Apparent solubility of hydroxyapatite in aqueous medium and its influence on the morphology of nanocrystallites with precipitation temperature

Two differing wet-chemical synthesis routes, Ca(OH)2 + H3PO4 and CaCl2 + Na3PO4/NaOH, were used to prepare hydroxyapatite (HA) at various temperatures ranging from 30 to 95 degrees C. The electrical conductivity of the solution was measured at regular intervals of time during H3PO4 and Na3PO4 addition to the suspension/solution containing Ca2+ ions. The rate of change of conductivity is used to note the end point of the reaction. X-ray diffraction of the dried, precipitated particles revealed HA as the predominant phase, and the FTIR spectroscopy studies indicated the presence of CO3(2-) groups which substituted PO4(3-) groups in the HA lattice (B-type). FESEM observations revealed that the aspect ratio of the particles decreased with increasing precipitation reaction temperature in one system [Ca(OH)2 + H3PO4] and in the other system it increased with increasing temperature. The changes in the morphology with temperature were analyzed through conductivity measurements and the thermochemical properties of the reaction systems. Conductivity measurements showed that the concentration of dissolved ions at the end point of the reaction between Ca(OH)2 and H3PO4 increased, indicating an increased apparent solubility of HA with increasing temperature, whereas the end-point conductivity did not increase noticeably in the other reaction system.     

蒸汽相中含硼多孔玻璃自转变合成沸石——Ⅱ．自转变动力学

以XRD为表征手段详细研究了408—453K温度范围、多孔玻璃粉在乙胺和水的蒸 汽相中自转变为B-Al-MFI型沸石的自转变动力学。由动力学曲线计算得到的成核活 化能和晶体生长活化能分别是102．0kJ／mol和34．7kJ／mol。沸石的结晶速率显 著受温度影响，低温有利于晶体成核和形成小晶体，高温有利于晶体生长和形成大 晶体。不同反应温度下的结晶速率几乎在相同时间内(cα．132h)达到最大值(νm) 。     

Kinetics of chlorine evolution on dimensionally stable anodes at high currents: Extending the concept of a self-accelerating electrode process

The chlorine evolution kinetics are studied by taking steady-state polarization measurements on rotating disc electrodes (dimensionally stable ones and those made of RuO₂) in solutions with various chloride concentration, at various acidity and temperatures, at current densities of up to 4 A/cm². At high temperatures and chloride concentrations, linear parts with an abnormally low slope of about 20 mV are observed in anodic polarization curves. Their appearance is attributed to the chlorine reaction self-acceleration due to a change in the chlorine mass transport mechanism in the anode’s porous coating.     

利用手持技术数字化实验促进学生对配合物概念的学习*——以铜氨配合物的形成和破坏过程为例

从高中“配合物”概念教学中值得探讨的问题出发,以手持技术TQVC 概念认知模型为理论依据,利用手持技术pH和电导率传感器对铜氨配合物的形成与破坏过程进行探究。首先以氨水滴定硫酸铜溶液,然后以硫酸反滴定铜氨配合物溶液。与此同时,利用手持技术获取反应过程中的pH和电导率变化曲线。再根据四重表征模型,从宏观、微观、符号、曲线4个角度对反应过程进行分析,从而揭示反应的本质,多维度实现“配合物”概念的有效建构。     

ω-氨基酸缩聚反应动力学研究

前曾报导9-氨基壬酸的缩聚,从开始到反应程度p增加到99％左右时止,均遵循二级反应。在这以后,由于体系粘稠的影响,迅速转为三级反应。本文报导在6-氨基己酸和11-氨基十一酸缩聚时,也出现同样现象,即在p逐渐到高于99％左右时,反应从二级转变为三级。可以说这一现象是AB型单体聚酰胺化的一般规律,从而澄清了文献中对这一问题的不同论点。     

Analyse thermique differentielle et cinetique de reaction

Equations are given for describing the differential temperature curves of phase transformation (fusion and solidification) and allotropic transformations, as well as of dismutation reactions proceeding without any change in weight. It is shown how the obtained relationships are modified according to whether the reaction proceeds with or without weight change. Finally, the properties of the derivative d/dt gDT are established theoretically that can be used to advantage in the investigation of heterogeneous kinetics on the basis of DTA curves.     

