TABLE OF CONTENTS Attempts to find a reaction which could serve as a standard for comparison of kinetic measurements have not been successful. The major requirements for such a reaction have been specified by Gallagher [89] as: (i) an irreversible reaction taking place in a single stage, with (ii) a low value of the enthalpy of reaction, to minimize self-heating or self-cooling effects; (iii) the temperature range required for reaction to proceed at a slow, but measurable rate should not be too low, so as to avoid large temperature calibration errors; (iv) there should be no reaction of the sample with the surrounding atmosphere; (v) no dependence of reaction on the method of sample preparation, pretreatment or particle size and distribution; and (vi) the changes to be measured to follow the course of reaction, e.g., mass, amounts of evolved gases, enthalpy change, should be large, to permit the use of small samples. Some of these requirements are not compatible with each other, so compromises are necessary.
The dehydration of lithium sulfate monohydrate as a kinetic standard was suggested, but ruled out [89], because: (i) the reaction is reversible at low temperatures, and (ii) is moderately endothermic; (iii) the reaction takes place at temperatures below 370 K and TG instruments are difficult to calibrate accurately in this range; (iv) rates of the reaction are very dependent upon particle-size and prehistory; (v) overall dehydration involves several rate processes, e.g., chemical reaction, diffusion, recrystallization, and the rate-determining step may not remain...
