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101.
Zusammenfassung Ein Verfahren zur Bestimmung von Tellurspuren mit Hilfe der Simultankomparationsmethode wurde ausgearbeitet, das auf der Bichromat-Ferriin-Reaktion beruht. Ferriin wird von Tellur(IV) reduziert; dieser Prozeß wird von Bichromat induziert. Bei diesem Verfahren werden die Tellurspuren durch Natriumhypophosphitreduktion mit Arsen oder Selen als Träger abgeschieden, der Metallniederschlag wird mit Salpetersäure zu Tellur(IV) oxydiert, die zu untersuchenden Proben und die Standardreihe gleichzeitig gestartet und verglichen. Mit dieser Methode kann Tellur auch in Mikrogrammengen mit einem Fehler von weniger als ±10% bestimmt werden. Störend wirken nur die Pt-Metalle und Au(III).
Determination of tellurium in microgram amounts with the aid of the simultaneous comparation method
Summary A procedure has been worked out for the determination of traces of tellurium with the aid of the simultaneous comparation method, and based on the bichromate-ferriin reaction. Ferriin is reduced by tellurium(IV); this process is induced by bichromate. In this procedure the traces of tellurium are precipitated by means of sodium hypophosphite-reduction with arsenic or selenium as carriers. The metal precipitate is oxidized to tellurium(IV) with nitric acid; the samples under investigation and the standard series are started at the same time and compared. By means of this method it is possible to determine tellurium, even in microgram amounts, with an error of less than ± 10%. Only the platinum metals and gold(III) interfere.相似文献
102.
Pavel Arsenyan Olga Pudova Juris Popelis Edmunds Lukevics 《Tetrahedron letters》2004,45(15):3109-3111
A series of thiophene-based homologues with a silicon core surrounded by mono-, bi-, terthiophene, and their derivatives with alkylsilyl linkages has been prepared using hydrosilylation and Stille coupling methods. 相似文献
103.
Reductive amination of 4′-formyl[benzo-15-crown-5] with sodium cyanoborohydride in the NH4Ac/MeOH medium followed by acid addition and extraction with CHCl3 unexpectedly lead to the isolation of the salt (B15C5-CH2-NH3)+-(H3BCNBH2CN)− with an unusual dicyanodiborohydride anion. The self-complementary 4′-ammoniomethyl[benzo-15-crown-5] cation was found to exist as a supramolecular dimer in the solid state, acetonitrile solution, and gas phase as revealed by X-ray crystallography, NMR and mass spectrometry, respectively. 相似文献
104.
105.
The optimization of lead hydride generation in aqueous ethanolic media and the influence on its generation of the wine components, both white and red, have been studied. These interferences were overcome by careful control of the parameters affecting hydride generation and the procedure was applied to the determination of Pb in wines. The method is fast, accurate and sensitive and can be used to quantify 24 ppb of Pb in wines. 相似文献
106.
Summary A simple spectrophotometric procedure for microdetermination of rhamnose is described. It is based on breakdown of the sugar with sulphuric acid followed by treatment withp-hydroxydiphenyl. The coloured product is measured spectrophotometrically at 560 nm. A linear relationship exists between the amount of rhamnose (10-0g–) and the absorbance. The method is recommended for determination of rhamnose in samples containing rhamnose, different neutral sugars, amino-sugars, sugar alcohols, uronic acids, and muramic acid.
Zusammenfassung Ein einfaches spektralphotometrisches Verfahren zur Mikrobestimmung von Rhamnose wurde beschrieben. Es beruht auf der Spaltung des Zuckers mit Schwefelsäure und nachfolgender Behandlung mit p-Hydroxydiphenyl. Das gefärbte Reaktionsprodukt wird spektralphotometrisch bei 560 nm gemessen. Die Extinktion steht mit der Rhamnosemenge in linearer Beziehung. Das Verfahren wird für die Rhamnosebestimmung in Proben empfohlen, die verschiedene Neutralzucker, Aminozucker, Zuckeralkohole, Uronsäuren und Muramsäure enthalten.相似文献
107.
