Effect of form of reagent on particle sizes of precipitates

The particle size of an ionic precipitate is much smaller when one of the reactants is introduced as a solid rather than as a solution The effect is enhanced as the particle size of the solid reagent is decreased. The presence of a freshly dissolved inert electrolyte also influences the particle size of a precipitate The affects are not attributable merely to the degree of supersaturation but rather to the provision by the freshly dissolved material of sites upon which nucleation can be induced.     

Shear modulus of sintered 'house of cards'-like assemblies of crystals

A cell model of a 'house of cards'-like assembly of crystals is used for the study of the evolution of the shear modulus during sintering. The crystals are assumed to have a lozenge shape. The cell model takes different crystal-crystal contacts into account. The force needed to separate two sintered crystals is calculated using the minimum surface area (MSA) approximation. By varying the thickness, long axis, and short axis of the crystals, it is possible to make space-filing configurations which have a nonzero shear modulus at crystal volume fraction that can be as low as phi = 0.03. This is significantly lower than the volume fractions phi > 0.52 that are found in studies where the MSA approximation is applied to assemblies of spherical particles. It is found that sintering may cause a nonlinear volume fraction dependence of the shear modulus, which depends on the shape of the crystals, the type of crystal-crystal contacts, and the character of the crystal assembly. The calculated shear modulus is analyzed using the phenomenological expression (phi - phi0)beta, where phi0 represents the volume fraction at the start of sintering. The exponent beta is found to vary between 1 and 2. The interpretation of the shear modulus using a fractal model is also discussed.     

掺杂剂对聚乙炔性质影响的理论研究

以多烯烃模拟聚乙炔链,用CNDO/2方法讨论了各种掺杂剂对聚乙炔性质的影响,掺杂剂使孤子宽度收缩变窄,且p型掺杂剂比n型掺杂剂的影响更大,掺杂剂影响聚乙炔链中的电荷密度波,使电荷主要集中分布于掺杂剂附近的碳原子上。掺杂碱金属时,掺杂剂原子的最高占据轨道与聚乙炔中孤子自旋轨道之间的作用由Li到K依次增强,解释了ESR实验结果。     

Kinetic study of the dehydration of modified Y zeolites

The dehydration of ferric exchanged Y zeolites is studied by thermal analysis. Their DTA shows three endotherms in the temperature range 80–450°C. The order of reaction and apparent energies of activation are calculated using various equations. The order of dehydration is nearly one and the apparent energy of activation is 4–8 kcal mole^?1. The effect of heating rate is studied. The energy of activation as determined by the Kissinger and Ozawa method is about 12 kcal mole^?1, which is comparable with the heat of adsorption of water determined by the gravimetric method, and is more acceptable.     

Quantitative Evaluation of the Influence of Physicochemical Properties of Solvents on the Kinetics of Formation of Phosphorus-Containing Semicarbazides

The kinetics of the reaction of diphenylphosphinic acid hydrazide with phenyl isocyanate in various solvents at 25°C was studied. The results can be quantitatively correlated with the physicochemical characteristics of solvents using the four-parameter Koppel-Palm equation. This relationship allows quantitative analysis of the solvation effects. The decisive influence on the reaction rate is exerted by the basicity and polarity of solvents. The reaction mechanism is discussed, and the possible scheme of solvation is suggested.     

Spectrophotometric determination of traces of selenium with diaminobenzidine

Diaminobenzidine is suitable for the spectrophotometric determination of traces of selenium. The reaction is very sensitive, the molecular extinction coefficient being as high as 10,200. The reaction is specific, the most common ions causing no perturbation. The procedure is especially valuable for the determination of traces selenium in tellurium and in sulfur, provided that the latter is present in the sulfate form.     

Infrared determination of traces of sulfate in reagent chemicals

A method for determining traces of sulfate in reagent-grade chemicals is presented. The sample is passed through an alumina column in acid solution to isolate the sulfate, which is then eluted with ammonia. Potassium bromide is added to an aliquot of the eluate, and the solution is freeze-dried. The resulting powder is pressed into a disk whose infrared spectrum is recorded. The method is reasonably accurate and requires a minimum of operator time per sample because a large number of samples can be processed together     

Mechanism of oligomerization reactions of silica

The mechanism of the oligomerization reaction of silica, the initial step of silica formation, has been studied by quantum chemical techniques. The solvent effect is included by using the COSMO model. The formation of various oligomers (from dimer to tetramer) was investigated. The calculations show that the anionic pathway is kinetically preferred over the neutral route. The first step in the anionic mechanism is the formation of the SiO-Si linkage between the reactants to form a five-coordinated silicon complex, which is an essential intermediate in the condensation reaction. The rate-limiting step is water removal leading to the oligomer product. The activation energies for dimer and trimer formation ( approximately 80 kJ/mol) are significantly higher than those of the subsequent oligermerization. The activation energies for the ring closure reaction ( approximately 100 kJ/mol) are even higher. The differences in activation energies can be related to the details in intra- and intermolecular hydrogen bonding of the oligomeric complexes.     

聚氨酯改性不饱和聚酯树脂的反应动力学

通过差示扫描量热法(DSC)和傅里叶红外光谱(FT-IR)分析,探讨了聚氨酯改性不饱和聚酯树脂(UPR)体系反应特征及固化反应动力学。结果表明:体系中聚氨酯网络的形成远快于UPR网络的形成。UPR和聚氨酯改性UPR体系的表观活化能分别为58.0 kJ/mol和64.8 kJ/mol,改性后体系的活化能并无较大变化;反应的级数分别为0.90和0.91。     

Thermal-diffusive behavior of a dilute solution of charged colloids

Thermal diffusion of a dilute solution of charged silica colloidal particles (Ludox) is studied by a holographic grating technique. The Soret coefficient of the charged colloids is measured as a function of the Debye screening length and the surface charge density of the colloids. The latter is varied by means of variation of the pH. The experimental Soret coefficients are compared with several theoretical predictions. The surface charge density is independently obtained from electrophoresis measurements, the size of the colloidal particles is obtained from electron microscopy, and the Debye length is calculated from ion concentrations. The only adjustable parameter in the comparison with theory is therefore the intercept at zero Debye length, which measures the contribution to the Soret coefficient of the solvation layer and possibly the colloid core material.     

