Stoichiometry and chemical metrology: Karl Fischer reaction

 Stoichiometry of analytical reactions is discussed as an important element of many primary methods (gravimetry, volumetry, spectrophotometry, etc.) and therefore, of chemical metrology as a whole. Variations in stoichiometry caused by non-equilibrium conditions, by changes in the reaction medium or by other factors can be a source of the dominant uncertainty component in the analytical measurement. Such a situation is illustrated with the Karl Fischer (KF) reaction which is widely used in aquametry. Two kinds of solvents used as a part of the reaction medium – alcohols and amides – are compared. The influence of the media on the stoichiometry of the reaction and, correspondingly, on the titre of the KF reagent is evaluated. Received: 4 November 1998 · Accepted: 22 December 1998     

Structure, intramolecular flexibility, and complexation of aza crown ethers to anions H2PO4 and HSO4 in nonprotic solvents

Both the structure and intramolecular flexibility of a series of aza crown ethers were studied by experimental NMR and theoretical molecular modeling. The stoichiometries of complexation to the anions H₂PO₄⁻ and HSO₄⁻ and resulting complex stabilities were determined by experimental NMR (¹H, ³¹P) titration and, in addition, the structure and mobility changes of the aza crown ethers upon complexation were also examined.     

Determination of aqueous inclusion complexation constants and stoichiometry of alkyl(methyl)-methylimidazolium-based ionic liquid cations and neutral cyclodextrins by affinity capillary electrophoresis

Affinity CE (ACE) method was developed to characterize the complex formation between seven alkyl(methyl)methylimidazolium-based ionic liquid (IL) cations and eight neutral cyclodextrins (CD). The effective mobility data of the IL cations were processed according to classical nonlinear and linear treatments to obtain the complex stoichiometry and formation constant K. The majority of systems followed a 1:1 complexation stoichiometry model but in four cases a 1:2 stoichiometry was better satisfied. The K values obtained for each IL were compared to elucidate the main influences of IL and CD nature. The availability of these data should lend support to various application areas, including the screening and tailoring of new interactions in the solution for CE.     

化学计量比和放氢背压对LiBH4+xMg2NiH4体系放氢性能的影响

研究了不同化学计量比(x=0.25, 0.5, 0.75, 1.0, 1.25)和放氢背压(1×10-4和0.4 MPa)对LiBH4+xMg2NiH4复合体系吸放氢性能的影响. 结果表明, 随着化学计量比(x)的增加, 复合体系的放氢温度逐渐降低, 放氢动力学性能得到提高, 但放氢容量逐渐降低; 其中, 在1×10-4和0.4 MPa初始放氢背压下, LiBH4+0.75Mg2NiH4体系具有最佳放氢动力学性能和较高的储氢容量. 结果表明, 放氢背压和化学计量比均会对高温下液相LiBH4 与固态Mg2NiH4 的润湿性产生影响, 进而影响复合体系的放氢路径和放氢动力学性能.     

α- and β-cyclodextrin complexes with n-alkyl carboxylic acids and n-alkyl p-hydroxy benzoates. A molecular mechanics study of 1:1 and 1:2 associations

Molecular mechanics (MM) methods were employed to evaluate stabilization upon formation of inclusion compounds between two different guest molecules and α- and β-cyclodextrins (CDs) for two different stoichiometries 1:1 and 1:2. The two guest molecules studied were n-alkyl carboxylic acids and n-alkyl p-hydroxy benzoates with variety of chain lengths. The computed stability for the inclusion compounds between α-CDs and n-alkyl carboxylic acids reproduced experimental data reported in the literature. The transition between 1:1/1:2 complexes occurred at an alkyl chain length of n_C=9. It was previously demonstrated by diffusion coefficients measures that a stable 1:2 stoichiometry inclusion compound could be formed between n-alkyl p-hydroxy benzoates and α-CD for the chain length n_C>4. The computed results reproduced the experimental ones. The combination between OPLS and GB/SA resulted in better agreements with experiments than those obtained with MM2 and MM3.     

