Thermodilatometkic analysis: some chemical applications

Some applications of thermodilatometric analysis are discussed. The technique may be used for the determination of solid-solid phase transitions and decomposition reactions. An inexpensive recording dilatometer is described.     

A graph-theoretical approach for the analysis and model reduction of complex-balanced chemical reaction networks

In this paper we derive a compact mathematical formulation describing the dynamics of chemical reaction networks that are complex-balanced and are governed by mass action kinetics. The formulation is based on the graph of (substrate and product) complexes and the stoichiometric information of these complexes, and crucially uses a balanced weighted Laplacian matrix. It is shown that this formulation leads to elegant methods for characterizing the space of all equilibria for complex-balanced networks and for deriving stability properties of such networks. We propose a method for model reduction of complex-balanced networks, which is similar to the Kron reduction method for electrical networks and involves the computation of Schur complements of the balanced weighted Laplacian matrix.     

Parametric sensitivity of complex temperature-programmed desorption,reaction and reduction

Through simulation of the temperature-programmed desorption, reaction and reduction in a system of parallel reactions. it is shown that the experimental conditions influence the simultaneous or consecutive occurrence of the individual reactions of the system. Adjustment of the concentration of a gaseous reaction component or optimization of the temperature program results in a shift to the desired consecutive occurrence of the reactions. Therefore, a better resolution of the individual reactions can be achieved through a judicious choice of the experimental conditions of the temperature-programmed methods.

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Zusammenfassung Durch Simulierung temperaturprogrammierter Desqrption, Reaktion und Reduzierung in einem System von Parallelreaktionen konnte gezeigt werden, daß die experimentellen Bedingungen die Simultanität bzw. Konsekutivität der Einzelreaktionen des Systems beeinflussen. Durch eine Einstellung der Konzentration der gasförmigen Reaktionsprodukte oder durch Optimalisierung des Temperaturprogrammes kann eine gewünschte Folge der Einzelreaktionen erreicht werden. Somit kann durch eine geschickte Wahl der experimentellen Bedingungen bei temperaturprogrammierten Verfahren eine bessere Separierung der Einzelreaktionen erreicht werden.

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Flow injection analysis as a diagnostic technique for development and testing of chemical sensors

An immobilized urease sensor is developed for continuous, on-line analysis. The sensor consists of the enzyme urease, cross-linked with bovine serum albumin into a cellulose pad, with an acid-base indicator dye covalently bound to the surface of the cellulose. The sensor is placed within a flow-injection optosensing system to monitor the change in pH, and subjected to a through evaluation, using the flow-injection technique; sensor stability (both dye and enzyme stability), speed of sensor response, sensor sensitivity, sensor-to-sensor reproducibility, response to a typical interferent, and sensor lifetime data are obtained. Sensor poisoning upon exposure to low levels of mercury, and subsequent regeneration of the immobilized enzyme pad, is investigated for use as an on-line mercury sensor. The urea sensor is also evaluated for use as a continuous monitor for urea in kidney dialysate. Enzyme Michaelis-Menten constants are determined for the immobilized urease, under given assay conditions, using a stopped-flow flow-injection technique.     

Recent advances in the development of colorimetric analysis and testing based on aggregation-induced nanozymes

Colorimetric sensing strategies as a powerful point-of-care testing(POCT) tool have attracted significant interest in various chem/biosensing applications.Taking the excellent bare-eye-detectable signaling feature,nanozymes-based colorimetric sensors enable more potential applications and have been a new forefront in the colorimetric POCT analysis toward different target analytes.However,the low catalytic activity of nanozymes in most cases limits their practical application.Recent efforts demonstrate that the aggregation-induced nanozymes provide a general means to modulate nanozymes activity and enhance colorimetric sensing performances of some nanozymes-based colorimetric sensors.But there are few reports are explored to discuss and review such aggregation-induced nanozymes and their colorimetric sensing applications.To highlight the advances and progress in aggregation-induced nanozymes based colorimetric assays,we herein summary the fundamentals,classify and applications of this newlydeveloping field,focusing on the aggregation-induced activity enhancement of nanozymes(AIAEnanozymes) with a significant "signal-on" feature and aggregation-induced activity inhibition of nanozymes(AIAI-nanozymes) with a dramatical "signal-of" characteristics.Finally,we also propose the current challenges and the future prospects on both AIAE-nanozymes and AIAI-nanozymes.     

Design and development of chemical ontologies for reaction representation

This paper describes the development of chemical ontologies applied to the representation of organic chemical reactions. The ontologies are built using the methodology known as methontology. The hierarchically structured set of terms describing the subdomains, namely, organic reactions, organic compounds, and reagents, are constructed into individual ontologies. The ontologies consist of about 200 concepts and around 125 individuals. A set of binary relations is defined in order to integrate the ontologies with applications. The ontologies are implemented as an XML application with a set of vocabulary describing the domain knowledge. This paper also features an easy-to-use chemical ontological support system (COSS) intended to represent organic chemical reactions automatically. As a model application, the automatic representation of aliphatic nucleophilic substitution reactions is demonstrated using COSS. The paper also describes a keyword-based search system whose functionality is backed with COSS.     

