The rate of the disproportionation reaction of iodous acid at different acidity values in aqueous sulfuric acid solution

The dependence of disproportionation reaction kinetics of iodous acid, HOIO, in aqueous solutions of sulfuric acid on the solution acidity is examined. The rate constants of the disproportionation reaction are determined at temperatures of 18, 25 and 30 °C, based on kinetic data obtained under stationary conditions. The average value of the activation energy is determined to be 42 kJ/mol.     

The activation energy as guiding factor in the fragmentation of substituted diphenylmethanes

The mass spectra of a series of substituted diphenylmethanes were obtained. The mass spectral features were compared with those of the corresponding 1,2 diphenylethanes and discussed in terms of the factors which influence the electron-impact fragmentation. In the series examined, it is shown that the activation energy is the most important factor in determining cracking patterns. The meaning of the averaged rate constants is criticised.     

The Kinetics of Heterogeneous Electron Transfer Reaction in Polar Solvents

Data for heterogeneous electron transfer reactions obtained in the last ten years have been examined within the context of contemporary theory. Special attention is focused on systems for which rate constants and activation parameters are available as functions of solvents and of temperataure. The role of reactant structure in deteriming the kinetic parameters is also considered. Double layer effects both at unmodified and modified electrodes are discussed in detail. Experimental techniques, especially those suited to measuring fast reactions, are also outlined.     

The effects of photo-degradation on the electrical properties of poly(ethylene terephthalate)

Poly(ethylene terephthalate) (PET) films have been exposed to uvradiation. Electrical property changes which occur in the films were examined. It was found that the relative dielectric constant and the dielectric strength decrease, while the dielectric loss factor increases. The rate of change of these parameters slowed down as the irradiation time increased. The parameter changes are presented graphically as a function of irradiation time and empirical equations are established. These empirical equations were obtained as second or third order polynomials by the least squares analysis method. The method of function straightening gave simpler power or exponential functions. These empirical equations represent the relationships between the electrical parameters and the irradiation time to a limit of relative error of less than 10%.     

DSC investigations on condensation polymers—I. Analysis of the curing process

Curing reactions in thermosetting resins and resin-composite systems have been examined using differential scanning calorimetry. In particular, phenol-formaldehyde and melamine-formaldehyde systems have been investigated using small pressure-sealed capsules. Curing exotherms obtained from a single temperature scan have been analysed to give reaction kinetic parameters associated with the overall cure process. The reproducibility of the derived parameters, their variation with scan speed and the absolute values of reaction rate constants obtained, are discussed. The kinetic values are shown to be reasonably reliable when used to calculate extents of cure resulting from thermal curing cycles.     

The application of Kohonen neural networks to diagnose calibration problems in atomic absorption spectrometry

In atomic absorption spectrometric measurements calibration lines are measured daily. These lines are not always acceptable. They can, for instance, contain outliers, have a bad precision or can be curved. To evaluate the quality of those lines a method which gives a fast diagnosis is recommended. In this study the use of Kohonen neural networks was examined as an automated procedure to classify these calibration lines. The results were compared with those obtained using a decision support system which uses classical statistical methods to classify the lines. The prediction capabilities of both approaches relative to a visual inspection and classification was found to be comparable, or even slightly better for the Kohonen networks, depending on the training set used. For both techniques a prediction error rate of <10% was obtained, relative to a visual classification.     

The determination of the number of electrons involved in irreversible reductions by microcoulometry

A coulometric method of determining the number of electrons taking part in a unit of reaction at the dropping mercury electrode, using only a polarograph and microcell is presented and critically examined. Results obtained with reversible and irreversible depositions and valency changes are discussed and recommendations are made.     

Demonstration of the Feasibility of Radially Compressed Microbore HPLC Columns

Abstract

This work involved the development of radially compressed, microbore high performance liquid chromatography (HPLC) columns. The design of the overall system and the column are described, and the problems associated with the design features are reported. Variables examined during the course of this work included the column material, column length, packing method, flow rate, radial compression pressure, and internal column pressure. Efficiencies (expressed as plates/meter) are shown for various combinations of those variables and are compared to those obtained using a commercial, steel microbore column.     

The anomalous temperature dependence of enzyme-catatlyzed reactions

Breaks or curvatures in the plot of log v_br vs. 1/T have been described in the literature for a number of reactions. If sufficiently small temperature intervals are used in the investigation of the temperature dependence (STI method) a trend line is obtained instead. It is then possible to distinguish regions of linear behavior and regions of anomalous behavior. The effects of e. g. the pH, the ionic strength, and added salts as well as of the size of the enzyme and substrate molecules on the anomalies were examined. The anomalous temperature dependence of the reaction rate was found to be a particularly sensitive indicator of the reaction event on the enzyme. The course of the reaction is evidently affected even by relatively small changes in conformation, particularly in the region of the active center.     

