Nonisothermal aqueous calorimetry: computation of process-dependent temperature change and aspects of calorimeter design.
A general method for determining the process-dependent (intrinsic) temperature change in a nonisothermal calorimeter is presented. The nonisothermal approach to calorimetric investigations requires an estimate of the magnitude of the process independent (extrinsic) temperature change during the reaction period. The proposed method can be applied to any calorimeter whose output is a discrete or continuous temperature--time profile. It is based on a first-derivative transformation of the temperature--time profile and the partitioning of the observed temperature variation into two components: pure extrinsic variation, which occurs outside the reaction period, and the combined extrinsic and intrinsic effects during the reaction period. Close examination of the pure extrinsic variation was considered essential, since it provided the basis for identifying the form of a descriptive mathematical function consistent with the observed extrinsic behavior. Once a suitable function was selected, parameters for the equation were determined through a linear regression procedure. The resulting equation was used to predict the extrinsic variation within the reaction period. Subtraction of predicted extrinsic variation from the observed total variation and integration over the time course of the experiment provide an estimate of the process-dependent temperature change. The differential approach was examined for processes performed in a calorimeter of simple design. Aspects of calorimeter design and advantages of the proposed method of data analysis are discussed.