Chemical calibration of both flow and insertion calorimetric vessels for biological applications: limitations and solutions

It is frequently stated that calorimetric vessels should be calibrated by a chemical reaction as well as electrically by the Joule effect. For calibrating relatively rapid, purely chemical reactions, there are many well-characterised systems. However, biological reactions and the decomposition of many compounds are slow and so far only the hydrolysis of triacetin has been suggested as a suitable candidate. Measurement of the 4-cm3 glass ampoule of a Thermometric TAM batch calorimeter gave a small thermal overestimate that was close to the quantity measured by others. In earlier work, it had been found that the thermal volume of the TAM standard and customised flow vessels varied in thermal size that depended on the rate of pumping through them. The analysis in this paper proved that it is preferable to use rigorous thermal kinetic equations rather than the empirical second-order polynomial expansion often advocated for fitting the experimental data of the type found in the hydrolysis of triacetin. By fitting the data to both the first-order equation and the one that was not first order, it was unequivocally demonstrated that the best fit for this reaction is first order rather than the assumed second order. Attention was drawn to the advantage of obtaining the absolute zero time for the reaction because only then will there be true values for the rate constant and the molar reaction enthalpy.