Morozova OB Korchak SE Sagdeev RZ Yurkovskaya AV 《The journal of physical chemistry. A》2005,109(45):10459-10466
Using time-resolved chemically induced dynamic nuclear polarization (CIDNP) techniques, we have studied the mechanism of the photoreactions of triplet excited 4-carboxybenzophenone (CBP) with l-methionine (Met) and 3-(methylthio)propylamine (MTPA) in aqueous solution and the details of the formation of CIDNP at pH from 6.7 to 13.6. At a pH below the pKa of the nitrogen atom of Met, the CIDNP is strongly affected by degenerate electron exchange between the S-S cationic radical dimer and the zwitterionic form of Met with the rate constant kex = 3.4 x 10(8) s(-1) providing an exhaustive explanation of the pH dependence of steady-state CIDNP that was previously interpreted as a manifestation of fast interconversion among three different methionine radical species (Goez, M.; Rozwadowski, J. J. Phys. Chem. A 1998, 102, 7945-7953). By analyzing the polarization of different protons formed in geminate recombination as a function of the pH, we obtained the branching ratio between two reaction pathways for oxidative quenching of (T)CBP via electron transfer from the sulfur and nitrogen atoms of Met and MTPA. Nuclear spin-lattice relaxation times were determined in the dimeric cation radical of Met (T1,S = 8.5 micros). In the cyclic radical cation of MTPA with a three-electron two-center S-N bond, the estimated paramagnetic relaxation is comparatively slow for all protons. Fast deprotonation of the primary aminium radical cation of MTPA and Met in strongly basic solution takes place on the submicrosecond time scale leading to efficient formation of CIDNP in the neutral aminyl radical. 相似文献
108.
Fernando Ferreira 《Tetrahedron letters》2004,45(33):6287-6290
Oligonucleotides protected with N-(trimethylsilylethoxycarbonyl) (Teoc) and P-(trimethylsilylethanol) (Tse) groups were synthesized and deprotected by a single ZnBr2 treatment. Teoc group stabilized dA against depurination. This strategy was applied to the synthesis of base-sensitive oligonucleotide prodrugs bearing S-acetyl-2-thioethyl (Sate) phosphotriesters. 相似文献
109.
Ferreira CM Guedes da Silva MF Fraústo da Silva JJ Pombeiro AJ Kukushkin VY Michelin RA 《Inorganic chemistry》2001,40(6):1134-1142
The dialkylcyanamide complexes cis-[PtCl(NCNR(2))(PPh(3))(2)][BF(4)] 1 and cis-[Pt(NCNR(2))(2)(PPh(3))(2)][BF(4)](2) 2 (R = Me or Et) have been prepared by treatment of a CH(2)Cl(2) solution of cis-[PtCl(2)(PPh(3))(2)] with the appropriate dialkylcyanamide and one or two equivalents of Ag[BF(4)], respectively. Compounds 2 can also be obtained from 1 by a similar procedure. Their reaction with oximes, HON=CR'R' ' (R'R' ' = Me(2) or C(4)H(8)), in CH(2)Cl(2) and in the presence of Ag[BF(4)] or Cu(CH(3)COO)(2), leads to the novel type of azametallacycles cis-[Pt(NH=C(ON=CR'R")-NR2)(PPh3)2][BF4]2 4 upon an unprecedented coupling of the organocyanamides with oximes, in a process that proceeds via the mixed oxime-organocyanamide species cis-[Pt(NCNR(2))(HON=CR'R' ')(PPh(3))(2)][BF(4)](2) 3, and is catalyzed by either Ag(+) or Cu(2+) which activate the ligating organocyanamide by Lewis acid addition to the amide group. In contrast, in the organonitrile complexes cis-[Pt(NCR)(2)(PPh(3))(2)][BF(4)](2) 5 (R = C(6)H(4)OMe-4 or Et), obtained in a similar way as 2 (but by using NCR instead of the cyanamide), the ligating NCR is not activated by the Lewis acid and does not couple with the oximes. The spectroscopic properties of those complexes are reported along with the molecular structures of 2b (R = Et), 4a1 (R = Me, R'R' ' = Me(2)), and 4b1 (R = Et, R'R' ' = Me(2)), as established by X-ray crystallography which indicates that in the former complex the amide-N-atoms are trigonal planar, whereas in the latter (4a1 and 4b1) the five-membered rings are planar with a localized N=C double bond (imine group derived from the cyanamide) and the exocyclic amide and alkylidene groups (in 4b1) are involved in two intramolecular H-bonds to the oxygen atom of the ring. 相似文献
110.
M. G. Ferreira Da Silva J. M. Fernàndez Navarro 《Journal of Sol-Gel Science and Technology》1994,2(1-3):895-901
The effect of thermal treatment on the structural incorporation of Cr2O3 in xCr2O3·(100 – x)SiO2 and 5R
n
O·xCr2O3·(95 – x)SiO2 (where x=0.01–1 mol.% and R=Li, Na and Ca) gel glasses was studied by optical absorption spectrophotometry, DTA-TG, XRD and electron microscopy.Samples heat treated at 60°C have green color due to the presence of Cr3+ in octahedral coordination. The optical transmission, and color (yellow, orange or ambar), of the samples heat treated between 200 and 700°C prove that Cr3+ (octahedrally coordinated) and Cr6+ (tetrahedrally coordinated) are both present. Segregation of Cr2O3 take place at temperatures above 800°C.In reducing conditions the gel glasses were green due to the presence of a high content of Cr3+ ions. Samples containing Li or Na show crystalline phases at temperatures below 850°C. 相似文献