固定pH滴定法测定弱酸弱碱性药物

报道一种水溶液吕测定弱酸弱碱性药物的新型滴定分析方法－固定ｐＨ滴定法，其理论基础是弱酸弱碱在水溶液中各种存在形式的分布系数在ｐＨ值固定时为常数，对甲硝唑，盐酸美西律等离散常数在１０＾－８至１０＾－１１范围内的７种弱酸弱碱性药物进行测定。结果与药典法吻合，回收率在９９．４％～１００．６％，方法简便，快速，准确，制剂中的赋形剂及色物不影响测定。     

Mechanistic Studies on the Oxidation of Hydroquinone by an Oxo-bridged Diiron(III,III) Complex in Weakly Acidic Aqueous Media

The kinetics of oxidation of hydroquinone (H₂Q) by a μ-oxo-bridged diiron(III,III) complex, Fe₂(μ-O)(phen)₄(H₂O)₂]⁴⁺ (1) has been investigated in aqueous media at 25.0 °C in presence of an excess of 1,10-phenanthroline (phen). The overall redox rate increases with increase in [H⁺]. The title complex (1) and its conjugate bases, [Fe₂(μ-O)(phen)₄(OH)₂]³⁺(2) and [Fe₂(μ-O)(phen)₄(OH)₂]²⁺ (3), participate in the reaction with H₂Q as the only kinetically reactive reducing species. Rate constants (in dm³ mol⁻¹ s⁻¹) for the parallel reactions (1) + H₂Q → Products, (2) + H₂Q → Products and that for (3) + H₂Q → Products are, respectively, 500 ± 40, 100 ± 6 and 30 ± 2. Substantial rate retardation in D₂O media in comparison to that in H₂O media suggests that electron transfer is coupled with proton movements in the rate-determining step.     

荧光各向异性法均相检测酸性、中性和碱性介质中的蛋白酶

荧光各向异性;罗丹明;蛋白酶;胰蛋白酶;胃蛋白酶;菠萝蛋白酶;蛋白酶抑制剂     

Kinetics of Hydrolysis of Bisoprolol Hemifumarate in Aqueous Acidic Solutions

The kinetics of hydrolysis of bisoprolol hemifumarate in acidic conditions was studied using high‐performance liquid chromatography. For this purpose, different hydrohalic acids and one weak carboxylic acid were used. The rate constants, the order of the reaction, and the activation parameters: enthalpy, entropy, and energy of activation were calculated. A proposition for the mechanism of degradation was provided.     

Hydrolysis of Chloro(silyl)olefins in Neutral and Acidic Media

Action of water on chloro[chloro(alkoxy)silyl]olefins with various number of chlorines in the unsaturated moiety was studied in acidic and neutral media. Linear and cyclic oligosiloxanes with unsaturated chlorine-containing groups were obtained. Correlation between the hydrolytic stability of the Si-Csp ² bond and the structure of the starting chlorosilylolefin was explored.     

Kinetics of Acid Blue 1 Adsorption from Aqueous Solution by Carbonaceous Substrate Produced from Biotic Precursor

Adsorption of acid blue 1 from aqueous solution onto carbonaceous substrate produced from the wood of Paulownia tomentosa was investigated. The samples characterized by FTIR, SEM, EDS and XRD techniques, indicated that the surface functional groups like carboxyl, lactones or phenols and ethers have disappeared at high activation temperature （800 ℃） and as a result porous structure was developed that has a positive effect on the adsorption capacity. Bangham and parabolic diffusion models were applied to the kinetic adsorption data, which show that the adsorption of acid blue 1 was a diffusion controlled process. The reaction rate increased with the increase in temperatures of both the adsorption and activation. Thermodynamic parameters like △E^≠, △H^≠, △S^≠ and △G^≠ were calculated from the kinetic data. The negative values of △S^≠ reflected the decrease in the disorder of the system at the solid-solution interface during adsorption. Gibbs free energy （△G^≠）, representing the driving force for the affinity of dye for the carbon surface, increased with the increase in sample activation and the adsorption temperatures.     

Kinetics and Mechanism of Permanganate Oxidations of Isosorbide in Different Acidic Media

The oxidation kinetics of isosorbide (S) by potassium permanganate in both perchloric and sulfuric acid solutions was investigated spectrophotometrically at a constant ionic strength of 2.0 mol·dm^?3 and at 25 °C. In both acids, the oxidation reactions showed a first-order dependence on \([{\text{MnO}}_{4}^{ - }]\), apparent a less than unit-order dependence with respect to [S] and a fractional-second-order dependence with respect to [H⁺]. Variation of either the ionic strength or dielectric constant of the reactions media did not significantly affect the oxidation rates. In both acids, the final oxidation product of isosorbide was identified by both spectroscopic and chemical tools as the corresponding monoketone derivative, namely (1S,4S,5R)-4-hydroxy-2,6-dioxabicyclo[3.3.0] octan-8-one. Under comparable experimental conditions, the oxidation rate of isosorbide in perchloric acid was lower than that in sulfuric acid. The oxidation mechanism describing the kinetic results was proposed and the rate law expression was derived. The activation parameters of the second-order rate constants were computed and are discussed.     