不同整比性VO2纳米粉体的合成

以V₂O₅,HCl,N₂H₄·2HCl和NH₄HCO₃为原料合成了紫红色多晶(NH₄)₅[(VO)₆(CO₃)₄(OH)₉]·10H₂O(I),产率80％～95％.在N₂气流中对前体I进行了TG/DTG和DTA热分析.研究了前体I在N₂-空气气流中热解时氧分压(po₂)和反应温度对产物VO₂整比性的影响.结果表明,在500℃条件下,VO_x中的x值与po₂呈正比关系,在po₂=30.3Pa条件下,在450～520℃区间,产物为缺氧VO₂;在t<450℃和t>520℃时,产物为富氧VO₂.在合适的条件下,成功地合成了从VO_1.927到VO_2.080之间不同整比性的VO₂粉体.产物组成均匀,其XRD谱未观察到非VO₂的杂质峰.VO₂粉体颗粒的平均粒径为20nm,粒度分布均匀并呈球形.     

Polarographic Study of Thallium(I) Complexes with Large Crown Ethers in Binary Acetonitrile-Water Mixtures

The complexation reactions between the Tl⁺ ion and large crown ethers dibenzo-30-crown-10 (DB30C10), dibenzo-27-crown-9 (DB27C9), dibenzo-24-crown-8 (DB24C8) and dibenzo-21-crown-7 (DB21C7) were studied in different acetonitrile-water mixtures at 25°C using an a.c. polarographic technique. The stoichiometry and stability of the complexes were determined by monitoring the shift in peak potential of the polarographic waves of the metal ion against the crown concentration. In all solvent mixtures used, the stability of the resulting 1:1 complexes was found to vary in the order DC24C8 » DB30C10 > DB21C7 > DB27C9 > DB24C8. There is an inverse relationship between the complex formation constants and the amount of water in the mixed solvent. In all cases, a linear relation was observed between log K_f and the mole fraction of acetonitrile in its mixtures with water.     

Magnetic phase diagram simulation of La1−xCaxMnO3 system by using Monte Carlo, Metropolis algorithm and Heisenberg model

This work presents a critical temperature study of La_1−xCa_xMnO₃ manganites in bulk by means of Monte Carlo method thermal activated magnetic properties. The analysis was carried out for stoichiometries in the range of 0≤x≤1. The model is based on a three-dimensional classical Heisenberg-Hamiltonian involving the presence of Mn^3+eg, Mn^3+eg′ and Mn⁴⁺ ions, and their nearest neighbor interaction. For this modeling, simple cubic lattice samples of size L³, with L=6, 15 and 30 were used. The values of exchange parameters were determined by using LaMnO₃ (x=0), La_0.5Ca_0.5MnO₃ and CaMnO₃ (x=1) phases. Relationships between exchange parameters and anisotropy constants for different hole densities were found. Results of transition temperatures for each phase showed good agreement with experimental reports, especially for L=30 and L→∞.     

Radical scavenging ability of polyphenolic compounds towards DPPH free radical

Free radical scavenging activity of different polyphenolic compounds commonly present in wine has been evaluated using DPPH method. The experiments were performed with different amounts of phenols within the linear interval of response and with an excess of DPPH in all cases. In these conditions, for most of the compounds tested, the reaction was biphasic. Total stoichiometry values n confirm the implication of more than one step in the process. Flavan-3-ol compounds showed the highest values, especially procyanidins B1 (9.8) and B2 (9.1). In this family, n values coincide with the number of hydroxyl groups available. EC₅₀ and TEC₅₀ parameters have been calculated. EC₅₀ values are extremely diverse, being the procyanidins B1 and B2 the most potent scavenging compounds and resveratrol the less one. TEC₅₀ considers the rate of reaction towards the free radical. (+)-Catechin and (−)-epicatechin are the phenolic compounds that need more time to react. In contrast, caftaric and caffeic acids are the phenolic acids that react more rapidly. Antioxidant efficacy (AE) is a parameter that combines both factors. Compounds as kaempferol, with a high EC₅₀ value, could be considered as an antioxidant with low relevance, but instead shows the highest AE value of the phenolic compounds tested, due to its fast rate of reaction, what is of great biological importance.     

Evaluation of departure from stoichiometry of gallium arsenide crystals

A method is described for determining arsenic in gallium arsenide. In order to avoid the conventional pre-reduction process, arsenic is directly titrated iodimetrically as As(V) in acidic solution. The determination is made with an automatic titrator and potentiometric detection of the equivalent point. Pure arsenic is used as a reference substance. Deviations from the stoichiometric composition of the order of 10^?3-10^?4% with a standard deviation of about 0.06% can be determined. The limits of chemical analysis for controlling the stoichiometry of the semiconductor are discussed.