Pitfall of an initial rate study: on the kinetics and mechanism of the reaction of periodate with iodide ions in a slightly acidic medium

The kinetics of the periodate-iodide reaction has a contradictory history dating back to almost a century. This reaction has been reinvestigated spectrophotometrically in the pH range 3.13-5.55 in both buffered (acetic acid/acetate) and unbuffered solution at T=25.0+/-0.1 degrees C with an I=0.5 M ionic strength. The spectra between 290 and 500 nm were recorded and the reaction was followed until at least 95% of one of the reactants was consumed. The stoichiometry has been found to be strongly dependent on pH, but the rate of the initial step is independent of pH within the pH range studied. An eight-step kinetic model is proposed with four fitted kinetic parameters to take all the important characteristics of the experimental curves into account. On the basis of the model, a perfect reconciliation of the previous contradictory results is presented. It is shown that the kinetic parameters obtained from the initial rate of formation of a product unavoidably leads to misinterpretation of the results in the case of a branching mechanism (and stoichiometry).     

Reasons for Structure-Acid Resistance Correlation in Series of Substituted Phenylpenicillins

The kinetics of acid hydrolysis of phenylpenicillins substituted in the benzene ring were studied at pH 3.0 and temperatures of 20, 25, and 30°C. A Hammett correlation analysis of the reaction was performed.     

Synthesis of silicon doped SrMO3 (M = Mn, Co): stabilization of the cubic perovskite and enhancement in conductivity

In this paper we report the successful incorporation of silicon into SrMO(3) (M = Co, Mn) leading to a structural change from a hexagonal to a cubic perovskite. For M = Co, the cubic phase was observed for low doping levels (3%), and these doped phases showed very high conductivities (up to ≈350 Scm(-1) at room temperature). However, annealing studies at intermediate temperatures (700-800 °C), indicated that the cubic phase was metastable with a gradual transformation to a hexagonal cell on annealing. Further work showed that co-doping with Fe resulted in improved stability of the cubic phase; a composition SrCo(0.85)Fe(0.1)Si(0.05)O(3-y) displayed good stability at intermediate temperatures and a high conductivity (≈150 Scm(-1) at room temperature). For M = Mn, the work showed that higher substitution levels were required to form the cubic perovskite (≈15% Si doping), although in these cases the phases were shown to be stable to annealing at intermediate temperatures. Conductivity measurements again showed an enhancement in the conductivity on Si doping, although the conductivities were lower (≈0.3-14 Scm(-1) in the range 20-800 °C) than the cobalt containing systems. The conductivities of both systems suggest potential for use as cathode materials in solid oxide fuel cells.     

仲辛基苯氧基乙酸从稀土中萃取分离钪及其机理

首次研究了一种新型萃取剂仲辛基苯氧基乙酸 (CA 12 )萃取钪的性能及其机理。通过研究平衡水相酸度、萃取剂浓度和温度对萃取平衡的影响 ,用斜率法和等摩尔系列法确定了CA 12萃取钪的机理 ,并求得了CA 12萃取钪的热力学函数 ,计算了平衡反应的浓度平衡常数及钪同其它稀土离子的分离系数 ,指出CA 12是一种从稀土 中分离钪的优良萃取剂     

Effect of pH on the Behavior of Hyaluronic Acid in Dilute and Semidilute Aqueous Solutions

Summary : Viscosity, asymmetric flow field-flow fractionation (AFFFF) methods, and dynamic light scattering (DLS) experiments were used to characterize the effect of pH on the behavior of dilute and semidilute aqueous buffered solutions of hyaluronic acid (HA). It is shown that degradation of HA occurs at pH < 4 and pH > 11, and in the domain 4 < pH < 11 virtually no disruption of the HA chains occurs. The pH-induced scission of HA is attributed to the cleavage of glycosidic bonds. In dilute solutions, intramolecular rupture of HA chains occurs and in the semidilute concentration regime network-fragmentation is observed at low and high pH values. The degree of degradation of HA is most marked at high pH. From the molecular weight and radius of gyration obtained from AFFFF at different pH values, it is clear that the degradation of HA starts at early times after preparation of the solution, and continues for a couple of days. The kinetics of degradation of HA is fastest at high pH.     

On the complexity of kinetics and the mechanism of the thiosulfate-periodate reaction

The thiosulfate-periodate reaction has been studied spectrophotometrically in a slightly acidic medium at 25.0 ± 0.1 °C in an acetate/acetic acid buffer by monitoring the absorbance in the 250-600 nm wavelength range at a constant ionic strength adjusted by the buffer component sodium acetate. In agreement with a previous study, we found that the reaction cannot be described by a single stoichiometric equation, tetrathionate and sulfate are simultaneously formed, and its ratio strongly depends on the pH. As expected at certain initial concentration ratios of the reactants, the reaction behaves as a clock reaction, but after its appearance, iodine is slowly consumed mainly because of the moderate tetrathionate-iodine reaction. It is also enlightened that the initial rate of the reaction is completely independent of the pH, which apparently contradicts a previous study, which postulates a "supercatalytic" behavior of the hydrogen ion on the title reaction. Significant buffer assistance that may change the absorbance-time profiles was also observed. On the basis of the kinetic data, a robust 28-step kinetic model with 22 fitted parameters is proposed and discussed to explain adequately all of the important characteristics of the kinetic curves.     

