Reaction kinetics, automated thermal

An Automated Thermal Analysis System for Reaction Kinetics," A.F. Kah, M.E. Koehler, T.H. Grentzer, T.F. Niemann, and T. Provder, Computer Applications... 

Kah, A. F. Koehler, M. E. Grentzer, T. H. Niemann, T. F. Provder, T. "An Automated Thermal Analysis System for Reaction Kinetics" in "Computer Applications in Applied Polymer Science" Provder, T., Ed. ACS SYMPOSIUM SERIES No. 197, American Chemical Society Washington, D.C., 1982 pp. 197-311. 

An Automated Thermal Analysis System for Reaction Kinetics... 

A DuPont Model 990 Thermal Analysis Console with Model 910 DSC accessory was interfaced to a minicomputer system by means of a microcomputer for automated data collection. A program to provide the analysis of reaction kinetics data by the single dynamic scan method for DSC kinetics was developed. Features of this program include a fit of the data to a single equation by multiple regression techniques to yield the reaction order, the energy of activation and the Arrhenius frequency factor. The rate constant k(T) is then calculated and conversion data as a function of time and temperature can be generated at the operator s option. 

Mathematical modeling of the cure process coupled with the automation of various thermal analytical instruments and Fourier Transform Infrared Spectroscopy (FT-IR) have made possible the determination of quantitative cure and chemical reaction kinetics from a single dynamic scan of the reaction process. This paper describes the application of FT-IR, differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA) in determining cure and reaction kinetics in some model organic coatings systems. 

The quantitative high-temperature chemistry of chlorine oxysalts is rather underdeveloped. There are very few thermodynamic data for these compounds above 298 K. Even when they exist, they must be applied cautiously, since there may be kinetic rather than thermodynamic factors that determine decomposition behavior. Although the thermal decomposition of a few compounds has been studied very carefully (e.g., the KC104 literature extends back for more than a hundred years because of the compound s use in explosives), the bulk of the available information is qualitative or semiquantitative. In recent years this has changed somewhat with increasing use of automated techniques such as DTA and TGA. Many of the reactions are complex, with mechanisms frequently controversial and not completely worked out. Decomposition products may depend on experimental conditions e.g., salts are frequently prepared by dehydration of their hydrates, and residual water may affect the course of the decomposition. 

Thermogravimetry is an attractive experimental technique for investigations of the thermal reactions of a wide range of initially solid or liquid substances, under controlled conditions of temperature and atmosphere. TG measurements probably provide more accurate kinetic (m, t, T) values than most other alternative laboratory methods available for the wide range of rate processes that involve a mass loss. The popularity of the method is due to the versatility and reliability of the apparatus, which provides results rapidly and is capable of automation. However, there have been relatively few critical studies of the accuracy, reproducibility, reliability, etc. of TG data based on quantitative comparisons with measurements made for the same reaction by alternative techniques, such as DTA, DSC, and EGA. One such comparison is by Brown et al. (69,70). This study of kinetic results obtained by different experimental methods contrasts with the often-reported use of multiple mathematical methods to calculate, from the same data, the kinetic model, rate equation g(a) = kt (29), the Arrhenius parameters, etc. In practice, the use of complementary kinetic observations, based on different measurable parameters of the chemical change occurring, provides a more secure foundation for kinetic data interpretation and formulation of a mechanism than multiple kinetic analyses based on a single set of experimental data. 

As in thermal kinetics, these examinations require measurements of high quality at as many times during the reaction as possible. Process control and automated equipment are state of the art nowadays to obtain data at more than 30-50 reaction times even in the case of such simple reactions as given by eq. (1.2). Otherwise different rate equations and mechanisms cannot be differentiated and trends in dependency of the photochemical quantum yields on reaction conditions will not be detected. This is the reason why - at least at the present time - data of flash photolysis result in too large standard deviations. Therefore here we focus on equipment measuring in the second or slightly sub-second time domain and on adequate examples of practical photoreactions. However, the formalism depends not on the time domain but only on the quality of data supplied. 

Thus, any successful kinetic analysis requires an optimal choice of the spectroscopic detection method, whereby special methods (e.g. pre-separation by chromatography) or modem approaches (e.g. interferometry) must also be considered. Therefore in the following sections different approaches in spectroscopy are presented, some combined irradiation and measurement devices introduced, and different set-ups classified with respect to their principle of operation (sequential, multiplex, single and double beam equipment). In addition some special devices are presented which allow an automated examination even of complex photochemical reactions (in part superimposed by thermal reactions) at a highly sophisticated level using various combinations of modem equipment and supplying data for multicomponent analysis. 

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