KINETIC CALORIMETRY IN THE STUDY OF THE MECHANISM OF LOW-TEMPERATURE CHEMICAL-REACTIONS

Chemical reactions are always followed by a change in the reacting system enthalpy, hence, calorimetry as a method of enthalpy and heat capacity measuring is a universal and, sometimes, even the only possible way of studying chemical reaction kinetics. Throughout its long history, the calorimeter, having preserved the positions of the main method of thermodynamic studies, has conquered a new field of application: that of kinetic study of chemical reactions. The advantages and disadvantages of the kinetic calorimeter are now obvious. First, the advantages are: (1) the possibility of measuring the rate of a chemical reaction without any special requirements being imposed on the reaction medium (solid, viscous, multicomponent systems); (2) the high efficiency: a large volume of kinetic information in one experiment and a non-destructive character of changes; (3) the possibility of measuring directly in the field of ionizing radiation (gamma-radiation, accelerated electrons) and light; and (4) recording to the chemical conversion directly at the time of its occurrence. The disadvantages of this method are: (1) the high inertia of standard calorimeter systems (tau(c) congruent-to 10(2)-10(3) s), which restricts the possibilities of studying fast processes; and (2) the complexity of the correct organization of the calorimeter experiment when the parameters of the process are changed (overheating in the sample, conversion of the process to explosive and auto wave regimens). One of the oldest and most universal methods of studying the mechanism of chemical reactions, calorimetry, is now passing through a period of turbulent development due to the advances in electronics and computerization. The wide variety of types of calorimeter set-ups and the large assortment of measurement schemes in the currently described methods complicate the experimental selection of the necessary instrument rather than facilitate it. The basic principles of the method, the types of calorimeters, and the measuring schemes are described [1-5]. However, despite the high working characteristics of modern calorimeters (Perkin-Elmer, Du Pont, LKB, etc.), all of them have one principal disadvantage: a cell with a sample is placed in them at room temperature. In cryochemical investigation, when the sample has metastable formations, the loading is made 'from nitrogen to nitrogen', i.e. the sample prepared at 77 K should be loaded into a calorimeter at 77 K. Besides, the existing installations do not allow measurements at the temperatures <110 K. For this reason, the Laboratory of Cryochemistry and Radiation Chemistry at the Institute of Chemical Physics in Chernogolovka has created original calorimetric techniques which allow: (1) the carrying out phase analysis and the determination of the main thermodynamic characteristic of individual substances and complicated systems in the temperature range 5-300 K. Sample loading can be conducted at 77 K that allows us to study the systems containing: tetrafluoroethylene, hexafluoropropylene, ethylene, carbon monoxide, nitrogen, methane, hydrogen, oxygen, ozone, formaldehyde and many other gaseous substances;