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971.
Elena Junquera Emilio Aicart 《Journal of inclusion phenomena and macrocyclic chemistry》1997,29(2):119-136
The encapsulation of the acid/base conjugated system salicylic acid(HSA)/salicylate (SA-) by hydroxypropyl--cyclodextrin (HPBCD) has beenstudied through fluorescence emission enhancement measurements in aqueoussolutions at 25 °C. With the aim of analyzing the crucial importance ofa proper and cautious choice of the pH of the medium (i.e. choice of thebuffer), the study has been carried out at pH = 1, 2, 4 and 7. Since the pKa of the HSA/SA- system is 2.95 at 25 °C, the presence of the protonated(HSA) and non-protonated (SA-) forms suitable for inclusion by cyclodextrinvary appreciably within the different pH conditions: 99% HSA,1% SA- at pH = 1, 90% HSA, 10% SA- at pH = 2,10% HSA, 90% SA- at pH = 4, and 0% HSA,100% SA- at pH =7. The association constants K CD : HSA and K CD: SA- have been determined in all cases by using a nonlinear regressionanalysis of the experimental data at three different em. The effectof the pH of the medium on all the equilibria involved as well as in the Kvalues is fully discussed. The 8-anilino-1-naphthalene sulfonate (ANS) +-cyclodextrin (-CD) system, widely reported in literature, hasinitially been studied to check the experimental protocol and the numericalmethod. 相似文献
972.
973.
A capillary zone electrophoresis (CZE) method has been developed for simultaneous determination of eukovoside, cinnamic acid and ferulic acid in Euphrasia regelii for the first time. The electrophoresis buffer was 20 mmol/L sodium borate containing 10% (v/v) methanol (pH 8.50). The effects of concentration of borate and electrolyte pH on electrophoretic behavior and separation were studied. Regression equations revealed linear relationships (correlation coefficients 0.9995-0.9998) between the peak area of each analyte and the concentration. The levels of analytes in the different parts of Euphrasia regelii were easily determined with recoveries ranging from 95.5 to 104.2%. 相似文献
974.
《Journal of separation science》2003,26(5):425-428
This paper describes a method for analysing some acids of milk whey by Capillary Zone Electrophoresis. After eliminating the whey proteins by ultrafiltration, the whey underwent electrophoretic separation in the presence of anodic electroosmotic flow. The following analytes were detected: citric, orotic, uric, and hippuric acids. A procedure is described for sample preparation and the operating conditions for electrophoretic capillary separation established. Finally, orotic acid is quantitatively determined. 相似文献
975.
Katarzyna Guzow Mariusz Szabelski Jerzy Karolczak Wiesaw Wiczk 《Journal of photochemistry and photobiology. A, Chemistry》2005,170(3):215-223
The photophysical properties of N-Boc-3-[2-(9-anthryl)benzoxazol-5-yl]-l-alanine methyl ester (BoxAnt) and N-Boc-3-[2-[4-(9′-(10′-butyl)anthryl)phenyl]benzoxazol-5-yl]-l-alanine methyl ester (BoxPhAnt) were studied in a series of solvents. Their absorption spectra are less sensitive to the solvent polarity than the corresponding fluorescence spectra which show a pronounced solvatochromic effect leading to large Stokes shifts. Using an efficient solvatochromic method, based on the empirical solvent polarity parameter , a large change of the dipole moment on excitation for BoxPhAnt has been found. From an analysis of the solvatochromic behaviour of the absorption and fluorescence spectra in terms of bulk solvent polarity functions, f(r, n) and g(n), a larger excited-state dipole moment (about 8 D, ψ = 56) was obtained for BoxPhAnt than for BoxAnt (about 3 D, ψ = 0). Both applied methods gave similar values of the excited-state dipole moments for both compounds studied. 相似文献
976.
SHEN Yu-Feng WANG Qing-Hai ZHU Dao-Qian ZHOU Liang-Mo Dalian Institute of Chemical Physics Chinese Academy of Sciences Dalian Liaoning China 《中国化学》1994,12(2):129-137
The study on retention behavior in supercritical fluid chromatography (SFC) is necessary to understand the mechanism of the various interactions in SFC. The retention of SFC in carboxylic acid methyl ester/polymethylsiloxane/CO2 system was studied systematically and the retention behavior of this kind of compounds under various typical operation conditions was described using the method of an alternative unified theory of chromatographic retention. The results illustrated that expression: Ink.= a + b/T + cp + dp/T + ep2/T can be used to describe quantitatively the retention behavior of carboxylic acid methyl ester/polymethylsiloxane/CO2 system in the ranges of reduced density from 0.549 to 1.411. It was also found that the entropy of solute in stationary phase is dependent on the density of supercritical fluid (SF) under typical operating conditions of SFC. 相似文献
977.
The dissolution of zinc in 0.48–1.49M HNO3 was studied at 15–25°C, by following simultaneously the concentration changes of the reactants (Zn and HNO3), intermediate (HNO2) and product (Zn2+) with time. Explicit mechanisms were suggested for the dissolution of zinc in nitric acid. The kinetics of the dissolution process show that it is of the first-order with respect to [Zn] and [HNO2]. The data obtained show that the dissolution process is diffusion-controlled. The mechanism of zinc dissolution is compared with the mechanism of copper dissolution.
Die Kinetik der Auflösung von Zink in Salpetersäure
Zusammenfassung Die Auflösung von Zink in 0.48–1.49M HNO3 wurde bei 15–25°C mittels gleichzeitiger Verfolgung der Konzentrationsänderungen der Reaktanden (Zn und HNO3), des intermediären HNO2 und des Produkts Zn2+ untersucht. Es wird ein Mechanismus vorgeschlagen. Die Kinetik der Auflösung ist erster Ordnung bezüglich [Zn] und [HNO2]. Die Daten zeigen, daß der Auflösungsvorgang diffusionskontrolliert ist. Der Mechanismus der Auflösung von Zink wird mit dem der Kupferauflösung verglichen.相似文献
978.
A series of binary complexes of Zn(II), Hg(II), Pb(II), La(III), Ce(III), Th(IV) and UO2(II) with 3,5-dinitrosalicylic acid and 5-sulphosalicylic acid have been isolated and characterized. The solution equilibria of these complexes have been studied pH-metrically. The formation constants (log K) at 25±1°C and ionic strength of 0.2M NaClC4 have been calculated. Stability of the complexes formed in relation to ligand molecular structure and nature of the metal ion has been examined and discussed. 相似文献
979.
980.
Fatty acid methyl esters from various fats and oils were separated by comprehensive two-dimensional supercritical fluid chromatography with conventional packed columns and FID detection. The first dimension was a silica gel column and the second dimension was an ODS column. This combination was largely orthogonal for the separation of fatty acid methyl esters. The first dimension separations were primarily based on the number of double bonds while the second dimension separations were based on the chain length. The highly-ordered chromatograms and improved resolution allowed the easy detection and identification of minor components. Although the first dimension separations were performed under isobaric conditions where the peak width increased in proportion to the retention, the programming of the sampling duration allowed us to maintain the optimum re-injection frequency (3–4 times) per peak into the second dimension and so to minimize the total analysis time without deteriorating the resolution. 相似文献