珍珠片剂中钙含量的测定

采用络滴定法对复方珍珠片剂中的钙含量进行了测定,相对标准偏差０．０２％,回收率９９．８％,方法简便易行,重现性好。     

Influence of the degree of ripeness of cotton seeds on the fatty acid composition of the lipids

The total fatty acids of cotton seeds include 31 components. The 18:3 acid that is the main component of the early-ripe seeds is present mostly in the mono-, di-, and triacylglycerides. The distribution of the fatty acids in the triacylglycerides of the early-ripe seeds is not characterized by selectivity.     

Applications of prompt particle spectrometry to studies of tinplate surfaces

The use of alpha backscattering is described for the study of oleo-resinous lacquers on tinplate. The lacquer composition is measured as a function of curing temperature, duration of stoving and ageing. The effect of reheating understoved specimens is evaluated and the decomposition during alpha bombardment is measured. The use of prompt proton spectrometry from (d, p) reactions is described for the analysis of stains on tinplate surfaces produced by interaction with protein-rich foods.     

Mechanism of ultraviolet bonding of perfluoropolyethers revisited

The mechanism of ultraviolet (UV) bonding of perfluoropolyether (PFPE) boundary lubricants on magnetic disk surfaces is re-examined experimentally. It is found that UV-emitted photoelectrons may contribute a negligible part, and instead the UV bonding correlates with the direct photodissociation of PFPE molecules. The UV-induced photodissociation is demonstrated to occur almost randomly on the PFPE molecular chain. The contribution from photooxidation is eliminated under nitrogen purge.     

Kinetics of the formation of Liesegang rings

The kinetics of the formation of Liesegang rings is considered and they are classified. The relationship between the position of the propagating diffusion front and the moments of the formation of Liesegang rings is shown. In order to describe the formation processes of the Liesegang rings a generalized model based on the diffusion equation hierarchy is proposed. The kinetics of changes in the dispersed phase sizes at the initial stage of the formation of Liesegang rings is studied.     

Formation of particles of metal hydroxides in water solution of polymer

The stages of the formation of metal hydroxide particles in water medium were described. The first stage is the formation of complexes between the stabilizer and metal ion or of metal polyions containing a few metal ions. Dependence between the number of metal ions in polyion and its charge on the pH is described. The second stage is the formation of the metal hydroxide particles by the aggregation of polyions or their adsorption on/in the stabilizer. The distribution of the polyion number in the particles in polyion aggregation is described by the equation p(k) = k(ks-1)(k-1)/ksk and in polyion adsorption on/in the stabilizer particles is more narrow and is described by the equation p(k) = exp(-ks)/ks(-k)/k!, where k and ks are the number and the average number of polyions in the particle.     

Calculation of the efficiency of purification by crystallization of ideal multicomponent stereoisomeric mixtures

A method for evaluating the maximum theoretical recovery of pure component from a stereoisomeric mixture by crystallization is discussed. The approach is based on the approximation of ideal behaviour as exemplified by the Schroeder–van Laar equation for conglomerates, and the Prigogine–Defay equation for racemic compounds. These equations are used to calculate the n-dimensional eutectic. It is shown that the n-dimensional eutectic is more soluble than the (n−1)-dimensional eutectic. A calculation is performed which removes only the n-dimensional eutectic from the sample and calculation of recovery through this discrete purification step is possible. After this, a system of n−1 components is obtained. The calculation may be repeated until only one component remains. The overall recovery of pure isomer is the product of the recoveries at each discrete purification step. The procedure is well suited to computer algorithm and a summary listing is presented. A series of example calculations are included for reference purposes. The methodology is intended to be useful in guiding efforts to improve process economics.     

Computation of the Effective Volumes of Covalently Bonded Molecules

An analytical algorithm for the calculation of molecular volume based on a model of partially overlapping spheres is presented. The algorithm takes into account the volume of overlapping caps of spheres and includes the contribution to the molecular volume by interstitial voids. The algorithm is constructed in a manner to ensure generality. The working of the algorithm is illustrated by examples of some different types of basic molecular structures. The Mathematica program is available.     

Characterization of liquid products of automobile tire pyrolysis

The liquid product of automobile tire pyrolysis and its fractions were studied. The amount of the liquid hydrocarbon fraction is 40–45% of the total weight of pyrolysis products. The hydrocarbon fraction is similar in its characteristics to low-sulfur heavy crude oil. Fractionation of the hydrocarbon fuel was performed. The maximal yield of the light distillation fraction is 39% (temperature interval 180–340°C). The extent to which the characteristics of the light fraction obtained meet the regulations was evaluated. The light fraction of hydrocarbon fuel produced by pyrolysis is recommended for use as a component of diesel motor fuel after additional purification.     

小环化合物中饱和碳质子化学位移的计算

小环化合物由于其张力、构型、构象和各向异性效应等原因,环碳上质子化学位移缺乏规律性,难以预测,对此作者曾提出一种近似算法。本文根据303种小环化合物中饱和碳质子的化学位移实验数据,将适于计算这类质子化学位移的公式表述为: