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A modified Lassaigne method was developed for N determination based on fusion of the organic substance with metallic Na, conversion of the cyanide in the aqueous leachate to thiocyanate by ammonium polysulfide treatment, and colorimetric measurement of the thiocyanate formed by the addition of excessive ferric ions in acidic medium. The mean molar absorptivity of the Fe(NCS)2+ complex at 480 nm is 2.96 x 10(3) L/mol x cm, enabling quantitation of 0.25-7.72 ppm N (linear range) in the final solution. The relative amounts of Na, (NH4)2S2, and Fe(III) with respect to nitrogen in the analyte were optimized. The developed method was successfully applied to the determination of N in various brands of baby food, and it was compared statistically with the conventional Kjeldahl and elemental analysis methods. Protein nitrogen in a number of meat products was also precisely determined by the developed method. Thus, the total digestion time of the conventional Kjeldahl method was reduced considerably (e.g., to approximately 15 min for a dried sample) with a relatively simple spectrophotometric method requiring no sophisticated instrumentation. 相似文献
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Birsen S. Buyuktas 《Transition Metal Chemistry》2006,31(6):786-791
Titanium tetra-n-butoxide, [Ti(OCH2CH2CH2CH3)4; Ti(OBun)4] was reacted with some unsaturated mono- and di-carboxylic acids such as t-cinnamic acid (TCA; C6H6CH=CHCOOH), maleic acid (MA; HOOCCH=CHCOOH) and itaconic acid [ITA; HOOCCH2C(=CH2)COOH]. Complexation ratios of the complexes (mol of the acid per Ti atom in the complexes) were determined by FTIR spectroscopy.
The structure of the complexes was supported by the 13C-n.m.r., 1H-n.m.r. and FTIR spectra. It was observed that MA, ITA and TCA behave as bidendate ligands. MA and ITA are dicarboxylic acids
that complexed with Ti(OBun)4 in a 2:1 mol ratio (Ti:MA and Ti:ITA). TCA is a monocarboxylic acid which was complexed with Ti(OBun)4 with a mol ratio of 1:1 (Ti:TCA). Hydrolysis reactions of these complexes were performed by Karl–Fischer titration. The hydrolysis
reaction rate of the modified Ti(OBun)4 was reduced with complexation by these carboxylic acids. 相似文献
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Journal of Sol-Gel Science and Technology - New silicon boron oxycarbide (SiBOC) preceramic polymers were synthesized via sol–gel method by using n-butyltrimethoxysilane and... 相似文献
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A Ce(IV)-based reducing capacity (CERAC) assay was developed to measure the total antioxidant capacity (TAC) of foods, in
which Ce(IV) would selectively oxidize antioxidant compounds but not citric acid and reducing sugars which are not classified
as antioxidants. The method is based on the electron-transfer (ET) reaction between Ce(IV) ion and antioxidants in optimized
acidic sulphate medium (i.e., 0.3 M H2SO4 and 0.7 M Na2SO4) and subsequent determination of the produced Ce(III) ions by a fluorometric method. The fluorescent product, Ce(III), exhibited
strong fluorescence at 360 nm with an excitation wavelength of 256 nm, the fluorescence intensity being correlated to antioxidant
power of the original sample. The linear concentration range for most antioxidants was quite wide, e.g., 5.0 × 10−7–1.0 × 10−5 M for quercetin. The developed procedure was successfully applied to the TAC assay of antioxidant compounds such as trolox,
quercetin, gallic acid, ascorbic acid, catechin, naringin, naringenin, caffeic acid, ferulic acid, glutathione, and cysteine.
The proposed method was reproducible, additive in terms of TAC values of constituents of complex mixtures, and the trolox
equivalent antioxidant capacities (TEAC coefficients) of the tested antioxidant compounds gave good correlations with those
found by reference methods such as ABTS and CUPRAC. 相似文献
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Comparative evaluation of various total antioxidant capacity assays applied to phenolic compounds with the CUPRAC assay 总被引:4,自引:0,他引:4
Apak R Güçlü K Demirata B Ozyürek M Celik SE Bektaşoğlu B Berker KI Ozyurt D 《Molecules (Basel, Switzerland)》2007,12(7):1496-1547
It would be desirable to establish and standardize methods that can measure the total antioxidant capacity level directly from vegetable extracts containing phenolics. Antioxidant capacity assays may be broadly classified as electron transfer (ET)- and hydrogen atom transfer (HAT)-based assays. The majority of HAT assays are kinetics-based, and involve a competitive reaction scheme in which antioxidant and substrate compete for peroxyl radicals thermally generated through the decomposition of azo compounds. ET-based assays measure the capacity of an antioxidant in the reduction of an oxidant, which changes colour when reduced. ET assays include the ABTS/TEAC, CUPRAC, DPPH, Folin-Ciocalteu and FRAP methods, each using different chromogenic redox reagents with different standard potentials. This review intends to offer a critical evaluation of existing antioxidant assays applied to phenolics, and reports the development by our research group of a simple and low-cost antioxidant capacity assay for dietary polyphenols, vitamins C and E, and human serum antioxidants, utilizing the copper(II)-neocuproine reagent as the chromogenic oxidizing agent, which we haved named the CUPRAC (cupric ion reducing antioxidant capacity) method. This method offers distinct advantages over other ET-based assays, namely the selection of working pH at physiological pH (as opposed to the Folin and FRAP methods, which work at alkaline and acidic pHs, respectively), applicability to both hydrophilic and lipophilic antioxidants (unlike Folin and DPPH), completion of the redox reactions for most common flavonoids (unlike FRAP), selective oxidation of antioxidant compounds without affecting sugars and citric acid commonly contained in foodstuffs and the capability to assay -SH bearing antioxidants (unlike FRAP). Other similar ET-based antioxidant assays that we have developed or modified for phenolics are the Fe(III)- and Ce(IV)-reducing capacity methods. 相似文献
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Birsen Demirata 《Mikrochimica acta》2001,136(3-4):143-146
A method is described for the quantitative preconcentration and separation of trace chromium in water by adsorption on melamine-urea-formaldehyde
resin. Cr(VI) is enriched from aqueous solutions on the resin. After elution the Cr(VI) is determined by FAAS. The capacity
of the resin is maximal at ∼ pH 2. Total chromium can be determined by the method after oxidation of Cr(III) to Cr(VI) by
hydrogen peroxide. The relative standard deviations (10 replicate analyses) for 10 mg/L levels of Cr(VI), Cr(III) and total
chromium were 1.5, 3.5 and 2.8% respectively. The procedure has been applied to the determination and speciation of chromium
in lake water, tap water and chromium-plating baths. 相似文献
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Catalytic Meerwein-Ponndorf-Verley (MPV) reduction of various aliphatic, aromatic, and unsaturated aldehydes and ketones to
corresponding alcohols (analyzed by GC-MS) in the presence of boron triethoxide (B(OEt)3) were studied. Kinetics of this reduction reaction was also studied and the respective rate constants were determined. It
was found that B(OEt)3 catalyzes the reduction of aliphatic aldehydes and ketones to alcohols at room temperature while aromatic aldehydes and ketones
were not reduced under the same conditions. In addition, MPV reduction using B(OEt)3 was found to be chemoselective as unsaturated aldehydes and ketones afforded the corresponding alcohols without affecting
unsaturated groups. The mechanism proposed involves a six-membered transition state in which both the alcohol and the carbonyl
are coordinated to the same boron centre of a boron alkoxide catalyst. 相似文献