica–alumina samples present continuously decreasing heats from 150 kJ mol−1 at zero coverage to 40 kJ mol−1 at high coverage, due to their surface heterogeneity. For amorphous silica–aluminas, the number of acid sites is dependent of the aluminum distribution at the surface.
The differences observed in the adsorption behavior of ammonia over the three zeolites arise from differences in their morphology, i.e. the total free volumes, pore geometries and electric field gradients at the adsorption sites. The adsorption isosteres have also been calculated from the adsorption isotherms, and the isosteric heats of adsorption have been compared with the heats measured by calorimetry. 相似文献
We introduced an approach to the ion chromatographic determination of common mono- and divalent cations including hydrogen ion and demonstrated the ability of a C30 column dynamically coated first with dodecylsulfate and then with 18-crown-6 ether to separate the cations by ion-exchange mechanism. Using an ethylenediamine solution containing a small concentration of 18-crown-6 ether and lithium dodecylsulfate at pH 6.2 as eluent, the cations were eluted in the order Li < Na+ < NH4+ < H+ < K+ < Mg2+ < Ca2+ with symmetrical peaks. The conductivity vs. concentration plots were linear about three orders of magnitude, from millimolar to micromolar; and the detection limits were all < 0.6 microM. Rainwater was analyzed directly using this ion chromatographic system with satisfactory results. 相似文献
Summary The acidity constants of adenosine-5-mono- and diphosphate (AMP andADP) were determined at 25.00±0.1°C by potentiometric titration in pure water and different solvent mixtures (methanol, ethanol, N,N-dimethylformamide, dimethylsulfoxide, acetone, and dioxane). The ionization ofAMP andADP depends on both the proportion and the nature of the organic solvent used. ThepKa1 values for bothAMP andADP are slightly influenced as the solvent is enriched in ethanol and methanol and remains practically constant in presence of different amounts ofDMF andDMSO. A pronounced change in thepKa1 values is observed as the solvent is enriched in acetone or dioxane. It is concluded that the electrostatic effect has only a relatively small influence on the dissociation equilibrium, whereas other solvent effects such as solvent basicity, hydrogen bonding and protonsolvent interactions play an important role.
Aciditätskonstanten von Adenosin-5-mono- und -diphosphat in verschiedenen organisch-wäßrigen Lösungsmittelgemischen
Zusammenfassung Die Aciditätskonstanten von Adenosin-5-mono- und -diphosphat wurden bei 25.0±0.1°C in reinem Wasser und in verschiedenen Lösungsmittelgemischen (Methanol, Ethanol, N,N-Dimethylformamid, Dimethylsulfoxid, Aceton und Dioxan) potentiometrisch bestimmt. Der Ionisierungsgrad vonAMP undADP hängt sowohl von der Menge als auch von der Art des organischen Lösungsmittels ab. DiepKa1-Werte vonAMP undADP werden durch Zugabe von Methanol und Ethanol nur wenig, durch verschiedene Mengen vonDMF undDMSO gar nicht, durch Aceton und Dioxan jedoch deutlich beeinflußt. Offensichtlich haben elektrostatische Effekte nur geringe Auswirkungen auf das Dissoziationsgleichgewicht, wogegen andere Faktoren wie Basizität des Lösungsmittels, Wasserstoffbrückenbindungen und Lösungsmittel-Proton-Wechselwirkungen eine bedeutende Rolle spielen.
Dissociation equilibria of 4‐(2‐pyridylazo) resorcinol (PAR) in aqueous micellar solutions were determined spectrophotometrically at 25 °C and at the constant ionic strength I = 0.1 M KNO3. For this purpose, the effect of nonionic (Brij‐35, Triton X‐100, Triton X‐114, Triton X‐405), and anionic (SDS) surfactants on the absorption spectra of PAR at different pH values was studied. Results show that the pKa values and pure spectra of each species of PAR are influenced by percentages of a neutral and an anionic surfactant such as Brij‐35, Triton X‐100, Triton X‐114, Triton X‐405 and SDS, respectively, added to the solution of this reagent. 相似文献