Electroactivity of Cacodylic Acid in Aqueous and Nonaqueous Media

Abstract

The electroreduction of dimethylarsenic acid (cacodylic acid), (CH₃)AsO₂H, and methylarsonic acid, (CH₃)ASO₃H₂, in both nonaqueous and aqueous buffer electrolytes, is described.     

Precipitation and Solubility of Calcium Soaps in Basic Aqueous Media

Measurements of the surface tension of aqueous media containing calcium salts of different fatty acids, whose structure only varied by the number of C=C unsaturations along their aliphatic chains, showed that their solubility increased as a function of this parameter going from near zero for calcium stearate to 4x10(-4) mol l(-1) for the linolenate homologue. The solubility product of the calcium soap corresponding to each tested fatty acid was estimated by using the anionic fatty acids adsorption isotherms in order to obtain the amount of dissolved ionic fatty acid at increasing concentration of calcium ions. These findings are discussed in terms of their serious repercussions on the operations related to flotation deinking in waste paper recycling. Copyright 2001 Academic Press.     

水介质中丙烯酸的分散聚合

丙烯酸聚合物及其与其它水性单体的共聚物是一类非常重要的水溶性高分子化合物, 具有许多优异的性能, 广泛应用于环保、 石油化工、 造纸和食品卫生等行业^[1]. 丙烯酸聚合物一般采用水溶液、 反相悬浮及反相乳液法制备, 但这些方法存在诸如反应体系粘度高, 不易散热、 使用不方便, 由于使用有机溶剂和表面活性剂易对环境造成二次污染等问题^[2].近些年, 由日本率先研制开发的以水为溶剂分散型高浓度﹑高分子量的新型水溶性高分子产品, 克服了传统合成方式和产品剂型等诸多问题, 极大地拓宽了其使用领域[^3~5]. 有关水介质中水溶性单体分散聚合的研究报道很少^[6~8].而针对于丙烯酸在水介质中的研究报道则更少[^9] , 大部分工作为专利文献.     

Enzyme Mimetic Active Intermediates for Nitrate Reduction in Neutral Aqueous Media

Nitrate is a pervasive aquatic contaminant of global environmental concern. In nature, the most effective nitrate reduction reaction (NRR) is catalyzed by nitrate reductase enzymes at neutral pH, using a highly‐conserved Mo center ligated mainly by oxo and thiolate groups. Mo‐based NRR catalysts mostly function in organic solvents with a low water stability. Recently, an oxo‐containing molybdenum sulfide nanoparticle that serves as an NRR catalyst at neutral pH was first reported. Herein, in a nanoparticle‐catalyzed NRR system a pentavalent Mo^V(=O)S₄ species, an enzyme mimetic, served as an active intermediate for the NRR. Potentiometric titration analysis revealed that a redox synergy among Mo^V?S, S radicals, and Mo^V(=O)S₄ likely play a key role in stabilizing Mo^V(=O)S₄, showing the importance of secondary interactions in facilitating NRR. The first identification and characterization of an oxo‐ and thiolate‐ligated Mo intermediates pave the way to the molecular design of efficient enzyme mimetic NRR catalysts in aqueous solution.     

Decomposition Products ofs-Triazine Herbicides by Electron-Transfer in Acidic Aqueous Media

The reduction of atrazine and terbutylazine was preceded by protonation equilibrium. Three protonation sites of thes-triazine molecule determined the structure of the final reduction product. Protonation was investigated by the change of UV–Vis spectra. Two slightly different pKs corresponding to protonation on N5 and N1 heteroatoms were evaluated. The principal reduction pathway involved the cleavage of a chlorine atom. A small quantity of desethylatrazine was detected in the most acidic media.     

Reactions of 2-Alkoxypropenals with Thiols in Neutral and Acid Media

Addition of thiols to 2-alkoxypropenal in neutral medium at 20°C in the absence of a catalyst occurs regioselectively, following the Markownikoff pattern. The resulting 2-alkoxy-2-R-thiopropanals are capable of undergoing spontaneous isomerization to 1-alkoxy-1-R-thiopropanones. The addition and isomerization processes are accelerated by heating to 60°C or in the presence of acid catalysts (TsOH, HCl). The reaction is also accompanied by partial disproportionation of 2-oxopropanal O,S-acetals to give O,O- and S,S-acetals.     