Swelling-shrinking behavior of a polyampholyte gel composed of positively charged networks with immobilized polyanions

Polyampholyte gels were prepared by free radical polymerization of aqueous monomer solutions with the following composition: 69% N-isopropylacrylamide (thermosensitive neutral monomer), 1% N,N'-methylenebisacrylamide (cross-linker), 15% 1-vinylimidazole (cationic monomer), and either 15% acrylic acid (AAc, anionic monomer) or poly(acrylic acid) (PAAc, polyanion). We thus obtained two sorts of polyampholyte gels; that is, G1 with immobilized PAAc and G2 with randomly copolymerized AAc. The equilibrium swelling ratio (Qe) was studied as a function of the pH, NaCl concentration, and temperature. Also studied was the kinetics of swelling and shrinking in response to a sudden pH change. The significant results obtained were as follows: (i) A fully collapsed state was observed at pH 4.5-9.0 for G1 and at pH 4.5-7.0 for G2. (ii) Below and above these pH ranges, both gels were in a swollen state; therefore, an isoelectric point (pI) appeared in a wide pH range. (iii) At alkaline pH regions where a hysteresis was observed in the Qe versus pH curves of G1 and G2 as the pH was first increased then decreased, G1 exhibited very slow swelling-shrinking kinetics. (iv) An increase in the NaCl concentration allowed the gel to swell at pH approximately pI (antipolyelectrolyte behavior) but to shrink at pHs below and above the pI range (polyelectrolyte behavior). (v) The magnitude of the salt-induced shrinking of G1 is smaller than that of G2 at pH 10 and at NaCl concentrations > 0.01 M. (vi) At pH 10, an increase in the temperature from 35 to 50 degrees C led to a shrinking change of G1 but not of G2. These results were found to be explicable in terms of a different distribution of negative charges within the polyampholyte gel network.     

Kinetics of Electrochemical Processes under Conditions of a Slow Chemical Reaction in Solution

The kinetics of electrochemical processes under the conditions of a slow chemical reaction in solution is analyzed in a general form, which admits a change in their limiting stages. The main feature of polarization curves for the electrochemical processes is the existence of two Tafel portions with slopes corresponding to a slow homogeneous chemical reaction (lower portion) and a preceding faster electrochemical stage (upper portion with a higher slope).     

Kinetic studies of nickel speciation in model solutions of a well-characterized humic acid using the competing ligand exchange method

Publications on the binding characteristics of metals with humic acid (HA) are sparse. Here we investigated the release of nickel from Ni(II)-HA complexes using model solutions of three different [Ni(II)]/[HA] mole ratios at three different pH values; we also compared the results with those of [Ni(II)]/[FA] complexes from previous work in this laboratory. Ligand exchange kinetics using the competing ligand exchange method (CLEM) were studied using two different techniques: graphite furnace atomic absorption spectrometry (GFAAS) with Chelex 100 resin as the competing ligand, and adsorptive cathodic stripping voltammetry (AdCSV) with dimethylglyoxime as the competing ligand to measure the rate of dissociation of Ni(II)-HA complexes. The results of the kinetic studies showed that as the [Ni(II)]/[HA] mole ratio was decreased, the rate of dissociation of Ni(II)-HA complexes decreased, and the proportion of free Ni²⁺ ions plus very labile nickel complexes decreased while the proportion of the less labile kinetically distinguishable components increased. Generally, the rate of dissociation of Ni(II)-HA complexes was slower than that of Ni(II)-FA complexes. Studies on the validity of the kinetic model showed that the concentrations of chemical species varied in a reasonable way with pH and the [Ni(II)]/[HA] mole ratios, indicating that the kinetically distinguishable components have chemical significance and the kinetic model is valid.     