Kinetic studies on the solvent extraction of uranium(VI) from phosphoric acid solution with HDEHP using laser-induced optical fiber fluorimetry

Laser-induced optical fiber fluorimetry has been first used to analyze uranium(VI) concentration in the kinetic studies on the extraction of uranium(VI) between 0.5 mol/l H₃PO₄ solution and HDEHP-cyclohexane system with a Lewis cell. The effects of stirring speed, temperature and concentrations of uranium(VI) and HDEHP on the rate of extraction were examined. These data show that the extraction rate of uranium(VI) in this system is controlled by the chemical reaction at the interface. The rate equations and the rate constants of forward and reverse extraction are obtained. The mechanism of the extraction is discussed.     

Estimation of first-order kinetic parameters by using the extended kalman filter

A model for first-order kinetics is derived for spectra obtained while a reaction is taking place. A technique for nonlinear-regression analysis known as the extended Kalman filter is used to estimate the initial concentrations of the reactants and the rate constant from the spectral data. The effects of the magnitude of the rate constant and the identity of the absorbing species are examined for synthetic spectra containing overlapped responses. The technique is used successfully to obtain the rate constant for the dissociation reaction of a praseodymium complex. The filter is also shown to be useful for the detection of erros in the kinetic model employed to fit the data. The extended Kalman filter can be used to fit kinetic models other than the one discussed here, and may prove to be a valuable technique for estimation of kinetic parameters.     

Study of relaxation rates of stable paramagnetic centers in gamma-irradiated alanine

The stable L-alanine radical induced by gamma-irradiation was examined by electron paramagnetic resonance (EPR), transfer saturation EPR and electron nuclear double resonance (ENDOR) in the temperature region of fast motion of the methyl group (180-320 K). From the obtained spectral line broadening and spectral intensity the correlation time for the methyl rotation was estimated. The complex processes determining the relaxation rate were examined in the same temperature interval. It was shown that important contributions to the relaxation rate arise from non-secular and pseudo-secular types of contributions. The non-secular contribution involves intramolecular dynamics while the pseudo-secular contribution originates from intermolecular motions. The obtained values for the dynamical parameters have been compared with those obtained by pulse EPR methods and by proton nuclear magnetic resonance (NMR) on undamaged crystals.     

On the Electrochemical Reduction of α-Cyanothioacetamide Derivatives (I) Polarographic Behaviour of 2-Arylhydrazono-2-Cyanothioacetamides

The polarographic reduction of a series of 2-aryl-hydrazono-2-cyanothioacetamides ((Ia-e) has been examined in 50% ethanolic buffer solutions. The obtained data indicate that these compounds are in the hydrazono form and that they are reduced to give an amino acid and the corresponding primary aromatic amine. A mechanism has been proposed and discussed. Recombination and dissociation rate constants (k_r) and (k_d) have been computed using the appropriate spectrophotometric, potentiometric and apparent acid equilibrium constants.     

The production of active carbon from corn cobs by chemical activation

Corn cobs obtained as waste from the corn industry, were analyzed by a TG-DTA unit in an atmosphere of flowing nitrogen. The carbonaceous products so formed were then produced on a preparative scale and activated chemically using potassium hydroxide. This resulted in the formation of a carbon with a very high surface area. The active carbon produced was then examined using thermal analysis in the temperature jump mode on a thermogravimetry unit. From this data the kinetics of degradation of the active carbon was determined using zero order rate kinetics. The pore structure of the active material was also examined using SEM. TheE _a for activated corn cobs was found to be 106 kJ mol_–1.     

The fluorescence quenching rate constant for the distance-dependent quenching processes in the presence of diffusion

It is shown that the fluorescence quenching constant is given by the Laplace transform of the time-dependent bimolecular rate constant k(t). The dependence of the quenching rate constant on the transformation parameter can be obtained from measurements of the fluorescence decay functions and a Stern-Volmer-type analysis in the Laplace domain. The advantages of this type of investigation of the quenching rate function are demonstrated by a numerical calculation for the distance-dependentquenching process in     

Dynamic nature of the transition state for the SN1 reaction mechanism of diphenylmethyl acetates

Picosecond absorption spectroscopy is employed in the study of the reaction dynamics for the contact ion pairs produced upon the photolysis of a series of substituted diphenylmethyl acetates in the solvents acetonitrile, dimethyl sulfoxide, and 2,2,2-trifluoroethanol. From the temperature dependence of the rate constants, the activation parameters associated with covalent bond formation and diffusional separation to the solvent-separated ion pair are obtained. The activation parameters for bond formation are examined within the context of the Hynes theory for solvent dynamical effects on the passage through the transition state; deviations from the transition-state theory are found to be large. Factors that control nucleophilicity are discussed. Finally the validity of applying the Marcus equation to the SN1 reaction mechanism is addressed.     