A new procedure for the wet chemical analysis of the lead—noble metals button

Thiobarbituric acid can be used as a group precipitant for the recovery of Pt, Pd, Rh and Au from the lead perchlorate—perchloric acid solution, obtained after the lead—noble metals button has been parted with perchloric acid. By integration of this procedure with the recovery of Ir, Ru and Os from the parting residue, a scheme is evolved which makes possible the determination of all the platinum metals and gold from a single button. The results of analyses of various platiniferous materials by the proposed lead—wet method and the classical lead-cupellation method are compared.     

Mechanism of reduction of double bond under chemical ionization conditions

The mechanism of double bond reduction occurring in certain conjugated ketones, nitriles, acids and esters under chemical ionization conditions has been studied. The results indicate that the hydrogen radicals present in the chemically ionized plasma are responsible for the reduction of the double bond. This is further supported by experiments with radical traps.     

A new approach to chemical concentration in activation analysis for some noble and rare elements

The possibility of concentrating Pt, Ir, Au, Ag, Re and some other elements from samples of chromites, sulfide ores, laterites, shales, titanium magnetites, and ultrabasic rocks was studied. A new simple procedure is based on sublimation of elements to be determined in air stream at 1200 °C in the presence of some powdered reagents /e.g., TiO₂, Nb₂O₅, Nb/ to enhance the yield, and on the use of chemical filters /CaO, MgO, TiO₂, Al₂O₃, Nb₂O₅/ absorbing the interfering volatile elements from the gas phase. Neutron activation and X-ray fluorescence isotopic excitation were used to analyze the obtained concentrates.     

Synthesis and chemical characterisation of some new diheteroaryl thienothiophene derivatives

Treatment of 1-(5-acetyl-3,4-dimethythieno[2,3-b]thiophene-2yl)ethanone (1) with dimethylformamide dimethyl acetal afforded enaminone derivative 2, which reacted with amino derivatives to give the corresponding bis-pyrimidine, bis-pyrazole, bis-triazolo-pyrimidine and bis-benzoimidazopyrimidine derivatives.     

Systematic procedure for reduction of kinetic mechanisms of complex chemical processes and its software implementation

Feasibility of multidimensional hydrodynamic modeling depends critically on the availability of accurate reduced kinetic mechanisms of physical and chemical processes taking place in the system. Such mechanisms should describe the processes under consideration within a specified error tolerance in the range of initial conditions of interest while keeping the number of species and reactions as small as possible. We have developed an advanced tool for reduction of detailed kinetic mechanisms with a minimal human effort. The tool includes 10 reduction and 2 analysis methods which are based on the results of zero-dimensional modeling. The methods can be combined and applied in sequence. The reduction tool has been implemented as a part the Chemical Workbench computational package and has been tested for a number of large kinetic mechanisms of gas-phase processes. Using this tool, we reduced the mechanism of tar gasification from 177 species and 879 reversible reactions to only 83 species and 278 reactions, while the mechanism of methane combustion initially involving 127 species and 1,206 reactions was reduced to 42 species and 173 reactions.     

Conformational analysis by kinetic methods: A critique : Theory and experimental development of procedures based on very fast chemical reactions

Two general methods of conformational analysis by kinetic methods are distinguished, and some definitive experiments are described concerning the validity of the reactivity-model method as employed for cyclohexanes. The fast chemical reaction method is analysed in detail, and potential applications to conformational equilibria of N-alkylpiperidines are discussed. The selectivity of electrophilic addition of some photogenerated carbenes and nitrenes to organic sulphides is examined.     

Comparative sensitivity analysis of transport properties and reaction rate coefficients