Kinetics and mechanism of tetrahydrofuran synthesis via 1,4-butanediol dehydration in high-temperature water

We conducted an experimental investigation into the kinetics and mechanism of tetrahydrofuran synthesis from 1,4-butanediol via dehydration in high-temperature liquid water (HTW) without added catalyst at 200-350 degrees C. The reaction was reversible, with tetrahydrofuran being produced at an equilibrium yield of 84% (at 200 degrees C) to 94% (at 350 degrees C). The addition of CO2 to the reaction mixture increased the reaction rate by a factor of 1.9-2.9, because of the increase in acidity resulting from the formation and dissociation of carbonic acid. This increase was much less than that expected (factor of 37-60) from a previously suggested acid-catalyzed mechanism. This disagreement prompted experiments with added acid (HCl) and base (NaOH) to investigate the influence of pH on the reaction rate. These experiments revealed three distinct regions of pH dependence. At high and low pH, the dehydration rate increased with increasing acidity. At near-neutral pH, however, the rate was essentially insensitive to changes in pH. This behavior is consistent with a mechanism where H2O, in addition to H+, serves as a proton donor. This work indicates that the relatively high native concentration of + (large KW), which has commonly been thought to lead to the occurrence of acid-catalyzed reactions in HTW without added catalyst, does not explain the dehydration of 1,4-butanediol in HTW without catalyst. Rather, H2O serves directly as the proton donor for the reaction.     

Stability study of simvastatin under hydrolytic conditions assessed by liquid chromatography

In this work, a liquid chromatography stability-indicating method was developed and applied to study the hydrolytic behavior of simvastatin in different pH values and temperatures. The selected chromatographic conditions were a C18 column; acetonitrile-28 mM phosphate buffer solution, pH 4 (65 + 35) as the mobile phase; 251 degrees C column temperature; and flow rate 1 mL/min. The developed method exhibited an adequate repeatability and reproducibility (coefficient of variation 0.54 and 0.74%, respectively) and a recovery higher than 98%. Furthermore, the detection and quantification limits were 9.1 x 10(-7) and 2.8 x 10(-6) M, respectively. The degradation of simvastatin fitted to pseudo-first order kinetics. The degradation was pH dependent, being much higher at alkaline pH than at acid pH. Activation energy, kinetic rate constants (k) at different temperatures, the half life (t1/2) and the time for 10% degradation to occur (t90) values are also reported.     

Poly(l‐lactic acid)‐modified silica stationary phase for reversed‐phase and hydrophilic interaction liquid chromatography

Poly(l ‐lactic acid) is a linear aliphatic thermoplastic polyester that can be produced from renewable resources. A poly(l ‐lactic acid)‐modified silica stationary phase was newly prepared by amide bond reaction between amino groups on aminopropyl silica and carboxylic acid groups at the end of the poly(l ‐lactic acid) chain. The poly(l ‐lactic acid)‐silica column was characterized in reversed‐phase liquid chromatography and hydrophilic interaction liquid chromatography with the use of different mobile phase compositions. The poly(l ‐lactic acid)‐silica column was found to work in both modes, and the retention of test compounds depending on acetonitrile content exhibited “U‐shaped” curves, which was an indicator of reversed‐phase liquid chromatography/hydrophilic interaction liquid chromatography mixed‐mode retention behavior. In addition, carbonyl groups included into the poly(l ‐lactic acid) backbone work as an electron‐accepting group toward a polycyclic aromatic hydrocarbon and provide π–π interactions.     

Thermal inactivation of immobilized enzymes: A kinetic study

The thermal inactivation kinetics of chymotrypsin, trypsin, and—-amylase bound to silica, polyacrylamide, and polystyrene were studied at different temperatures. The inactivation curves were analyzed by a kinetic model, assuming a first-order reaction of differently stable enzyme fractions on the matrix. In all cases the assumption of two enzyme fractions with distinctly different inactivation constants was sufficient for describing the inactivation progress (standard deviations between experimental and calculated inactivation curves, 1-4%). Both the inactivation constants as well as the relative concentrations of the enzyme fractions were found to change in dependence on temperature.     

Molecular beam measurements and Monte Carlo simulations of the kinetics of N2O decomposition on Rh(111) single-crystal surfaces

The kinetics of N2O decomposition on Rh(111) single-crystal surfaces were investigated both experimentally by isothermal molecular beam measurements and theoretically using a Monte Carlo algorithm. The present work was directed to the understanding of two unusual observations derived from our previous work on this system, namely, (1) the lower rates for N2O decomposition seen at higher reaction temperatures, and (2) the lower total nitrogen yields and final oxygen surface coverages that accompany that behavior. Experimentally, it was determined here that after the rhodium surface is rendered inactive by N2O decomposition at high (520 K) temperatures, significant activity is still possible at lower (350 K) temperatures. The Monte Carlo simulations explain these observations by assuming that the surface sites required for the activation of adsorbed N2O increase in size with increasing reaction temperature.