The form of the rate constant for elementary reactions at equilibrium from MD: framework and proposals for thermokinetics

The rates of formation and concentration distributions of a dimer reaction showing hysteresis behavior are examined in an ab initio chemical reaction designed as elementary and where the hysteresis structure precludes the formation of transition states (TS) with pre-equilibrium and internal sub-reactions. It was discovered that the the reactivity coefficients, defined as a measure of departure from the zero density rate constant for the forward and backward steps had a ratio that was equal to the activity coefficient ratio for the product and reactant species. This surprising result, never formally incorporated in elementary rate expressions over approximately one and a half centuries of quantitative chemical kinetics measurement and calculation is accepted axiomatically and leads to an outline of a theory for the form of the rate constant, in any one given substrate—here the vacuum state. A major deduction is that the long-standing definition of the rate constant for elementary reactions is not complete and is nonlinear, where previous works almost always implicitly refer to the zero density limit for strictly irreducible elementary reactions without any attending concatenation of side-reactions. This is shown directly from MD simulation, where for specially designed elementary reactions without any transition states, density dependence of reactants and products always feature, in contrast to current practice of writing rate equations. It is argued that the rate constant expression without reactant and product dependence is due to historical conventions used for strictly elementary reactions. From the above observations, a theory is developed with the aid of some proven elementary theorems in thermodynamics, and expressions under different state conditions are derived whereby a feasible experimental and computational method for determining the activity coefficients from the rate constants may be obtained under various approximations and conditions. Elementary relations for subspecies equilibria and its relation to the bulk activity coefficient are discussed. From one choice of reaction conditions, estimates of activity coefficients are given which are in at least semi-quantitative agreement with the data for non-reacting Lennard-Jones (LJ) particles for the atomic component. The theory developed is applied to ionic reactions where the standard Brönsted-Bjerrum rate equation and exceptions to this are rationalized.     

Photopolymerization of acrylamide initiated by the three‐component system safranine/triethanolamine/diphenyliodonium chloride: The effect of the aggregation of the salt

The effects of diphenyliodonium chloride (DPI) on the polymerization of acrylamide photoinitiated by the dye safranine with triethanolamine as a coinitiator were investigated in aqueous solutions. The salt notably increased the polymerization rate. This accelerating effects increased appreciably at concentrations of DPI at which the light scattering of the solutions became important. The effects of DPI on the photophysical properties of the dye were also investigated. Although the absorption and fluorescence were scarcely affected, the triplet yield increased by 60%. However, the polymerization rate increased by a factor much higher than that of the triplet yield. The results obtained at high concentrations of the salt could be ascribed to the presence of aggregates of the hydrophobic cations. The lower limit established for the formation of the aggregates was a DPI concentration of approximately 1 × 10⁻³ M. Possible mechanisms for the action of the salt were examined. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 4916–4920, 2004     

Effect of pH and gaseous ammonia flow rate on the composition and structure of ammonium uranate

Ammonium uranate was precipitated from uranyl nitrate solution using gaseous ammonia, then filtered, washed with demineralized water and dried. The influence of pH and ammonia flow rate on their composition and structure were examined by X-ray diffraction analysis, making use of additional information obtained from infrared analysis.     

The influence of physical aging on the creep rupture behavior of polystyrene

The effects of postannealing aging time on the brittle fracture behavior of polystyrene were studied. A combination of mechanical properties, including creep and creep rupture under constant load and the behavior under constant extension rate deformation were examined for polystyrene samples of different prior aging times (from 1h to 2 months). The specimens and fracture surfaces were examined by optical microscopy and SEM to observe any change in the fracture behavior. It was found that longer aging times caused not only a change in the time-dependent modulus of the material but also a significant decrease in the creep rupture life and a decrease in strain to failure. It was found that the reasons for this are that although aging delays craze formation, craze breakdown and ultimate failure are accelerated by aging. The importance of these findings are discussed, particularly in relation to failure criteria involving the use of critical strains. © 1993 John Wiley & Sons, Inc.