Transport properties of chemical species are required for many combustion models. A sensitivity analysis is conducted to assess the significance of transport properties and their underlying molecular parameterizations for atmospheric pressure premixed laminar flames for three different fuels and two different approaches to transport property calculations. The analysis is performed at both the macroscopic level of Arrhenius A-factors and transport coefficients as well as at the molecular scale. First- and second-order sensitivities of reactant, intermediate, and product species concentrations, temperature, and flame velocity were calculated with respect to various parameters, all within the mixture approximation using ADIFOR 2.0, a software package that supports exact differentiation. Parameters considered were the binary diffusion coefficients, pure species thermal conductivity coefficients, and thermal diffusion ratios. The more fundamental molecular parameters: collision diameters, well depths, dipole moments, polarizabilities, and the rotational relaxation collision numbers were also considered. Influential transport properties are found to be as important in flame modeling as influential reaction rates, and both should be taken into account when building chemical mechanisms. Transport parameter importance was found to vary according to the independent variable being considered and the flame type. Magnitudes of sensitivities appear to be more influenced by the underlying molecular parameters than the approach used to compute the transport properties. The number of significant sensitivities to transport parameters increases for the progression: flame temperature, flame velocity, reactant species, product species, and intermediate radical species. Many dependent variables have significant sensitivities to the pure species thermal conductivities of N₂, O₂, and the fuel. At the molecular level, large sensitivities to the collision diameters of several species are also observed, but significant sensitivity to well depths, although observed is less and more rare. Large sensitivities are not observed to the rotational relaxation collision number, the dipole moment, or to the molecular polarizability. Second-order sensitivities are significant for a number of dependent variables. © 2005 Wiley Periodicals, Inc. Int J Chem Kinet 37: 538–553, 2005     

Parameter sensitivity analysis of stochastic models: Application to catalytic reaction networks

A general numerical methodology for parametric sensitivity analysis is proposed, which allows to determine the parameters exerting the greatest influence on the output of a stochastic computational model, especially when the knowledge about the actual value of a parameter is insufficient. An application of the procedure is performed on a model of protocell, in order to detect the kinetic rates mainly affecting the capability of a catalytic reaction network enclosed in a semi-permeable membrane to retain material from its environment and to generate a variety of molecular species within its boundaries. It is shown that the former capability is scarcely sensitive to variations in the model parameters, whereas a kinetic rate responsible for profound modifications of the latter can be identified and it depends on the specific reaction network. A faster uptaking of limited resources from the environment may have represented a significant advantage from an evolutionary point of view and this result is a first indication in order to decipher which kind of structures are more suitable to achieve a viable evolution.     

Adaptive approach for nonlinear sensitivity analysis of reaction kinetics

We present a unified approach for linear and nonlinear sensitivity analysis for models of reaction kinetics that are stated in terms of systems of ordinary differential equations (ODEs). The approach is based on the reformulation of the ODE problem as a density transport problem described by a Fokker-Planck equation. The resulting multidimensional partial differential equation is herein solved by extending the TRAIL algorithm originally introduced by Horenko and Weiser in the context of molecular dynamics (J. Comp. Chem. 2003, 24, 1921) and discussed it in comparison with Monte Carlo techniques. The extended TRAIL approach is fully adaptive and easily allows to study the influence of nonlinear dynamical effects. We illustrate the scheme in application to an enzyme-substrate model problem for sensitivity analysis w.r.t. to initial concentrations and parameter values.     

High-speed carbon nanotube actuators based on an oxidation/reduction reaction

Actuators with a high-speed response under a high-frequency (more than 100 Hz) applied square-wave voltage of ±2 V have been developed with an electrode composed of millimeter-long single-walled carbon nanotubes synthesized by the "supergrowth method" (SG-SWNTs) and ionic liquids (ILs). Detailed studies concerning induced electric current and transferred charge in the electrode as well as cyclic voltammetric studies of the electrode revealed that the high-speed response originates from the electric current generated by an oxidation/reduction (redox) reaction in addition to electric double-layer charging. The contribution of the redox reactions of SG-SWNTs to the actuation is sensitive to the presence of supporting polymers, the thickness of the electrolyte, and the amplitude of the applied voltage.     

Headspace analysis with cryogenic focusing: A procedure for increasing the sensitivity of automated capillary headspace analysis

Summary A convenient procedure has been described for enhancing the sensitivity of equilibrium headspace gas chromatography. The technique involves the cryogenic focusing of headspace vapors at the head of a 0.32 mm fusedsilica capillary column which has been coupled to a packed column injection port. Cryofocusing is accomplished automatically by cooling the GC oven and results in enhanced sensitivity with improved resolution. With this simple focusing technique no sample splitting is required which gives at least a 10-fold increase in sensitivity. A Multiple Headspace Injection (MHI) technique has been described which allows several rapid headspace injections to be made at the start of a single chromatographic run. Cryofocusing causes the injections to be superimposed, thus increasing the sensitivity even further. These procedures have been demonstrated for flavor and aroma analyses of toothpaste and soft drink samples. Presented at the Symposium on Headspace GC, National American Chemical Society Meeting, Chicago, Ill., September 8–13, 1985     

Reference procedures for testing and chemical analysis provided by BAM

Recently the German Federal Institute for Materials Research and Testing (BAM) has made an inventory of the reference procedures for testing and chemical analysis provided by its laboratories. This compilation was first published as a printed catalogue [1] but is now also available on the Internet at the BAM website (see http://www.bam.de/service, direct link: http://www.bam.de/ reference_procedures.htm). The intention of this article is to give an introduction to the catalogue and provide relevant background information.