Experimental techniques continued temperature measurement

From the discussion presented of reactions in solids, it should be apparent that it is not practical in most cases to determine the concentration of some species during a kinetic study. In fact, it may be necessary to perform the analysis in a continuous way as the sample reacts with no separation necessary or even possible. Experimental methods that allow measurement of the progress of the reaction, especially as the temperature is increased, are particularly valuable. Two such techniques are thermo-gravimetric analysis (TGA) and differential scanning calorimetry (DSC). These techniques have become widely used to characterize solids, determine thermal stability, study phase changes, and so forth. Because they are so versatile in studies on solids, these techniques will be described briefly. 

Experimental technique. The experiments were carried out in small, cylindrical, stainless. steel autoclaves such as shown in Fig. 4-24. Thermocouples welded into the bottom closure and a 20-mil-ID steel capillary in the top permitted a continuous measurement of temperature and pressure. Continuous stirring was accomplished by means of a dashpot type stirrer of Armco iron clad with stainless steel, with a stainless steel stirring head. The capacity of the autoclave with stirrer was approximately 14 ml. 

As noted above, not all techniques which provide information regarding crystallinity are useful to follow the rate of crystallization. In addition to sufficient sensitivity to monitor small changes, the method must be rapid and suitable for isothermal regulation, quite possibly over a range of different temperatures. Specific volume measurements are especially convenient for this purpose. We shall continue our discussion using specific volume as the experimental method. 

This chapter describes use of solid-surface room temperature phosphorimetry (SSRTP) as a detection technique in the liquid chromatographic (LC) analysis of caffeine, theophylline, and theobromine. Measurements were made in a continuous mode, using a 2-nebulizer automatic system for SSRTP analysis (previously optimized for LC detection). Use of SSRTP and UV absorption detection was compared under identical experimental conditions.38... 

In common with other spectroscopic techniques, UV spectroscopy can be used to measure the kinetics of chemical reactions, including biochemical reactions catalyzed by enzymes. For example, suppose that two compounds A and B react to form a third compound C. If the third compound absorbs UV radiation, its concentration can be measured continuously. The original concentrations of A and B can be measured at the start of the experiment. By measuring the concentration of C at different time intervals, the kinetics of the reaction A -I- B C can be calculated. Enzyme reactions are important biochemically and also analytically an enzyme is very selective, even specific, for a given compound. The compound with which the enzyme reacts is called the substrate. If the enzyme assay is correctly designed, any change in absorbance of the sample will result only from reaction of the substrate with the enzyme. The rate of an enzyme reaction depends on temperature, pH, enzyme concentration and activity, and substrate concentration. If conditions are selected such that all of the substrate is converted to product in a short period of time, the amount of substrate can be calculated from the difference between the initial absorbance of the solution and the final absorbance. Alternatively, the other experimental variables can be controlled so that the rate of the enzyme reaction is directly proportional to substrate concentration. 

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