弱碱性大孔吸附树脂对腐殖酸的吸附

研究了腐殖酸分子量对弱碱性大孔树脂吸附腐殖酸的影响, 阐明了溶液中小分子芳环化合物(苯酚)及盐含量对树脂吸附腐殖酸的影响机制. 结果表明, 树脂对低分子量腐殖酸的吸附效果要优于高分子量腐殖酸; 低浓度苯酚在溶液中可以促进树脂吸附腐殖酸, 但溶液中苯酚浓度过高会对树脂吸附腐殖酸产生抑制作用; 溶液中的盐对树脂吸附腐殖酸的影响取决于溶液的pH值.     

Critical Switching of Cyclization Modes of Polyepoxides in Acidic Aqueous Media and Neutral Water: Synthesis and Revised Structure of a Nerolidol‐Type Sesquiterpenoid

Biomimetic epoxide‐opening cascades of polyepoxides enable the efficient and rapid construction of polyether frameworks. Herein, we show that the epoxide‐opening cascade cyclization that affords tetrahydrofuran products in acidic aqueous media produces tetrahydropyran (THP) in neutral water. THP formation proceeded by simply heating polyepoxides in neutral water and followed a different cyclization mode from those observed so far. The novel cascade cyclization in H₂O was applied to the synthesis of a new nerolidol‐type sesquiterpenoid, resulting in revision of the proposed structure and determination of the absolute configuration.     

Formation and Out-of-Equilibrium,High/Low State Switching of a Nitroaldol Dynamer in Neutral Aqueous Media

The nitroaldol reaction is demonstrated as an efficient dynamic covalent reaction in phosphate buffers at neutral pH. Rapid equilibration was recorded with pyridine-based aldehydes, and dynamic oligomerization could be achieved, leading to nitroaldol dynamers of up to 17 repeating units. The dynamers were applied in a coherent stimuli-responsive molecular system in which larger dynamers transiently existed out-of-equilibrium in a neutral aqueous system rich in formaldehyde, controlled by nitromethane.     

Spontaneous Polymerization of 2-Acrylamido-2-methylpropanesulfonic Acid in Acidic Aqueous Solutions

Spontaneous polymerization of 2-acrylamido-2-methylpropanesulfonic acid in the presence of inorganic and organic neutralizing agents in concentrated aqueous solutions is studied. The hypothesis of redox mechanism of the process initiation is discussed.     

Kinetics and Modeling of Methacrylic Acid Radical Polymerization in Aqueous Solution

Batch radical polymerization of non‐ionized methacrylic acid, 30 wt.‐% in aqueous solution, has been studied at 50 °C and ambient pressure with 2‐mercaptoethanol (ME) as the chain‐transfer agent (CTA). Initial polymerization rate decreases with CTA concentration, which has been varied up to 20 mol‐%. A kinetic model is presented which includes chain‐length‐dependent termination and uses an empirical function to account for the dependence of termination rate on both monomer conversion and molar mass of the polymeric product. In conjunction with PREDICI simulation, this model affords for an adequate representation of the measured monomer conversion vs. time profiles.

     

Kinetics and Mechanism of Oxidation of Captopril by Hexacyanoferrate(III) in Aqueous Acidic Medium

Captopril (Capt, 1-[2(s)-3-mercapto-2-methyl-1-oxopropyl]-l-proline) was oxidized by hexacyanoferrate(III) (HCF). The kinetics of the oxidation was studied spectrophotometrically at 420 nm. The reaction was found to be first order in [HCF] and [Capt] and to have a negative fractional order in [H⁺]. Oxidation was followed by generation of a free radical from captopril, and the oxidative product of catpotpril was identified as captopril disulfide. It was characterized by IR, GCMS and ESI–MS spectra. Initially added product, hexacyanoferrate(II), retarded the rate of reaction with an order of ?0.5. The retarding effect of added [H⁺] indicated that unprotonated hexacyanoferrate(III) is the active species. A suitable free radical mechanism was proposed. The rate law was derived and verified.     

Kinetics of Oxidation of Diphenylmethane and Derivatives with Ce(IV) in Aqueous Acidic Acetonitrile

In aqueous acidic acetonitrilc solution cerium(IV) oxidizes diphenylmethane (Ph₂CH₂) to produce diphenylmethanol (Ph₂CHOH) first and then benzophenone (Ph₂CO). With the organic reductant in great excess, both the Ce(IV)-Ph₂CH₂ and Ce(IV)-Ph₂CHOH rcactions follow second-order kinetics. The rates of both reactions increase nonlinearly with increasing [H⁺] or with decreasing [H₂O]. Both kinetic and spectrophotometrc results indicate that replacement of water molecules by complex formation through the phenyl or hydroxyl group plays an important role in activating the reaction. Under similar conditions, the order of relative reactivities toward Ce(IV) oxidation is PhCH₂OH > CH₃CH₂OH > Ph2CHOH > Ph₂CH₂ > PhCH₃> (Ph₂CO, C₆H₆). Mechanistic interpretations of the kinetic results are presented.